text:
This file documents the GNU debugger GDB.
This is the Tenth Edition, of Debugging with GDB: the GNU Source-Level Debugger for GDB (GDB) Version 18.104.22.16870330-git.
Copyright © 1988-2017 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being "Free Software" and "Free Software Needs Free Documentation", with the Front-Cover Texts being "A GNU Manual," and with the Back-Cover Texts as in (a) below.
(a) The FSF's Back-Cover Text is: "You are free to copy and modify this GNU Manual. Buying copies from GNU Press supports the FSF in developing GNU and promoting software freedom."
This file describes GDB, the GNU symbolic debugger.
This is the Tenth Edition, for GDB (GDB) Version 126.96.36.19970330-git.
Copyright (C) 1988-2017 Free Software Foundation, Inc.
This edition of the GDB manual is dedicated to the memory of Fred Fish. Fred was a long-standing contributor to GDB and to Free software in general. We will miss him.
• Summary: | Summary of GDB | |
• Sample Session: | A sample GDB session | |
• Invocation: | Getting in and out of GDB | |
• Commands: | GDB commands | |
• Running: | Running programs under GDB | |
• Stopping: | Stopping and continuing | |
• Reverse Execution: | Running programs backward | |
• Process Record and Replay: | Recording inferior's execution and replaying it | |
• Stack: | Examining the stack | |
• Source: | Examining source files | |
• Data: | Examining data | |
• Optimized Code: | Debugging optimized code | |
• Macros: | Preprocessor Macros | |
• Tracepoints: | Debugging remote targets non-intrusively | |
• Overlays: | Debugging programs that use overlays | |
• Languages: | Using GDB with different languages | |
• Symbols: | Examining the symbol table | |
• Altering: | Altering execution | |
• GDB Files: | GDB files | |
• Targets: | Specifying a debugging target | |
• Remote Debugging: | Debugging remote programs | |
• Configurations: | Configuration-specific information | |
• Controlling GDB: | Controlling GDB | |
• Extending GDB: | Extending GDB | |
• Interpreters: | Command Interpreters | |
• TUI: | GDB Text User Interface | |
• Emacs: | Using GDB under GNU Emacs | |
• GDB/MI: | GDB's Machine Interface. | |
• Annotations: | GDB's annotation interface. | |
• JIT Interface: | Using the JIT debugging interface. | |
• In-Process Agent: | In-Process Agent | |
• GDB Bugs: | Reporting bugs in GDB | |
• Command Line Editing: | Command Line Editing | |
• Using History Interactively: | Using History Interactively | |
• In Memoriam: | In Memoriam | |
• Formatting Documentation: | How to format and print GDB documentation | |
• Installing GDB: | Installing GDB | |
• Maintenance Commands: | Maintenance Commands | |
• Remote Protocol: | GDB Remote Serial Protocol | |
• Agent Expressions: | The GDB Agent Expression Mechanism | |
• Target Descriptions: | How targets can describe themselves to GDB | |
• Operating System Information: | Getting additional information from the operating system | |
• Trace File Format: | GDB trace file format | |
• Index Section Format: | .gdb_index section format | |
• Man Pages: | Manual pages | |
• Copying: | GNU General Public License says how you can copy and share GDB | |
• GNU Free Documentation License: | The license for this documentation | |
• Concept Index: | Index of GDB concepts | |
• Command and Variable Index: | Index of GDB commands, variables, functions, and Python data types |
section formatNext: Sample Session, Up: Top [Contents][Index]
The purpose of a debugger such as GDB is to allow you to see what is going on "inside" another program while it executes—or what another program was doing at the moment it crashed.
GDB can do four main kinds of things (plus other things in support of these) to help you catch bugs in the act:
You can use GDB to debug programs written in C and C++. For more information, see Supported Languages. For more information, see C and C++.
Support for D is partial. For information on D, see D.
Support for Modula-2 is partial. For information on Modula-2, see Modula-2.
Support for OpenCL C is partial. For information on OpenCL C, see OpenCL C.
Debugging Pascal programs which use sets, subranges, file variables, or nested functions does not currently work. GDB does not support entering expressions, printing values, or similar features using Pascal syntax.
GDB can be used to debug programs written in Fortran, although it may be necessary to refer to some variables with a trailing underscore.
GDB can be used to debug programs written in Objective-C, using either the Apple/NeXT or the GNU Objective-C runtime.
• Free Software: | Freely redistributable software | |
• Free Documentation: | Free Software Needs Free Documentation | |
• Contributors: | Contributors to GDB |
Next: Free Documentation, Up: Summary [Contents][Index]
GDB is free software, protected by the GNU General Public License (GPL). The GPL gives you the freedom to copy or adapt a licensed program—but every person getting a copy also gets with it the freedom to modify that copy (which means that they must get access to the source code), and the freedom to distribute further copies. Typical software companies use copyrights to limit your freedoms; the Free Software Foundation uses the GPL to preserve these freedoms.
Fundamentally, the General Public License is a license which says that you have these freedoms and that you cannot take these freedoms away from anyone else.
Next: Contributors, Previous: Free Software, Up: Summary [Contents][Index]
The biggest deficiency in the free software community today is not in the software—it is the lack of good free documentation that we can include with the free software. Many of our most important programs do not come with free reference manuals and free introductory texts. Documentation is an essential part of any software package; when an important free software package does not come with a free manual and a free tutorial, that is a major gap. We have many such gaps today.
Consider Perl, for instance. The tutorial manuals that people normally use are non-free. How did this come about? Because the authors of those manuals published them with restrictive terms—no copying, no modification, source files not available—which exclude them from the free software world.
That wasn't the first time this sort of thing happened, and it was far from the last. Many times we have heard a GNU user eagerly describe a manual that he is writing, his intended contribution to the community, only to learn that he had ruined everything by signing a publication contract to make it non-free.
Free documentation, like free software, is a matter of freedom, not price. The problem with the non-free manual is not that publishers charge a price for printed copies—that in itself is fine. (The Free Software Foundation sells printed copies of manuals, too.) The problem is the restrictions on the use of the manual. Free manuals are available in source code form, and give you permission to copy and modify. Non-free manuals do not allow this.
The criteria of freedom for a free manual are roughly the same as for free software. Redistribution (including the normal kinds of commercial redistribution) must be permitted, so that the manual can accompany every copy of the program, both on-line and on paper.
Permission for modification of the technical content is crucial too. When people modify the software, adding or changing features, if they are conscientious they will change the manual too—so they can provide accurate and clear documentation for the modified program. A manual that leaves you no choice but to write a new manual to document a changed version of the program is not really available to our community.
Some kinds of limits on the way modification is handled are acceptable. For example, requirements to preserve the original author's copyright notice, the distribution terms, or the list of authors, are ok. It is also no problem to require modified versions to include notice that they were modified. Even entire sections that may not be deleted or changed are acceptable, as long as they deal with nontechnical topics (like this one). These kinds of restrictions are acceptable because they don't obstruct the community's normal use of the manual.
However, it must be possible to modify all the technical content of the manual, and then distribute the result in all the usual media, through all the usual channels. Otherwise, the restrictions obstruct the use of the manual, it is not free, and we need another manual to replace it.
Please spread the word about this issue. Our community continues to lose manuals to proprietary publishing. If we spread the word that free software needs free reference manuals and free tutorials, perhaps the next person who wants to contribute by writing documentation will realize, before it is too late, that only free manuals contribute to the free software community.
If you are writing documentation, please insist on publishing it under the GNU Free Documentation License or another free documentation license. Remember that this decision requires your approval—you don't have to let the publisher decide. Some commercial publishers will use a free license if you insist, but they will not propose the option; it is up to you to raise the issue and say firmly that this is what you want. If the publisher you are dealing with refuses, please try other publishers. If you're not sure whether a proposed license is free, write to email@example.com.
You can encourage commercial publishers to sell more free, copylefted manuals and tutorials by buying them, and particularly by buying copies from the publishers that paid for their writing or for major improvements. Meanwhile, try to avoid buying non-free documentation at all. Check the distribution terms of a manual before you buy it, and insist that whoever seeks your business must respect your freedom. Check the history of the book, and try to reward the publishers that have paid or pay the authors to work on it.
The Free Software Foundation maintains a list of free documentation published by other publishers, at http://www.fsf.org/doc/other-free-books.html.
Previous: Free Documentation, Up: Summary [Contents][Index]
Richard Stallman was the original author of GDB, and of many other GNU programs. Many others have contributed to its development. This section attempts to credit major contributors. One of the virtues of free software is that everyone is free to contribute to it; with regret, we cannot actually acknowledge everyone here. The file ChangeLog in the GDB distribution approximates a blow-by-blow account.
Changes much prior to version 2.0 are lost in the mists of time.
Plea: Additions to this section are particularly welcome. If you or your friends (or enemies, to be evenhanded) have been unfairly omitted from this list, we would like to add your names!
So that they may not regard their many labors as thankless, we particularly thank those who shepherded GDB through major releases: Andrew Cagney (releases 6.3, 6.2, 6.1, 6.0, 5.3, 5.2, 5.1 and 5.0); Jim Blandy (release 4.18); Jason Molenda (release 4.17); Stan Shebs (release 4.14); Fred Fish (releases 4.16, 4.15, 4.13, 4.12, 4.11, 4.10, and 4.9); Stu Grossman and John Gilmore (releases 4.8, 4.7, 4.6, 4.5, and 4.4); John Gilmore (releases 4.3, 4.2, 4.1, 4.0, and 3.9); Jim Kingdon (releases 3.5, 3.4, and 3.3); and Randy Smith (releases 3.2, 3.1, and 3.0).
Richard Stallman, assisted at various times by Peter TerMaat, Chris Hanson, and Richard Mlynarik, handled releases through 2.8.
Michael Tiemann is the author of most of the GNU C++ support in GDB, with significant additional contributions from Per Bothner and Daniel Berlin. James Clark wrote the GNU C++ demangler. Early work on C++ was by Peter TerMaat (who also did much general update work leading to release 3.0).
GDB uses the BFD subroutine library to examine multiple object-file formats; BFD was a joint project of David V. Henkel-Wallace, Rich Pixley, Steve Chamberlain, and John Gilmore.
David Johnson wrote the original COFF support; Pace Willison did the original support for encapsulated COFF.
Brent Benson of Harris Computer Systems contributed DWARF 2 support.
Adam de Boor and Bradley Davis contributed the ISI Optimum V support. Per Bothner, Noboyuki Hikichi, and Alessandro Forin contributed MIPS support. Jean-Daniel Fekete contributed Sun 386i support. Chris Hanson improved the HP9000 support. Noboyuki Hikichi and Tomoyuki Hasei contributed Sony/News OS 3 support. David Johnson contributed Encore Umax support. Jyrki Kuoppala contributed Altos 3068 support. Jeff Law contributed HP PA and SOM support. Keith Packard contributed NS32K support. Doug Rabson contributed Acorn Risc Machine support. Bob Rusk contributed Harris Nighthawk CX-UX support. Chris Smith contributed Convex support (and Fortran debugging). Jonathan Stone contributed Pyramid support. Michael Tiemann contributed SPARC support. Tim Tucker contributed support for the Gould NP1 and Gould Powernode. Pace Willison contributed Intel 386 support. Jay Vosburgh contributed Symmetry support. Marko Mlinar contributed OpenRISC 1000 support.
Andreas Schwab contributed M68K GNU/Linux support.
Rich Schaefer and Peter Schauer helped with support of SunOS shared libraries.
Jay Fenlason and Roland McGrath ensured that GDB and GAS agree about several machine instruction sets.
Patrick Duval, Ted Goldstein, Vikram Koka and Glenn Engel helped develop remote debugging. Intel Corporation, Wind River Systems, AMD, and ARM contributed remote debugging modules for the i960, VxWorks, A29K UDI, and RDI targets, respectively.
Brian Fox is the author of the readline libraries providing command-line editing and command history.
Andrew Beers of SUNY Buffalo wrote the language-switching code, the Modula-2 support, and contributed the Languages chapter of this manual.
Fred Fish wrote most of the support for Unix System Vr4. He also enhanced the command-completion support to cover C++ overloaded symbols.
Hitachi America (now Renesas America), Ltd. sponsored the support for H8/300, H8/500, and Super-H processors.
NEC sponsored the support for the v850, Vr4xxx, and Vr5xxx processors.
Mitsubishi (now Renesas) sponsored the support for D10V, D30V, and M32R/D processors.
Toshiba sponsored the support for the TX39 Mips processor.
Matsushita sponsored the support for the MN10200 and MN10300 processors.
Fujitsu sponsored the support for SPARClite and FR30 processors.
Kung Hsu, Jeff Law, and Rick Sladkey added support for hardware watchpoints.
Michael Snyder added support for tracepoints.
Stu Grossman wrote gdbserver.
Jim Kingdon, Peter Schauer, Ian Taylor, and Stu Grossman made nearly innumerable bug fixes and cleanups throughout GDB.
The following people at the Hewlett-Packard Company contributed support for the PA-RISC 2.0 architecture, HP-UX 10.20, 10.30, and 11.0 (narrow mode), HP's implementation of kernel threads, HP's aC++ compiler, and the Text User Interface (nee Terminal User Interface): Ben Krepp, Richard Title, John Bishop, Susan Macchia, Kathy Mann, Satish Pai, India Paul, Steve Rehrauer, and Elena Zannoni. Kim Haase provided HP-specific information in this manual.
DJ Delorie ported GDB to MS-DOS, for the DJGPP project. Robert Hoehne made significant contributions to the DJGPP port.
Cygnus Solutions has sponsored GDB maintenance and much of its development since 1991. Cygnus engineers who have worked on GDB fulltime include Mark Alexander, Jim Blandy, Per Bothner, Kevin Buettner, Edith Epstein, Chris Faylor, Fred Fish, Martin Hunt, Jim Ingham, John Gilmore, Stu Grossman, Kung Hsu, Jim Kingdon, John Metzler, Fernando Nasser, Geoffrey Noer, Dawn Perchik, Rich Pixley, Zdenek Radouch, Keith Seitz, Stan Shebs, David Taylor, and Elena Zannoni. In addition, Dave Brolley, Ian Carmichael, Steve Chamberlain, Nick Clifton, JT Conklin, Stan Cox, DJ Delorie, Ulrich Drepper, Frank Eigler, Doug Evans, Sean Fagan, David Henkel-Wallace, Richard Henderson, Jeff Holcomb, Jeff Law, Jim Lemke, Tom Lord, Bob Manson, Michael Meissner, Jason Merrill, Catherine Moore, Drew Moseley, Ken Raeburn, Gavin Romig-Koch, Rob Savoye, Jamie Smith, Mike Stump, Ian Taylor, Angela Thomas, Michael Tiemann, Tom Tromey, Ron Unrau, Jim Wilson, and David Zuhn have made contributions both large and small.
Andrew Cagney, Fernando Nasser, and Elena Zannoni, while working for Cygnus Solutions, implemented the original GDB/MI interface.
Jim Blandy added support for preprocessor macros, while working for Red Hat.
Andrew Cagney designed GDB's architecture vector. Many people including Andrew Cagney, Stephane Carrez, Randolph Chung, Nick Duffek, Richard Henderson, Mark Kettenis, Grace Sainsbury, Kei Sakamoto, Yoshinori Sato, Michael Snyder, Andreas Schwab, Jason Thorpe, Corinna Vinschen, Ulrich Weigand, and Elena Zannoni, helped with the migration of old architectures to this new framework.
Andrew Cagney completely re-designed and re-implemented GDB's unwinder framework, this consisting of a fresh new design featuring frame IDs, independent frame sniffers, and the sentinel frame. Mark Kettenis implemented the DWARF 2 unwinder, Jeff Johnston the libunwind unwinder, and Andrew Cagney the dummy, sentinel, tramp, and trad unwinders. The architecture-specific changes, each involving a complete rewrite of the architecture's frame code, were carried out by Jim Blandy, Joel Brobecker, Kevin Buettner, Andrew Cagney, Stephane Carrez, Randolph Chung, Orjan Friberg, Richard Henderson, Daniel Jacobowitz, Jeff Johnston, Mark Kettenis, Theodore A. Roth, Kei Sakamoto, Yoshinori Sato, Michael Snyder, Corinna Vinschen, and Ulrich Weigand.
Christian Zankel, Ross Morley, Bob Wilson, and Maxim Grigoriev from Tensilica, Inc. contributed support for Xtensa processors. Others who have worked on the Xtensa port of GDB in the past include Steve Tjiang, John Newlin, and Scott Foehner.
Michael Eager and staff of Xilinx, Inc., contributed support for the Xilinx MicroBlaze architecture.
Next: Invocation, Previous: Summary, Up: Top [Contents][Index]
You can use this manual at your leisure to read all about GDB. However, a handful of commands are enough to get started using the debugger. This chapter illustrates those commands.
One of the preliminary versions of GNU m4
(a generic macro
processor) exhibits the following bug: sometimes, when we change its
quote strings from the default, the commands used to capture one macro
definition within another stop working. In the following short m4
session, we define a macro foo
which expands to 0000
; we
then use the m4
built-in defn
to define bar
as the
same thing. However, when we change the open quote string to
and the close quote string to <UNQUOTE>
, the same
procedure fails to define a new synonym baz
$ cd gnu/m4 $ ./m4 define(foo,0000) foo 0000 define(bar,defn('foo')) bar 0000 changequote(<QUOTE>,<UNQUOTE>) define(baz,defn(<QUOTE>foo<UNQUOTE>)) baz Ctrl-d m4: End of input: 0: fatal error: EOF in string
Let us use GDB to try to see what is going on.
$ gdb m4 GDB is free software and you are welcome to distribute copies of it under certain conditions; type "show copying" to see the conditions. There is absolutely no warranty for GDB; type "show warranty" for details. GDB 18.104.22.16870330-git, Copyright 1999 Free Software Foundation, Inc... (gdb)
GDB reads only enough symbol data to know where to find the rest when needed; as a result, the first prompt comes up very quickly. We now tell GDB to use a narrower display width than usual, so that examples fit in this manual.
(gdb) set width 70
We need to see how the m4
built-in changequote
Having looked at the source, we know the relevant subroutine is
, so we set a breakpoint there with the GDB
(gdb) break m4_changequote Breakpoint 1 at 0x62f4: file builtin.c, line 879.
Using the run
command, we start m4
running under GDB
control; as long as control does not reach the m4_changequote
subroutine, the program runs as usual:
(gdb) run Starting program: /work/Editorial/gdb/gnu/m4/m4 define(foo,0000) foo 0000
To trigger the breakpoint, we call changequote
suspends execution of m4
, displaying information about the
context where it stops.
changequote(<QUOTE>,<UNQUOTE>) Breakpoint 1, m4_changequote (argc=3, argv=0x33c70) at builtin.c:879 879 if (bad_argc(TOKEN_DATA_TEXT(argv[0]),argc,1,3))
Now we use the command n
) to advance execution to
the next line of the current function.
(gdb) n 882 set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1])\ : nil,
looks like a promising subroutine. We can go into it
by using the command s
) instead of next
goes to the next line to be executed in any
subroutine, so it steps into set_quotes
(gdb) s set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>") at input.c:530 530 if (lquote != def_lquote)
The display that shows the subroutine where m4
is now
suspended (and its arguments) is called a stack frame display. It
shows a summary of the stack. We can use the backtrace
command (which can also be spelled bt
), to see where we are
in the stack as a whole: the backtrace
command displays a
stack frame for each active subroutine.
(gdb) bt #0 set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>") at input.c:530 #1 0x6344 in m4_changequote (argc=3, argv=0x33c70) at builtin.c:882 #2 0x8174 in expand_macro (sym=0x33320) at macro.c:242 #3 0x7a88 in expand_token (obs=0x0, t=209696, td=0xf7fffa30) at macro.c:71 #4 0x79dc in expand_input () at macro.c:40 #5 0x2930 in main (argc=0, argv=0xf7fffb20) at m4.c:195
We step through a few more lines to see what happens. The first two
times, we can use 's'; the next two times we use n
to avoid
falling into the xstrdup
(gdb) s 0x3b5c 532 if (rquote != def_rquote) (gdb) s 0x3b80 535 lquote = (lq == nil || *lq == '\0') ? \ def_lquote : xstrdup(lq); (gdb) n 536 rquote = (rq == nil || *rq == '\0') ? def_rquote\ : xstrdup(rq); (gdb) n 538 len_lquote = strlen(rquote);
The last line displayed looks a little odd; we can examine the variables
and rquote
to see if they are in fact the new left
and right quotes we specified. We use the command p
) to see their values.
(gdb) p lquote $1 = 0x35d40 "<QUOTE>" (gdb) p rquote $2 = 0x35d50 "<UNQUOTE>"
and rquote
are indeed the new left and right quotes.
To look at some context, we can display ten lines of source
surrounding the current line with the l
) command.
(gdb) l 533 xfree(rquote); 534 535 lquote = (lq == nil || *lq == '\0') ? def_lquote\ : xstrdup (lq); 536 rquote = (rq == nil || *rq == '\0') ? def_rquote\ : xstrdup (rq); 537 538 len_lquote = strlen(rquote); 539 len_rquote = strlen(lquote); 540 } 541 542 void
Let us step past the two lines that set len_lquote
, and then examine the values of those variables.
(gdb) n 539 len_rquote = strlen(lquote); (gdb) n 540 } (gdb) p len_lquote $3 = 9 (gdb) p len_rquote $4 = 7
That certainly looks wrong, assuming len_lquote
are meant to be the lengths of lquote
respectively. We can set them to better values using
the p
command, since it can print the value of
any expression—and that expression can include subroutine calls and
(gdb) p len_lquote=strlen(lquote) $5 = 7 (gdb) p len_rquote=strlen(rquote) $6 = 9
Is that enough to fix the problem of using the new quotes with the
built-in defn
? We can allow m4
to continue
executing with the c
) command, and then try the
example that caused trouble initially:
(gdb) c Continuing. define(baz,defn(<QUOTE>foo<UNQUOTE>)) baz 0000
Success! The new quotes now work just as well as the default ones. The
problem seems to have been just the two typos defining the wrong
lengths. We allow m4
exit by giving it an EOF as input:
Ctrl-d Program exited normally.
The message 'Program exited normally.' is from GDB; it
indicates m4
has finished executing. We can end our GDB
session with the GDB quit
(gdb) quit
Next: Commands, Previous: Sample Session, Up: Top [Contents][Index]
This chapter discusses how to start GDB, and how to get out of it. The essentials are:
• Invoking GDB: | How to start GDB | |
• Quitting GDB: | How to quit GDB | |
• Shell Commands: | How to use shell commands inside GDB | |
• Logging Output: | How to log GDB's output to a file |
Next: Quitting GDB, Up: Invocation [Contents][Index]
Invoke GDB by running the program gdb
. Once started,
GDB reads commands from the terminal until you tell it to exit.
You can also run gdb
with a variety of arguments and options,
to specify more of your debugging environment at the outset.
The command-line options described here are designed to cover a variety of situations; in some environments, some of these options may effectively be unavailable.
The most usual way to start GDB is with one argument, specifying an executable program:
gdb program
You can also start with both an executable program and a core file specified:
gdb program core
You can, instead, specify a process ID as a second argument, if you want to debug a running process:
gdb program 1234
would attach GDB to process 1234
(unless you also have a file
named 1234; GDB does check for a core file first).
Taking advantage of the second command-line argument requires a fairly complete operating system; when you use GDB as a remote debugger attached to a bare board, there may not be any notion of "process", and there is often no way to get a core dump. GDB will warn you if it is unable to attach or to read core dumps.
You can optionally have gdb
pass any arguments after the
executable file to the inferior using --args
. This option stops
option processing.
gdb --args gcc -O2 -c foo.c
This will cause gdb
to debug gcc
, and to set
's command-line arguments (see Arguments) to '-O2 -c foo.c'.
You can run gdb
without printing the front material, which describes
GDB's non-warranty, by specifying --silent
(or -q
gdb --silent
You can further control how GDB starts up by using command-line options. GDB itself can remind you of the options available.
gdb -help
to display all available options and briefly describe their use ('gdb -h' is a shorter equivalent).
All options and command line arguments you give are processed in sequential order. The order makes a difference when the '-x' option is used.
• File Options: | Choosing files | |
• Mode Options: | Choosing modes | |
• Startup: | What GDB does during startup |
Next: Mode Options, Up: Invoking GDB [Contents][Index]
When GDB starts, it reads any arguments other than options as specifying an executable file and core file (or process ID). This is the same as if the arguments were specified by the '-se' and '-c' (or '-p') options respectively. (GDB reads the first argument that does not have an associated option flag as equivalent to the '-se' option followed by that argument; and the second argument that does not have an associated option flag, if any, as equivalent to the '-c'/'-p' option followed by that argument.) If the second argument begins with a decimal digit, GDB will first attempt to attach to it as a process, and if that fails, attempt to open it as a corefile. If you have a corefile whose name begins with a digit, you can prevent GDB from treating it as a pid by prefixing it with ./, e.g. ./12345.
If GDB has not been configured to included core file support, such as for most embedded targets, then it will complain about a second argument and ignore it.
Many options have both long and short forms; both are shown in the following list. GDB also recognizes the long forms if you truncate them, so long as enough of the option is present to be unambiguous. (If you prefer, you can flag option arguments with '--' rather than '-', though we illustrate the more usual convention.)
-symbols file
-s file
Read symbol table from file file.
-exec file
-e file
Use file file as the executable file to execute when appropriate, and for examining pure data in conjunction with a core dump.
-se file
Read symbol table from file file and use it as the executable file.
-core file
-c file
Use file file as a core dump to examine.
-pid number
-p number
Connect to process ID number, as with the attach
-command file
-x file
Execute commands from file file. The contents of this file is
evaluated exactly as the source
command would.
See Command files.
-eval-command command
-ex command
Execute a single GDB command.
This option may be used multiple times to call multiple commands. It may also be interleaved with '-command' as required.
gdb -ex 'target sim' -ex 'load' \ -x setbreakpoints -ex 'run' a.out
-init-command file
-ix file
Execute commands from file file before loading the inferior (but after loading gdbinit files). See Startup.
-init-eval-command command
-iex command
Execute a single GDB command before loading the inferior (but after loading gdbinit files). See Startup.
-directory directory
-d directory
Add directory to the path to search for source and script files.
Read each symbol file's entire symbol table immediately, rather than the default, which is to read it incrementally as it is needed. This makes startup slower, but makes future operations faster.
Next: Startup, Previous: File Options, Up: Invoking GDB [Contents][Index]
You can run GDB in various alternative modes—for example, in batch mode or quiet mode.
Do not execute commands found in any initialization file. There are three init files, loaded in the following order:
This is the system-wide init file.
Its location is specified with the --with-system-gdbinit
configure option (see System-wide configuration).
It is loaded first when GDB starts, before command line options
have been processed.
This is the init file in your home directory. It is loaded next, after system.gdbinit, and before command options have been processed.
This is the init file in the current directory.
It is loaded last, after command line options other than -x
have been processed. Command line options -x
are processed last, after ./.gdbinit has been loaded.
For further documentation on startup processing, See Startup. For documentation on how to write command files, See Command Files.
Do not execute commands found in ~/.gdbinit, the init file in your home directory. See Startup.
"Quiet". Do not print the introductory and copyright messages. These messages are also suppressed in batch mode.
Run in batch mode. Exit with status 0
after processing all the
command files specified with '-x' (and all commands from
initialization files, if not inhibited with '-n'). Exit with
nonzero status if an error occurs in executing the GDB commands
in the command files. Batch mode also disables pagination, sets unlimited
terminal width and height see Screen Size, and acts as if set confirm
off were in effect (see Messages/Warnings).
Batch mode may be useful for running GDB as a filter, for example to download and run a program on another computer; in order to make this more useful, the message
Program exited normally.
(which is ordinarily issued whenever a program running under GDB control terminates) is not issued when running in batch mode.
Run in batch mode exactly like '-batch', but totally silently. All
GDB output to stdout
is prevented (stderr
unaffected). This is much quieter than '-silent' and would be useless
for an interactive session.
This is particularly useful when using targets that give 'Loading section' messages, for example.
Note that targets that give their output via GDB, as opposed to
writing directly to stdout
, will also be made silent.
The return code from GDB will be the return code from the child process (the process being debugged), with the following exceptions:
This option is useful in conjunction with '-batch' or '-batch-silent', when GDB is being used as a remote program loader or simulator interface.
"No windows". If GDB comes with a graphical user interface (GUI) built in, then this option tells GDB to only use the command-line interface. If no GUI is available, this option has no effect.
If GDB includes a GUI, then this option requires it to be used if possible.
-cd directory
Run GDB using directory as its working directory, instead of the current directory.
-data-directory directory
-D directory
Run GDB using directory as its data directory. The data directory is where GDB searches for its auxiliary files. See Data Files.
GNU Emacs sets this option when it runs GDB as a subprocess. It tells GDB to output the full file name and line number in a standard, recognizable fashion each time a stack frame is displayed (which includes each time your program stops). This recognizable format looks like two '\032' characters, followed by the file name, line number and character position separated by colons, and a newline. The Emacs-to-GDB interface program uses the two '\032' characters as a signal to display the source code for the frame.
-annotate level
This option sets the annotation level inside GDB. Its effect is identical to using 'set annotate level' (see Annotations). The annotation level controls how much information GDB prints together with its prompt, values of expressions, source lines, and other types of output. Level 0 is the normal, level 1 is for use when GDB is run as a subprocess of GNU Emacs, level 3 is the maximum annotation suitable for programs that control GDB, and level 2 has been deprecated.
The annotation mechanism has largely been superseded by GDB/MI (see GDB/MI).
Change interpretation of command line so that arguments following the executable file are passed as command line arguments to the inferior. This option stops option processing.
-baud bps
-b bps
Set the line speed (baud rate or bits per second) of any serial interface used by GDB for remote debugging.
-l timeout
Set the timeout (in seconds) of any communication used by GDB for remote debugging.
-tty device
-t device
Run using device for your program's standard input and output.
Activate the Text User Interface when starting. The Text User Interface manages several text windows on the terminal, showing source, assembly, registers and GDB command outputs (see GDB Text User Interface). Do not use this option if you run GDB from Emacs (see Using GDB under GNU Emacs).
-interpreter interp
Use the interpreter interp for interface with the controlling program or device. This option is meant to be set by programs which communicate with GDB using it as a back end. See Command Interpreters.
'--interpreter=mi' (or '--interpreter=mi2') causes GDB to use the GDB/MI interface (see The GDB/MI Interface) included since GDB version 6.0. The previous GDB/MI interface, included in GDB version 5.3 and selected with '--interpreter=mi1', is deprecated. Earlier GDB/MI interfaces are no longer supported.
Open the executable and core files for both reading and writing. This is equivalent to the 'set write on' command inside GDB (see Patching).
This option causes GDB to print statistics about time and memory usage after it completes each command and returns to the prompt.
This option causes GDB to print its version number and no-warranty blurb, and exit.
This option causes GDB to print details about its build-time configuration parameters, and then exit. These details can be important when reporting GDB bugs (see GDB Bugs).
Previous: Mode Options, Up: Invoking GDB [Contents][Index]
Here's the description of what GDB does during session startup:
If you wish to disable the auto-loading during startup, you must do something like the following:
$ gdb -iex "set auto-load python-scripts off" myprogram
Option '-ex' does not work because the auto-loading is then turned off too late.
Init files use the same syntax as command files (see Command Files) and are processed by GDB in the same way. The init file in your home directory can set options (such as 'set complaints') that affect subsequent processing of command line options and operands. Init files are not executed if you use the '-nx' option (see Choosing Modes).
To display the list of init files loaded by gdb at startup, you can use gdb --help.
The GDB init files are normally called .gdbinit. The DJGPP port of GDB uses the name gdb.ini, due to the limitations of file names imposed by DOS filesystems. The Windows port of GDB uses the standard name, but if it finds a gdb.ini file in your home directory, it warns you about that and suggests to rename the file to the standard name.
Next: Shell Commands, Previous: Invoking GDB, Up: Invocation [Contents][Index]
quit [expression]
To exit GDB, use the quit
command (abbreviated
), or type an end-of-file character (usually Ctrl-d). If you
do not supply expression, GDB will terminate normally;
otherwise it will terminate using the result of expression as the
error code.
An interrupt (often Ctrl-c) does not exit from GDB, but rather terminates the action of any GDB command that is in progress and returns to GDB command level. It is safe to type the interrupt character at any time because GDB does not allow it to take effect until a time when it is safe.
If you have been using GDB to control an attached process or
device, you can release it with the detach
(see Debugging an Already-running Process).
Next: Logging Output, Previous: Quitting GDB, Up: Invocation [Contents][Index]
If you need to execute occasional shell commands during your
debugging session, there is no need to leave or suspend GDB; you can
just use the shell
shell command-string
Invoke a standard shell to execute command-string.
Note that no space is needed between !
and command-string.
If it exists, the environment variable SHELL
determines which
shell to run. Otherwise GDB uses the default shell
(/bin/sh on Unix systems, COMMAND.COM on MS-DOS, etc.).
The utility make
is often needed in development environments.
You do not have to use the shell
command for this purpose in
make make-args
Execute the make
program with the specified
arguments. This is equivalent to 'shell make make-args'.
Previous: Shell Commands, Up: Invocation [Contents][Index]
You may want to save the output of GDB commands to a file. There are several commands to control GDB's logging.
set logging on
Enable logging.
set logging off
set logging file file
Change the name of the current logfile. The default logfile is gdb.txt.
set logging overwrite [on|off]
By default, GDB will append to the logfile. Set overwrite
you want set logging on
to overwrite the logfile instead.
set logging redirect [on|off]
By default, GDB output will go to both the terminal and the logfile.
Set redirect
if you want output to go only to the log file.
show logging
Show the current values of the logging settings.
Next: Running, Previous: Invocation, Up: Top [Contents][Index]
You can abbreviate a GDB command to the first few letters of the command name, if that abbreviation is unambiguous; and you can repeat certain GDB commands by typing just RET. You can also use the TAB key to get GDB to fill out the rest of a word in a command (or to show you the alternatives available, if there is more than one possibility).
• Command Syntax: | How to give commands to GDB | |
• Completion: | Command completion | |
• Help: | How to ask GDB for help |
Next: Completion, Up: Commands [Contents][Index]
A GDB command is a single line of input. There is no limit on
how long it can be. It starts with a command name, which is followed by
arguments whose meaning depends on the command name. For example, the
command step
accepts an argument which is the number of times to
step, as in 'step 5'. You can also use the step
with no arguments. Some commands do not allow any arguments.
GDB command names may always be truncated if that abbreviation is
unambiguous. Other possible command abbreviations are listed in the
documentation for individual commands. In some cases, even ambiguous
abbreviations are allowed; for example, s
is specially defined as
equivalent to step
even though there are other commands whose
names start with s
. You can test abbreviations by using them as
arguments to the help
A blank line as input to GDB (typing just RET) means to
repeat the previous command. Certain commands (for example, run
will not repeat this way; these are commands whose unintentional
repetition might cause trouble and which you are unlikely to want to
repeat. User-defined commands can disable this feature; see
The list
and x
commands, when you repeat them with
RET, construct new arguments rather than repeating
exactly as typed. This permits easy scanning of source or memory.
GDB can also use RET in another way: to partition lengthy
output, in a way similar to the common utility more
(see Screen Size). Since it is easy to press one
RET too many in this situation, GDB disables command
repetition after any command that generates this sort of display.
Any text from a # to the end of the line is a comment; it does nothing. This is useful mainly in command files (see Command Files).
The Ctrl-o binding is useful for repeating a complex sequence of commands. This command accepts the current line, like RET, and then fetches the next line relative to the current line from the history for editing.
Next: Help, Previous: Command Syntax, Up: Commands [Contents][Index]
GDB can fill in the rest of a word in a command for you, if there is only one possibility; it can also show you what the valid possibilities are for the next word in a command, at any time. This works for GDB commands, GDB subcommands, and the names of symbols in your program.
Press the TAB key whenever you want GDB to fill out the rest of a word. If there is only one possibility, GDB fills in the word, and waits for you to finish the command (or press RET to enter it). For example, if you type
(gdb) info bre TAB
GDB fills in the rest of the word 'breakpoints', since that is
the only info
subcommand beginning with 'bre':
(gdb) info breakpoints
You can either press RET at this point, to run the info
command, or backspace and enter something else, if
'breakpoints' does not look like the command you expected. (If you
were sure you wanted info breakpoints
in the first place, you
might as well just type RET immediately after 'info bre',
to exploit command abbreviations rather than command completion).
If there is more than one possibility for the next word when you press TAB, GDB sounds a bell. You can either supply more characters and try again, or just press TAB a second time; GDB displays all the possible completions for that word. For example, you might want to set a breakpoint on a subroutine whose name begins with 'make_', but when you type b make_TAB GDB just sounds the bell. Typing TAB again displays all the function names in your program that begin with those characters, for example:
(gdb) b make_ TAB
GDB sounds bell; press TAB again, to see:
make_a_section_from_file make_environ make_abs_section make_function_type make_blockvector make_pointer_type make_cleanup make_reference_type make_command make_symbol_completion_list (gdb) b make_
After displaying the available possibilities, GDB copies your partial input ('b make_' in the example) so you can finish the command.
If you just want to see the list of alternatives in the first place, you can press M-? rather than pressing TAB twice. M-? means META ?. You can type this either by holding down a key designated as the META shift on your keyboard (if there is one) while typing ?, or as ESC followed by ?.
If the number of possible completions is large, GDB will print as much of the list as it has collected, as well as a message indicating that the list may be truncated.
(gdb) b mTABTAB main <... the rest of the possible completions ...> *** List may be truncated, max-completions reached. *** (gdb) b m
This behavior can be controlled with the following commands:
set max-completions limit
set max-completions unlimited
Set the maximum number of completion candidates. GDB will stop looking for more completions once it collects this many candidates. This is useful when completing on things like function names as collecting all the possible candidates can be time consuming. The default value is 200. A value of zero disables tab-completion. Note that setting either no limit or a very large limit can make completion slow.
show max-completions
Show the maximum number of candidates that GDB will collect and show during completion.
Sometimes the string you need, while logically a "word", may contain
parentheses or other characters that GDB normally excludes from
its notion of a word. To permit word completion to work in this
situation, you may enclose words in '
(single quote marks) in
GDB commands.
The most likely situation where you might need this is in typing the
name of a C++ function. This is because C++ allows function
overloading (multiple definitions of the same function, distinguished
by argument type). For example, when you want to set a breakpoint you
may need to distinguish whether you mean the version of name
that takes an int
parameter, name(int)
, or the version
that takes a float
parameter, name(float)
. To use the
word-completion facilities in this situation, type a single quote
at the beginning of the function name. This alerts
GDB that it may need to consider more information than usual
when you press TAB or M-? to request word completion:
(gdb) b 'bubble( M-? bubble(double,double) bubble(int,int) (gdb) b 'bubble(
In some cases, GDB can tell that completing a name requires using quotes. When this happens, GDB inserts the quote for you (while completing as much as it can) if you do not type the quote in the first place:
(gdb) b bub TAB
GDB alters your input line to the following, and rings a bell:
(gdb) b 'bubble(
In general, GDB can tell that a quote is needed (and inserts it) if you have not yet started typing the argument list when you ask for completion on an overloaded symbol.
For more information about overloaded functions, see C++ Expressions. You can use the command set
overload-resolution off
to disable overload resolution;
see GDB Features for C++.
When completing in an expression which looks up a field in a structure, GDB also tries2 to limit completions to the field names available in the type of the left-hand-side:
(gdb) p gdb_stdout.M-? magic to_fputs to_rewind to_data to_isatty to_write to_delete to_put to_write_async_safe to_flush to_read
This is because the gdb_stdout
is a variable of the type
struct ui_file
that is defined in GDB sources as
struct ui_file { int *magic; ui_file_flush_ftype *to_flush; ui_file_write_ftype *to_write; ui_file_write_async_safe_ftype *to_write_async_safe; ui_file_fputs_ftype *to_fputs; ui_file_read_ftype *to_read; ui_file_delete_ftype *to_delete; ui_file_isatty_ftype *to_isatty; ui_file_rewind_ftype *to_rewind; ui_file_put_ftype *to_put; void *to_data; }
Previous: Completion, Up: Commands [Contents][Index]
You can always ask GDB itself for information on its commands,
using the command help
You can use help
(abbreviated h
) with no arguments to
display a short list of named classes of commands:
(gdb) help List of classes of commands: aliases -- Aliases of other commands breakpoints -- Making program stop at certain points data -- Examining data files -- Specifying and examining files internals -- Maintenance commands obscure -- Obscure features running -- Running the program stack -- Examining the stack status -- Status inquiries support -- Support facilities tracepoints -- Tracing of program execution without stopping the program user-defined -- User-defined commands Type "help" followed by a class name for a list of commands in that class. Type "help" followed by command name for full documentation. Command name abbreviations are allowed if unambiguous. (gdb)
help class
Using one of the general help classes as an argument, you can get a
list of the individual commands in that class. For example, here is the
help display for the class status
(gdb) help status Status inquiries. List of commands: info -- Generic command for showing things about the program being debugged show -- Generic command for showing things about the debugger Type "help" followed by command name for full documentation. Command name abbreviations are allowed if unambiguous. (gdb)
help command
With a command name as help
argument, GDB displays a
short paragraph on how to use that command.
apropos args
The apropos
command searches through all of the GDB
commands, and their documentation, for the regular expression specified in
args. It prints out all matches found. For example:
apropos alias
results in:
alias -- Define a new command that is an alias of an existing command aliases -- Aliases of other commands d -- Delete some breakpoints or auto-display expressions del -- Delete some breakpoints or auto-display expressions delete -- Delete some breakpoints or auto-display expressions
complete args
The complete args
command lists all the possible completions
for the beginning of a command. Use args to specify the beginning of the
command you want completed. For example:
complete i
results in:
if ignore info inspect
This is intended for use by GNU Emacs.
In addition to help
, you can use the GDB commands info
and show
to inquire about the state of your program, or the state
of GDB itself. Each command supports many topics of inquiry; this
manual introduces each of them in the appropriate context. The listings
under info
and under show
in the Command, Variable, and
Function Index point to all the sub-commands. See Command and Variable Index.
This command (abbreviated i
) is for describing the state of your
program. For example, you can show the arguments passed to a function
with info args
, list the registers currently in use with info
, or list the breakpoints you have set with info breakpoints
You can get a complete list of the info
sub-commands with
help info
You can assign the result of an expression to an environment variable with
. For example, you can set the GDB prompt to a $-sign with
set prompt $
In contrast to info
, show
is for describing the state of
GDB itself.
You can change most of the things you can show
, by using the
related command set
; for example, you can control what number
system is used for displays with set radix
, or simply inquire
which is currently in use with show radix
To display all the settable parameters and their current
values, you can use show
with no arguments; you may also use
info set
. Both commands produce the same display.
Here are several miscellaneous show
subcommands, all of which are
exceptional in lacking corresponding set
show version
Show what version of GDB is running. You should include this information in GDB bug-reports. If multiple versions of GDB are in use at your site, you may need to determine which version of GDB you are running; as GDB evolves, new commands are introduced, and old ones may wither away. Also, many system vendors ship variant versions of GDB, and there are variant versions of GDB in GNU/Linux distributions as well. The version number is the same as the one announced when you start GDB.
show copying
info copying
Display information about permission for copying GDB.
show warranty
info warranty
Display the GNU "NO WARRANTY" statement, or a warranty, if your version of GDB comes with one.
show configuration
Display detailed information about the way GDB was configured
when it was built. This displays the optional arguments passed to the
configure script and also configuration parameters detected
automatically by configure
. When reporting a GDB
bug (see GDB Bugs), it is important to include this information in
your report.
When you run a program under GDB, you must first generate debugging information when you compile it.
You may start GDB with its arguments, if any, in an environment of your choice. If you are doing native debugging, you may redirect your program's input and output, debug an already running process, or kill a child process.
• Compilation: | Compiling for debugging | |
• Starting: | Starting your program | |
• Arguments: | Your program's arguments | |
• Environment: | Your program's environment | |
• Working Directory: | Your program's working directory | |
• Input/Output: | Your program's input and output | |
• Attach: | Debugging an already-running process | |
• Kill Process: | Killing the child process | |
• Inferiors and Programs: | Debugging multiple inferiors and programs | |
• Threads: | Debugging programs with multiple threads | |
• Forks: | Debugging forks | |
• Checkpoint/Restart: | Setting a bookmark to return to later |
In order to debug a program effectively, you need to generate debugging information when you compile it. This debugging information is stored in the object file; it describes the data type of each variable or function and the correspondence between source line numbers and addresses in the executable code.
To request debugging information, specify the '-g' option when you run the compiler.
Programs that are to be shipped to your customers are compiled with optimizations, using the '-O' compiler option. However, some compilers are unable to handle the '-g' and '-O' options together. Using those compilers, you cannot generate optimized executables containing debugging information.
GCC, the GNU C/C++ compiler, supports '-g' with or without '-O', making it possible to debug optimized code. We recommend that you always use '-g' whenever you compile a program. You may think your program is correct, but there is no sense in pushing your luck. For more information, see Optimized Code.
Older versions of the GNU C compiler permitted a variant option '-gg' for debugging information. GDB no longer supports this format; if your GNU C compiler has this option, do not use it.
GDB knows about preprocessor macros and can show you their expansion (see Macros). Most compilers do not include information about preprocessor macros in the debugging information if you specify the -g flag alone. Version 3.1 and later of GCC, the GNU C compiler, provides macro information if you are using the DWARF debugging format, and specify the option -g3.
See Options for Debugging Your Program or GCC in Using the GNU Compiler Collection (GCC), for more information on GCC options affecting debug information.
You will have the best debugging experience if you use the latest version of the DWARF debugging format that your compiler supports. DWARF is currently the most expressive and best supported debugging format in GDB.
Next: Arguments, Previous: Compilation, Up: Running [Contents][Index]
Use the run
command to start your program under GDB.
You must first specify the program name with an argument to
GDB (see Getting In and Out of
GDB), or by using the file
or exec-file
command (see Commands to Specify Files).
If you are running your program in an execution environment that
supports processes, run
creates an inferior process and makes
that process run your program. In some environments without processes,
jumps to the start of your program. Other targets,
like 'remote', are always running. If you get an error
message like this one:
The "remote" target does not support "run". Try "help target" or "continue".
then use continue
to run your program. You may need load
first (see load).
The execution of a program is affected by certain information it receives from its superior. GDB provides ways to specify this information, which you must do before starting your program. (You can change it after starting your program, but such changes only affect your program the next time you start it.) This information may be divided into four categories:
Specify the arguments to give your program as the arguments of the
command. If a shell is available on your target, the shell
is used to pass the arguments, so that you may use normal conventions
(such as wildcard expansion or variable substitution) in describing
the arguments.
In Unix systems, you can control which shell is used with the
environment variable. If you do not define SHELL
GDB uses the default shell (/bin/sh). You can disable
use of any shell with the set startup-with-shell
command (see
below for details).
Your program normally inherits its environment from GDB, but you can
use the GDB commands set environment
and unset
to change parts of the environment that affect
your program. See Your Program's Environment.
Your program inherits its working directory from GDB. You can set
the GDB working directory with the cd
command in GDB.
See Your Program's Working Directory.
Your program normally uses the same device for standard input and
standard output as GDB is using. You can redirect input and output
in the run
command line, or you can use the tty
command to
set a different device for your program.
See Your Program's Input and Output.
Warning: While input and output redirection work, you cannot use pipes to pass the output of the program you are debugging to another program; if you attempt this, GDB is likely to wind up debugging the wrong program.
When you issue the run
command, your program begins to execute
immediately. See Stopping and Continuing, for discussion
of how to arrange for your program to stop. Once your program has
stopped, you may call functions in your program, using the print
or call
commands. See Examining Data.
If the modification time of your symbol file has changed since the last time GDB read its symbols, GDB discards its symbol table, and reads it again. When it does this, GDB tries to retain your current breakpoints.
The name of the main procedure can vary from language to language.
With C or C++, the main procedure name is always main
, but
other languages such as Ada do not require a specific name for their
main procedure. The debugger provides a convenient way to start the
execution of the program and to stop at the beginning of the main
procedure, depending on the language used.
The 'start' command does the equivalent of setting a temporary breakpoint at the beginning of the main procedure and then invoking the 'run' command.
Some programs contain an elaboration phase where some startup code is
executed before the main procedure is called. This depends on the
languages used to write your program. In C++, for instance,
constructors for static and global objects are executed before
is called. It is therefore possible that the debugger stops
before reaching the main procedure. However, the temporary breakpoint
will remain to halt execution.
Specify the arguments to give to your program as arguments to the 'start' command. These arguments will be given verbatim to the underlying 'run' command. Note that the same arguments will be reused if no argument is provided during subsequent calls to 'start' or 'run'.
It is sometimes necessary to debug the program during elaboration. In
these cases, using the start
command would stop the execution of
your program too late, as the program would have already completed the
elaboration phase. Under these circumstances, insert breakpoints in your
elaboration code before running your program.
set exec-wrapper wrapper
show exec-wrapper
unset exec-wrapper
When 'exec-wrapper' is set, the specified wrapper is used to launch programs for debugging. GDB starts your program with a shell command of the form exec wrapper program. Quoting is added to program and its arguments, but not to wrapper, so you should add quotes if appropriate for your shell. The wrapper runs until it executes your program, and then GDB takes control.
You can use any program that eventually calls execve
its arguments as a wrapper. Several standard Unix utilities do
this, e.g. env
and nohup
. Any Unix shell script ending
with exec "$@"
will also work.
For example, you can use env
to pass an environment variable to
the debugged program, without setting the variable in your shell's
(gdb) set exec-wrapper env 'LD_PRELOAD=libtest.so' (gdb) run
This command is available when debugging locally on most targets, excluding DJGPP, Cygwin, MS Windows, and QNX Neutrino.
set startup-with-shell
set startup-with-shell on
set startup-with-shell off
show set startup-with-shell
On Unix systems, by default, if a shell is available on your target,
GDB) uses it to start your program. Arguments of the
command are passed to the shell, which does variable
substitution, expands wildcard characters and performs redirection of
I/O. In some circumstances, it may be useful to disable such use of a
shell, for example, when debugging the shell itself or diagnosing
startup failures such as:
(gdb) run Starting program: ./a.out During startup program terminated with signal SIGSEGV, Segmentation fault.
which indicates the shell or the wrapper specified with 'exec-wrapper' crashed, not your program. Most often, this is caused by something odd in your shell's non-interactive mode initialization file—such as .cshrc for C-shell, $.zshenv for the Z shell, or the file specified in the 'BASH_ENV' environment variable for BASH.
set auto-connect-native-target
set auto-connect-native-target on
set auto-connect-native-target off
show auto-connect-native-target
By default, if not connected to any target yet (e.g., with
target remote
), the run
command starts your program as a
native process under GDB, on your local machine. If you're
sure you don't want to debug programs on your local machine, you can
tell GDB to not connect to the native target automatically
with the set auto-connect-native-target off
If on
, which is the default, and if GDB is not
connected to a target already, the run
command automaticaly
connects to the native target, if one is available.
If off
, and if GDB is not connected to a target
already, the run
command fails with an error:
(gdb) run Don't know how to run. Try "help target".
If GDB is already connected to a target, GDB always
uses it with the run
In any case, you can explicitly connect to the native target with the
target native
command. For example,
(gdb) set auto-connect-native-target off (gdb) run Don't know how to run. Try "help target". (gdb) target native (gdb) run Starting program: ./a.out [Inferior 1 (process 10421) exited normally]
In case you connected explicitly to the native
GDB remains connected even if all inferiors exit, ready for
the next run
command. Use the disconnect
command to
Examples of other commands that likewise respect the
setting: attach
, info
, info os
set disable-randomization
set disable-randomization on
This option (enabled by default in GDB) will turn off the native randomization of the virtual address space of the started program. This option is useful for multiple debugging sessions to make the execution better reproducible and memory addresses reusable across debugging sessions.
This feature is implemented only on certain targets, including GNU/Linux. On GNU/Linux you can get the same behavior using
(gdb) set exec-wrapper setarch `uname -m` -R
set disable-randomization off
Leave the behavior of the started executable unchanged. Some bugs rear their ugly heads only when the program is loaded at certain addresses. If your bug disappears when you run the program under GDB, that might be because GDB by default disables the address randomization on platforms, such as GNU/Linux, which do that for stand-alone programs. Use set disable-randomization off to try to reproduce such elusive bugs.
On targets where it is available, virtual address space randomization protects the programs against certain kinds of security attacks. In these cases the attacker needs to know the exact location of a concrete executable code. Randomizing its location makes it impossible to inject jumps misusing a code at its expected addresses.
Prelinking shared libraries provides a startup performance advantage but it makes addresses in these libraries predictable for privileged processes by having just unprivileged access at the target system. Reading the shared library binary gives enough information for assembling the malicious code misusing it. Still even a prelinked shared library can get loaded at a new random address just requiring the regular relocation process during the startup. Shared libraries not already prelinked are always loaded at a randomly chosen address.
Position independent executables (PIE) contain position independent code
similar to the shared libraries and therefore such executables get loaded at
a randomly chosen address upon startup. PIE executables always load even
already prelinked shared libraries at a random address. You can build such
executable using gcc -fPIE -pie
Heap (malloc storage), stack and custom mmap areas are always placed randomly (as long as the randomization is enabled).
show disable-randomization
Show the current setting of the explicit disable of the native randomization of the virtual address space of the started program.
Next: Environment, Previous: Starting, Up: Running [Contents][Index]
The arguments to your program can be specified by the arguments of the
They are passed to a shell, which expands wildcard characters and
performs redirection of I/O, and thence to your program. Your
environment variable (if it exists) specifies what shell
GDB uses. If you do not define SHELL
, GDB uses
the default shell (/bin/sh on Unix).
On non-Unix systems, the program is usually invoked directly by GDB, which emulates I/O redirection via the appropriate system calls, and the wildcard characters are expanded by the startup code of the program, not by the shell.
with no arguments uses the same arguments used by the previous
, or those set by the set args
set args
Specify the arguments to be used the next time your program is run. If
set args
has no arguments, run
executes your program
with no arguments. Once you have run your program with arguments,
using set args
before the next run
is the only way to run
it again without arguments.
show args
Show the arguments to give your program when it is started.
Next: Working Directory, Previous: Arguments, Up: Running [Contents][Index]
The environment consists of a set of environment variables and their values. Environment variables conventionally record such things as your user name, your home directory, your terminal type, and your search path for programs to run. Usually you set up environment variables with the shell and they are inherited by all the other programs you run. When debugging, it can be useful to try running your program with a modified environment without having to start GDB over again.
path directory
Add directory to the front of the PATH
environment variable
(the search path for executables) that will be passed to your program.
The value of PATH
used by GDB does not change.
You may specify several directory names, separated by whitespace or by a
system-dependent separator character (':' on Unix, ';' on
MS-DOS and MS-Windows). If directory is already in the path, it
is moved to the front, so it is searched sooner.
You can use the string '$cwd' to refer to whatever is the current
working directory at the time GDB searches the path. If you
use '.' instead, it refers to the directory where you executed the
command. GDB replaces '.' in the
directory argument (with the current path) before adding
directory to the search path.
show paths
Display the list of search paths for executables (the PATH
environment variable).
show environment [varname]
Print the value of environment variable varname to be given to
your program when it starts. If you do not supply varname,
print the names and values of all environment variables to be given to
your program. You can abbreviate environment
as env
set environment varname [=value]
Set environment variable varname to value. The value changes for your program (and the shell GDB uses to launch it), not for GDB itself. The value may be any string; the values of environment variables are just strings, and any interpretation is supplied by your program itself. The value parameter is optional; if it is eliminated, the variable is set to a null value.
For example, this command:
set env USER = foo
tells the debugged program, when subsequently run, that its user is named 'foo'. (The spaces around '=' are used for clarity here; they are not actually required.)
Note that on Unix systems, GDB runs your program via a shell,
which also inherits the environment set with set environment
If necessary, you can avoid that by using the 'env' program as a
wrapper instead of using set environment
. See set exec-wrapper, for an example doing just that.
unset environment varname
Remove variable varname from the environment to be passed to your
program. This is different from 'set env varname =';
unset environment
removes the variable from the environment,
rather than assigning it an empty value.
Warning: On Unix systems, GDB runs your program using
the shell indicated by your SHELL
environment variable if it
exists (or /bin/sh
if not). If your SHELL
names a shell that runs an initialization file when started
non-interactively—such as .cshrc for C-shell, $.zshenv
for the Z shell, or the file specified in the 'BASH_ENV'
environment variable for BASH—any variables you set in that file
affect your program. You may wish to move setting of environment
variables to files that are only run when you sign on, such as
.login or .profile.
Next: Input/Output, Previous: Environment, Up: Running [Contents][Index]
Each time you start your program with run
, it inherits its
working directory from the current working directory of GDB.
The GDB working directory is initially whatever it inherited
from its parent process (typically the shell), but you can specify a new
working directory in GDB with the cd
The GDB working directory also serves as a default for the commands that specify files for GDB to operate on. See Commands to Specify Files.
cd [directory]
Set the GDB working directory to directory. If not given, directory uses '~'.
Print the GDB working directory.
It is generally impossible to find the current working directory of
the process being debugged (since a program can change its directory
during its run). If you work on a system where GDB is
configured with the /proc support, you can use the info
command (see SVR4 Process Information) to find out the
current working directory of the debuggee.
Next: Attach, Previous: Working Directory, Up: Running [Contents][Index]
By default, the program you run under GDB does input and output to the same terminal that GDB uses. GDB switches the terminal to its own terminal modes to interact with you, but it records the terminal modes your program was using and switches back to them when you continue running your program.
info terminal
Displays information recorded by GDB about the terminal modes your program is using.
You can redirect your program's input and/or output using shell
redirection with the run
command. For example,
run > outfile
starts your program, diverting its output to the file outfile.
Another way to specify where your program should do input and output is
with the tty
command. This command accepts a file name as
argument, and causes this file to be the default for future run
commands. It also resets the controlling terminal for the child
process, for future run
commands. For example,
tty /dev/ttyb
directs that processes started with subsequent run
default to do input and output on the terminal /dev/ttyb and have
that as their controlling terminal.
An explicit redirection in run
overrides the tty
effect on the input/output device, but not its effect on the controlling
When you use the tty
command or redirect input in the run
command, only the input for your program is affected. The input
for GDB still comes from your terminal. tty
is an alias
for set inferior-tty
You can use the show inferior-tty
command to tell GDB to
display the name of the terminal that will be used for future runs of your
set inferior-tty [ tty ]
Set the tty for the program being debugged to tty. Omitting tty restores the default behavior, which is to use the same terminal as GDB.
show inferior-tty
Show the current tty for the program being debugged.
Next: Kill Process, Previous: Input/Output, Up: Running [Contents][Index]
attach process-id
This command attaches to a running process—one that was started
outside GDB. (info files
shows your active
targets.) The command takes as argument a process ID. The usual way to
find out the process-id of a Unix process is with the ps
or with the 'jobs -l' shell command.
does not repeat if you press RET a second time after
executing the command.
To use attach
, your program must be running in an environment
which supports processes; for example, attach
does not work for
programs on bare-board targets that lack an operating system. You must
also have permission to send the process a signal.
When you use attach
, the debugger finds the program running in
the process first by looking in the current working directory, then (if
the program is not found) by using the source file search path
(see Specifying Source Directories). You can also use
the file
command to load the program. See Commands to
Specify Files.
The first thing GDB does after arranging to debug the specified
process is to stop it. You can examine and modify an attached process
with all the GDB commands that are ordinarily available when
you start processes with run
. You can insert breakpoints; you
can step and continue; you can modify storage. If you would rather the
process continue running, you may use the continue
command after
attaching GDB to the process.
When you have finished debugging the attached process, you can use the
command to release it from GDB control. Detaching
the process continues its execution. After the detach
that process and GDB become completely independent once more, and you
are ready to attach
another process or start one with run
does not repeat if you press RET again after
executing the command.
If you exit GDB while you have an attached process, you detach
that process. If you use the run
command, you kill that process.
By default, GDB asks for confirmation if you try to do either of these
things; you can control whether or not you need to confirm by using the
set confirm
command (see Optional Warnings and
Next: Inferiors and Programs, Previous: Attach, Up: Running [Contents][Index]
Kill the child process in which your program is running under GDB.
This command is useful if you wish to debug a core dump instead of a running process. GDB ignores any core dump file while your program is running.
On some operating systems, a program cannot be executed outside GDB
while you have breakpoints set on it inside GDB. You can use the
command in this situation to permit running your program
outside the debugger.
The kill
command is also useful if you wish to recompile and
relink your program, since on many systems it is impossible to modify an
executable file while it is running in a process. In this case, when you
next type run
, GDB notices that the file has changed, and
reads the symbol table again (while trying to preserve your current
breakpoint settings).
Next: Threads, Previous: Kill Process, Up: Running [Contents][Index]
GDB lets you run and debug multiple programs in a single session. In addition, GDB on some systems may let you run several programs simultaneously (otherwise you have to exit from one before starting another). In the most general case, you can have multiple threads of execution in each of multiple processes, launched from multiple executables.
GDB represents the state of each program execution with an object called an inferior. An inferior typically corresponds to a process, but is more general and applies also to targets that do not have processes. Inferiors may be created before a process runs, and may be retained after a process exits. Inferiors have unique identifiers that are different from process ids. Usually each inferior will also have its own distinct address space, although some embedded targets may have several inferiors running in different parts of a single address space. Each inferior may in turn have multiple threads running in it.
To find out what inferiors exist at any moment, use info inferiors
info inferiors
Print a list of all inferiors currently being managed by GDB.
GDB displays for each inferior (in this order):
An asterisk '*' preceding the GDB inferior number indicates the current inferior.
For example,
(gdb) info inferiors Num Description Executable 2 process 2307 hello * 1 process 3401 goodbye
To switch focus between inferiors, use the inferior
inferior infno
Make inferior number infno the current inferior. The argument infno is the inferior number assigned by GDB, as shown in the first field of the 'info inferiors' display.
The debugger convenience variable '$_inferior' contains the number of the current inferior. You may find this useful in writing breakpoint conditional expressions, command scripts, and so forth. See Convenience Variables, for general information on convenience variables.
You can get multiple executables into a debugging session via the
and clone-inferior
commands. On some
systems GDB can add inferiors to the debug session
automatically by following calls to fork
and exec
. To
remove inferiors from the debugging session use the
add-inferior [ -copies n ] [ -exec executable ]
Adds n inferiors to be run using executable as the
executable; n defaults to 1. If no executable is specified,
the inferiors begins empty, with no program. You can still assign or
change the program assigned to the inferior at any time by using the
command with the executable name as its argument.
clone-inferior [ -copies n ] [ infno ]
Adds n inferiors ready to execute the same program as inferior infno; n defaults to 1, and infno defaults to the number of the current inferior. This is a convenient command when you want to run another instance of the inferior you are debugging.
(gdb) info inferiors Num Description Executable * 1 process 29964 helloworld (gdb) clone-inferior Added inferior 2. 1 inferiors added. (gdb) info inferiors Num Description Executable 2 <null> helloworld * 1 process 29964 helloworld
You can now simply switch focus to inferior 2 and run it.
remove-inferiors infno…
Removes the inferior or inferiors infno…. It is not
possible to remove an inferior that is running with this command. For
those, use the kill
or detach
command first.
To quit debugging one of the running inferiors that is not the current
inferior, you can either detach from it by using the detach inferior
command (allowing it to run independently), or kill it
using the kill inferiors
detach inferior infno…
Detach from the inferior or inferiors identified by GDB
inferior number(s) infno…. Note that the inferior's entry
still stays on the list of inferiors shown by info inferiors
but its Description will show '<null>'.
kill inferiors infno…
Kill the inferior or inferiors identified by GDB inferior
number(s) infno…. Note that the inferior's entry still
stays on the list of inferiors shown by info inferiors
, but its
Description will show '<null>'.
After the successful completion of a command such as detach
detach inferiors
, kill
or kill inferiors
, or after
a normal process exit, the inferior is still valid and listed with
info inferiors
, ready to be restarted.
To be notified when inferiors are started or exit under GDB's
control use set print inferior-events
set print inferior-events
set print inferior-events on
set print inferior-events off
The set print inferior-events
command allows you to enable or
disable printing of messages when GDB notices that new
inferiors have started or that inferiors have exited or have been
detached. By default, these messages will not be printed.
show print inferior-events
Show whether messages will be printed when GDB detects that inferiors have started, exited or have been detached.
Many commands will work the same with multiple programs as with a
single program: e.g., print myglobal
will simply display the
value of myglobal
in the current inferior.
Occasionaly, when debugging GDB itself, it may be useful to
get more info about the relationship of inferiors, programs, address
spaces in a debug session. You can do that with the maint info program-spaces
maint info program-spaces
Print a list of all program spaces currently being managed by GDB.
GDB displays for each program space (in this order):
An asterisk '*' preceding the GDB program space number indicates the current program space.
In addition, below each program space line, GDB prints extra information that isn't suitable to display in tabular form. For example, the list of inferiors bound to the program space.
(gdb) maint info program-spaces Id Executable * 1 hello 2 goodbye Bound inferiors: ID 1 (process 21561)
Here we can see that no inferior is running the program hello
while process 21561
is running the program goodbye
. On
some targets, it is possible that multiple inferiors are bound to the
same program space. The most common example is that of debugging both
the parent and child processes of a vfork
call. For example,
(gdb) maint info program-spaces Id Executable * 1 vfork-test Bound inferiors: ID 2 (process 18050), ID 1 (process 18045)
Here, both inferior 2 and inferior 1 are running in the same program
space as a result of inferior 1 having executed a vfork
Next: Forks, Previous: Inferiors and Programs, Up: Running [Contents][Index]
In some operating systems, such as GNU/Linux and Solaris, a single program may have more than one thread of execution. The precise semantics of threads differ from one operating system to another, but in general the threads of a single program are akin to multiple processes—except that they share one address space (that is, they can all examine and modify the same variables). On the other hand, each thread has its own registers and execution stack, and perhaps private memory.
GDB provides these facilities for debugging multi-thread programs:
to use if the default choice
isn't compatible with the program.
The GDB thread debugging facility allows you to observe all threads while your program runs—but whenever GDB takes control, one thread in particular is always the focus of debugging. This thread is called the current thread. Debugging commands show program information from the perspective of the current thread.
Whenever GDB detects a new thread in your program, it displays the target system's identification for the thread with a message in the form '[New systag]', where systag is a thread identifier whose form varies depending on the particular system. For example, on GNU/Linux, you might see
[New Thread 0x41e02940 (LWP 25582)]
when GDB notices a new thread. In contrast, on other systems, the systag is simply something like 'process 368', with no further qualifier.
For debugging purposes, GDB associates its own thread number —always a single integer—with each thread of an inferior. This number is unique between all threads of an inferior, but not unique between threads of different inferiors.
You can refer to a given thread in an inferior using the qualified
inferior-num.thread-num syntax, also known as
qualified thread ID, with inferior-num being the inferior
number and thread-num being the thread number of the given
inferior. For example, thread 2.3
refers to thread number 3 of
inferior 2. If you omit inferior-num (e.g., thread 3
then GDB infers you're referring to a thread of the current
Until you create a second inferior, GDB does not show the inferior-num part of thread IDs, even though you can always use the full inferior-num.thread-num form to refer to threads of inferior 1, the initial inferior.
Some commands accept a space-separated thread ID list as argument. A list element can be:
'1.*') or *
. The former refers to all threads of the
given inferior, and the latter form without an inferior qualifier
refers to all threads of the current inferior.
For example, if the current inferior is 1, and inferior 7 has one thread with ID 7.1, the thread list '1 2-3 4.5 6.7-9 7.*' includes threads 1 to 3 of inferior 1, thread 5 of inferior 4, threads 7 to 9 of inferior 6 and all threads of inferior 7. That is, in expanded qualified form, the same as '1.1 1.2 1.3 4.5 6.7 6.8 6.9 7.1'.
In addition to a per-inferior number, each thread is also assigned a unique global number, also known as global thread ID, a single integer. Unlike the thread number component of the thread ID, no two threads have the same global ID, even when you're debugging multiple inferiors.
From GDB's perspective, a process always has at least one thread. In other words, GDB assigns a thread number to the program's "main thread" even if the program is not multi-threaded.
The debugger convenience variables '$_thread' and '$_gthread' contain, respectively, the per-inferior thread number and the global thread number of the current thread. You may find this useful in writing breakpoint conditional expressions, command scripts, and so forth. See Convenience Variables, for general information on convenience variables.
If GDB detects the program is multi-threaded, it augments the usual message about stopping at a breakpoint with the ID and name of the thread that hit the breakpoint.
Thread 2 "client" hit Breakpoint 1, send_message () at client.c:68
Likewise when the program receives a signal:
Thread 1 "main" received signal SIGINT, Interrupt.
info threads [thread-id-list]
Display information about one or more threads. With no arguments displays information about all threads. You can specify the list of threads that you want to display using the thread ID list syntax (see thread ID lists).
GDB displays for each thread (in this order):
thread name
, below), or, in some cases, by the
program itself.
An asterisk '*' to the left of the GDB thread number indicates the current thread.
For example,
(gdb) info threads Id Target Id Frame * 1 process 35 thread 13 main (argc=1, argv=0x7ffffff8) 2 process 35 thread 23 0x34e5 in sigpause () 3 process 35 thread 27 0x34e5 in sigpause () at threadtest.c:68
If you're debugging multiple inferiors, GDB displays thread IDs using the qualified inferior-num.thread-num format. Otherwise, only thread-num is shown.
If you specify the '-gid' option, GDB displays a column indicating each thread's global thread ID:
(gdb) info threads Id GId Target Id Frame 1.1 1 process 35 thread 13 main (argc=1, argv=0x7ffffff8) 1.2 3 process 35 thread 23 0x34e5 in sigpause () 1.3 4 process 35 thread 27 0x34e5 in sigpause () * 2.1 2 process 65 thread 1 main (argc=1, argv=0x7ffffff8)
On Solaris, you can display more information about user threads with a Solaris-specific command:
maint info sol-threads
Display info on Solaris user threads.
thread thread-id
Make thread ID thread-id the current thread. The command argument thread-id is the GDB thread ID, as shown in the first field of the 'info threads' display, with or without an inferior qualifier (e.g., '2.1' or '1').
GDB responds by displaying the system identifier of the thread you selected, and its current stack frame summary:
(gdb) thread 2 [Switching to thread 2 (Thread 0xb7fdab70 (LWP 12747))] #0 some_function (ignore=0x0) at example.c:8 8 printf ("hello\n");
As with the '[New …]' message, the form of the text after 'Switching to' depends on your system's conventions for identifying threads.
thread apply [thread-id-list | all [-ascending]] command
The thread apply
command allows you to apply the named
command to one or more threads. Specify the threads that you
want affected using the thread ID list syntax (see thread ID lists), or specify all
to apply to all threads. To apply a
command to all threads in descending order, type thread apply all
command. To apply a command to all threads in ascending order,
type thread apply all -ascending command.
thread name [name]
This command assigns a name to the current thread. If no argument is given, any existing user-specified name is removed. The thread name appears in the 'info threads' display.
On some systems, such as GNU/Linux, GDB is able to determine the name of the thread as given by the OS. On these systems, a name specified with 'thread name' will override the system-give name, and removing the user-specified name will cause GDB to once again display the system-specified name.
thread find [regexp]
Search for and display thread ids whose name or systag matches the supplied regular expression.
As well as being the complement to the 'thread name' command, this command also allows you to identify a thread by its target systag. For instance, on GNU/Linux, the target systag is the LWP id.
(GDB) thread find 26688 Thread 4 has target id 'Thread 0x41e02940 (LWP 26688)' (GDB) info thread 4 Id Target Id Frame 4 Thread 0x41e02940 (LWP 26688) 0x00000031ca6cd372 in select ()
set print thread-events
set print thread-events on
set print thread-events off
The set print thread-events
command allows you to enable or
disable printing of messages when GDB notices that new threads have
started or that threads have exited. By default, these messages will
be printed if detection of these events is supported by the target.
Note that these messages cannot be disabled on all targets.
show print thread-events
Show whether messages will be printed when GDB detects that threads have started and exited.
See Stopping and Starting Multi-thread Programs, for more information about how GDB behaves when you stop and start programs with multiple threads.
See Setting Watchpoints, for information about watchpoints in programs with multiple threads.
set libthread-db-search-path [path]
If this variable is set, path is a colon-separated list of
directories GDB will use to search for libthread_db
If you omit path, 'libthread-db-search-path' will be reset to
its default value ($sdir:$pdir
on GNU/Linux and Solaris systems).
Internally, the default value comes from the LIBTHREAD_DB_SEARCH_PATH
On GNU/Linux and Solaris systems, GDB uses a "helper"
library to obtain information about threads in the
inferior process. GDB will use 'libthread-db-search-path'
to find libthread_db
. GDB also consults first if inferior
specific thread debugging library loading is enabled
by 'set auto-load libthread-db' (see libthread_db.so.1 file).
A special entry '$sdir' for 'libthread-db-search-path' refers to the default system directories that are normally searched for loading shared libraries. The '$sdir' entry is the only kind not needing to be enabled by 'set auto-load libthread-db' (see libthread_db.so.1 file).
A special entry '$pdir' for 'libthread-db-search-path'
refers to the directory from which libpthread
was loaded in the inferior process.
For any libthread_db
library GDB finds in above directories,
GDB attempts to initialize it with the current inferior process.
If this initialization fails (which could happen because of a version
mismatch between libthread_db
and libpthread
will unload libthread_db
, and continue with the next directory.
If none of libthread_db
libraries initialize successfully,
GDB will issue a warning and thread debugging will be disabled.
Setting libthread-db-search-path
is currently implemented
only on some platforms.
show libthread-db-search-path
Display current libthread_db search path.
set debug libthread-db
show debug libthread-db
Turns on or off display of libthread_db
-related events.
Use 1
to enable, 0
to disable.
Next: Checkpoint/Restart, Previous: Threads, Up: Running [Contents][Index]
On most systems, GDB has no special support for debugging
programs which create additional processes using the fork
function. When a program forks, GDB will continue to debug the
parent process and the child process will run unimpeded. If you have
set a breakpoint in any code which the child then executes, the child
will get a SIGTRAP
signal which (unless it catches the signal)
will cause it to terminate.
However, if you want to debug the child process there is a workaround
which isn't too painful. Put a call to sleep
in the code which
the child process executes after the fork. It may be useful to sleep
only if a certain environment variable is set, or a certain file exists,
so that the delay need not occur when you don't want to run GDB
on the child. While the child is sleeping, use the ps
program to
get its process ID. Then tell GDB (a new invocation of
GDB if you are also debugging the parent process) to attach to
the child process (see Attach). From that point on you can debug
the child process just like any other process which you attached to.
On some systems, GDB provides support for debugging programs
that create additional processes using the fork
or vfork
functions. On GNU/Linux platforms, this feature is supported
with kernel version 2.5.46 and later.
The fork debugging commands are supported in native mode and when
connected to gdbserver
in either target remote
mode or
target extended-remote
By default, when a program forks, GDB will continue to debug the parent process and the child process will run unimpeded.
If you want to follow the child process instead of the parent process,
use the command set follow-fork-mode
set follow-fork-mode mode
Set the debugger response to a program call of fork
. A call to fork
or vfork
creates a new
process. The mode argument can be:
The original process is debugged after a fork. The child process runs unimpeded. This is the default.
The new process is debugged after a fork. The parent process runs unimpeded.
show follow-fork-mode
Display the current debugger response to a fork
or vfork
On Linux, if you want to debug both the parent and child processes, use the
command set detach-on-fork
set detach-on-fork mode
Tells gdb whether to detach one of the processes after a fork, or retain debugger control over them both.
The child process (or parent process, depending on the value of
) will be detached and allowed to run
independently. This is the default.
Both processes will be held under the control of GDB.
One process (child or parent, depending on the value of
) is debugged as usual, while the other
is held suspended.
show detach-on-fork
Show whether detach-on-fork mode is on/off.
If you choose to set 'detach-on-fork' mode off, then GDB
will retain control of all forked processes (including nested forks).
You can list the forked processes under the control of GDB by
using the info inferiors
command, and switch from one fork
to another by using the inferior
command (see Debugging Multiple Inferiors and Programs).
To quit debugging one of the forked processes, you can either detach
from it by using the detach inferiors
command (allowing it
to run independently), or kill it using the kill inferiors
command. See Debugging Multiple Inferiors
and Programs.
If you ask to debug a child process and a vfork
is followed by an
, GDB executes the new target up to the first
breakpoint in the new target. If you have a breakpoint set on
in your original program, the breakpoint will also be set on
the child process's main
On some systems, when a child process is spawned by vfork
, you
cannot debug the child or parent until an exec
call completes.
If you issue a run
command to GDB after an exec
call executes, the new target restarts. To restart the parent
process, use the file
command with the parent executable name
as its argument. By default, after an exec
call executes,
GDB discards the symbols of the previous executable image.
You can change this behaviour with the set follow-exec-mode
set follow-exec-mode mode
Set debugger response to a program call of exec
. An
call replaces the program image of a process.
can be:
GDB creates a new inferior and rebinds the process to this
new inferior. The program the process was running before the
call can be restarted afterwards by restarting the
original inferior.
For example:
(gdb) info inferiors (gdb) info inferior Id Description Executable * 1 <null> prog1 (gdb) run process 12020 is executing new program: prog2 Program exited normally. (gdb) info inferiors Id Description Executable 1 <null> prog1 * 2 <null> prog2
GDB keeps the process bound to the same inferior. The new
executable image replaces the previous executable loaded in the
inferior. Restarting the inferior after the exec
call, with
e.g., the run
command, restarts the executable the process was
running after the exec
call. This is the default mode.
For example:
(gdb) info inferiors Id Description Executable * 1 <null> prog1 (gdb) run process 12020 is executing new program: prog2 Program exited normally. (gdb) info inferiors Id Description Executable * 1 <null> prog2
is supported in native mode and
target extended-remote
You can use the catch
command to make GDB stop whenever
a fork
, vfork
, or exec
call is made. See Setting Catchpoints.
On certain operating systems3, GDB is able to save a snapshot of a program's state, called a checkpoint, and come back to it later.
Returning to a checkpoint effectively undoes everything that has
happened in the program since the checkpoint
was saved. This
includes changes in memory, registers, and even (within some limits)
system state. Effectively, it is like going back in time to the
moment when the checkpoint was saved.
Thus, if you're stepping thru a program and you think you're getting close to the point where things go wrong, you can save a checkpoint. Then, if you accidentally go too far and miss the critical statement, instead of having to restart your program from the beginning, you can just go back to the checkpoint and start again from there.
This can be especially useful if it takes a lot of time or steps to reach the point where you think the bug occurs.
To use the checkpoint
method of debugging:
Save a snapshot of the debugged program's current execution state.
The checkpoint
command takes no arguments, but each checkpoint
is assigned a small integer id, similar to a breakpoint id.
info checkpoints
List the checkpoints that have been saved in the current debugging session. For each checkpoint, the following information will be listed:
Checkpoint ID
Process ID
Code Address
Source line, or label
restart checkpoint-id
Restore the program state that was saved as checkpoint number checkpoint-id. All program variables, registers, stack frames etc. will be returned to the values that they had when the checkpoint was saved. In essence, gdb will "wind back the clock" to the point in time when the checkpoint was saved.
Note that breakpoints, GDB variables, command history etc. are not affected by restoring a checkpoint. In general, a checkpoint only restores things that reside in the program being debugged, not in the debugger.
delete checkpoint checkpoint-id
Delete the previously-saved checkpoint identified by checkpoint-id.
Returning to a previously saved checkpoint will restore the user state of the program being debugged, plus a significant subset of the system (OS) state, including file pointers. It won't "un-write" data from a file, but it will rewind the file pointer to the previous location, so that the previously written data can be overwritten. For files opened in read mode, the pointer will also be restored so that the previously read data can be read again.
Of course, characters that have been sent to a printer (or other external device) cannot be "snatched back", and characters received from eg. a serial device can be removed from internal program buffers, but they cannot be "pushed back" into the serial pipeline, ready to be received again. Similarly, the actual contents of files that have been changed cannot be restored (at this time).
However, within those constraints, you actually can "rewind" your program to a previously saved point in time, and begin debugging it again — and you can change the course of events so as to debug a different execution path this time.
Finally, there is one bit of internal program state that will be different when you return to a checkpoint — the program's process id. Each checkpoint will have a unique process id (or pid), and each will be different from the program's original pid. If your program has saved a local copy of its process id, this could potentially pose a problem.
On some systems such as GNU/Linux, address space randomization is performed on new processes for security reasons. This makes it difficult or impossible to set a breakpoint, or watchpoint, on an absolute address if you have to restart the program, since the absolute location of a symbol will change from one execution to the next.
A checkpoint, however, is an identical copy of a process. Therefore if you create a checkpoint at (eg.) the start of main, and simply return to that checkpoint instead of restarting the process, you can avoid the effects of address randomization and your symbols will all stay in the same place.
Next: Reverse Execution, Previous: Running, Up: Top [Contents][Index]
The principal purposes of using a debugger are so that you can stop your program before it terminates; or so that, if your program runs into trouble, you can investigate and find out why.
Inside GDB, your program may stop for any of several reasons,
such as a signal, a breakpoint, or reaching a new line after a
GDB command such as step
. You may then examine and
change variables, set new breakpoints or remove old ones, and then
continue execution. Usually, the messages shown by GDB provide
ample explanation of the status of your program—but you can also
explicitly request this information at any time.
info program
Display information about the status of your program: whether it is running or not, what process it is, and why it stopped.
• Breakpoints: | Breakpoints, watchpoints, and catchpoints | |
• Continuing and Stepping: | Resuming execution | |
• Skipping Over Functions and Files: | Skipping over functions and files | |
• Signals: | Signals | |
• Thread Stops: | Stopping and starting multi-thread programs |
Next: Continuing and Stepping, Up: Stopping [Contents][Index]
A breakpoint makes your program stop whenever a certain point in
the program is reached. For each breakpoint, you can add conditions to
control in finer detail whether your program stops. You can set
breakpoints with the break
command and its variants (see Setting Breakpoints), to specify the place where your program
should stop by line number, function name or exact address in the
On some systems, you can set breakpoints in shared libraries before the executable is run.
A watchpoint is a special breakpoint that stops your program when the value of an expression changes. The expression may be a value of a variable, or it could involve values of one or more variables combined by operators, such as 'a + b'. This is sometimes called data breakpoints. You must use a different command to set watchpoints (see Setting Watchpoints), but aside from that, you can manage a watchpoint like any other breakpoint: you enable, disable, and delete both breakpoints and watchpoints using the same commands.
You can arrange to have values from your program displayed automatically whenever GDB stops at a breakpoint. See Automatic Display.
A catchpoint is another special breakpoint that stops your program
when a certain kind of event occurs, such as the throwing of a C++
exception or the loading of a library. As with watchpoints, you use a
different command to set a catchpoint (see Setting
Catchpoints), but aside from that, you can manage a catchpoint like any
other breakpoint. (To stop when your program receives a signal, use the
command; see Signals.)
GDB assigns a number to each breakpoint, watchpoint, or catchpoint when you create it; these numbers are successive integers starting with one. In many of the commands for controlling various features of breakpoints you use the breakpoint number to say which breakpoint you want to change. Each breakpoint may be enabled or disabled; if disabled, it has no effect on your program until you enable it again.
Some GDB commands accept a range of breakpoints on which to operate. A breakpoint range is either a single breakpoint number, like '5', or two such numbers, in increasing order, separated by a hyphen, like '5-7'. When a breakpoint range is given to a command, all breakpoints in that range are operated on.
• Set Breaks: | Setting breakpoints | |
• Set Watchpoints: | Setting watchpoints | |
• Set Catchpoints: | Setting catchpoints | |
• Delete Breaks: | Deleting breakpoints | |
• Disabling: | Disabling breakpoints | |
• Conditions: | Break conditions | |
• Break Commands: | Breakpoint command lists | |
• Dynamic Printf: | Dynamic printf | |
• Save Breakpoints: | How to save breakpoints in a file | |
• Static Probe Points: | Listing static probe points | |
• Error in Breakpoints: | "Cannot insert breakpoints" | |
• Breakpoint-related Warnings: | "Breakpoint address adjusted..." |
Next: Set Watchpoints, Up: Breakpoints [Contents][Index]
Breakpoints are set with the break
command (abbreviated
). The debugger convenience variable '$bpnum' records the
number of the breakpoint you've set most recently; see Convenience Variables, for a discussion of what you can do with
convenience variables.
break location
Set a breakpoint at the given location, which can specify a function name, a line number, or an address of an instruction. (See Specify Location, for a list of all the possible ways to specify a location.) The breakpoint will stop your program just before it executes any of the code in the specified location.
When using source languages that permit overloading of symbols, such as C++, a function name may refer to more than one possible place to break. See Ambiguous Expressions, for a discussion of that situation.
It is also possible to insert a breakpoint that will stop the program only if a specific thread (see Thread-Specific Breakpoints) or a specific task (see Ada Tasks) hits that breakpoint.
When called without any arguments, break
sets a breakpoint at
the next instruction to be executed in the selected stack frame
(see Examining the Stack). In any selected frame but the
innermost, this makes your program stop as soon as control
returns to that frame. This is similar to the effect of a
command in the frame inside the selected frame—except
that finish
does not leave an active breakpoint. If you use
without an argument in the innermost frame, GDB stops
the next time it reaches the current location; this may be useful
inside loops.
GDB normally ignores breakpoints when it resumes execution, until at least one instruction has been executed. If it did not do this, you would be unable to proceed past a breakpoint without first disabling the breakpoint. This rule applies whether or not the breakpoint already existed when your program stopped.
break … if cond
Set a breakpoint with condition cond; evaluate the expression cond each time the breakpoint is reached, and stop only if the value is nonzero—that is, if cond evaluates as true. '…' stands for one of the possible arguments described above (or no argument) specifying where to break. See Break Conditions, for more information on breakpoint conditions.
tbreak args
Set a breakpoint enabled only for one stop. The args are the
same as for the break
command, and the breakpoint is set in the same
way, but the breakpoint is automatically deleted after the first time your
program stops there. See Disabling Breakpoints.
hbreak args
Set a hardware-assisted breakpoint. The args are the same as for the
command and the breakpoint is set in the same way, but the
breakpoint requires hardware support and some target hardware may not
have this support. The main purpose of this is EPROM/ROM code
debugging, so you can set a breakpoint at an instruction without
changing the instruction. This can be used with the new trap-generation
provided by SPARClite DSU and most x86-based targets. These targets
will generate traps when a program accesses some data or instruction
address that is assigned to the debug registers. However the hardware
breakpoint registers can take a limited number of breakpoints. For
example, on the DSU, only two data breakpoints can be set at a time, and
GDB will reject this command if more than two are used. Delete
or disable unused hardware breakpoints before setting new ones
(see Disabling Breakpoints).
See Break Conditions.
For remote targets, you can restrict the number of hardware
breakpoints GDB will use, see set remote hardware-breakpoint-limit.
thbreak args
Set a hardware-assisted breakpoint enabled only for one stop. The args
are the same as for the hbreak
command and the breakpoint is set in
the same way. However, like the tbreak
the breakpoint is automatically deleted after the
first time your program stops there. Also, like the hbreak
command, the breakpoint requires hardware support and some target hardware
may not have this support. See Disabling Breakpoints.
See also Break Conditions.
rbreak regex
Set breakpoints on all functions matching the regular expression
regex. This command sets an unconditional breakpoint on all
matches, printing a list of all breakpoints it set. Once these
breakpoints are set, they are treated just like the breakpoints set with
the break
command. You can delete them, disable them, or make
them conditional the same way as any other breakpoint.
The syntax of the regular expression is the standard one used with tools
like grep. Note that this is different from the syntax used by
shells, so for instance foo*
matches all functions that include
an fo
followed by zero or more o
s. There is an implicit
leading and trailing the regular expression you supply, so to
match only functions that begin with foo
, use ^foo
When debugging C++ programs, rbreak
is useful for setting
breakpoints on overloaded functions that are not members of any special
The rbreak
command can be used to set breakpoints in
all the functions in a program, like this:
(gdb) rbreak .
rbreak file:regex
If rbreak
is called with a filename qualification, it limits
the search for functions matching the given regular expression to the
specified file. This can be used, for example, to set breakpoints on
every function in a given file:
(gdb) rbreak file.c:.
The colon separating the filename qualifier from the regex may optionally be surrounded by spaces.
info breakpoints [n…]
info break [n…]
Print a table of all breakpoints, watchpoints, and catchpoints set and not deleted. Optional argument n means print information only about the specified breakpoint(s) (or watchpoint(s) or catchpoint(s)). For each breakpoint, following columns are printed:
Breakpoint, watchpoint, or catchpoint.
Whether the breakpoint is marked to be disabled or deleted when hit.
Enabled breakpoints are marked with 'y'. 'n' marks breakpoints that are not enabled.
Where the breakpoint is in your program, as a memory address. For a pending breakpoint whose address is not yet known, this field will contain '<PENDING>'. Such breakpoint won't fire until a shared library that has the symbol or line referred by breakpoint is loaded. See below for details. A breakpoint with several locations will have '<MULTIPLE>' in this field—see below for details.
Where the breakpoint is in the source for your program, as a file and line number. For a pending breakpoint, the original string passed to the breakpoint command will be listed as it cannot be resolved until the appropriate shared library is loaded in the future.
If a breakpoint is conditional, there are two evaluation modes: "host" and
"target". If mode is "host", breakpoint condition evaluation is done by
GDB on the host's side. If it is "target", then the condition
is evaluated by the target. The info break
command shows
the condition on the line following the affected breakpoint, together with
its condition evaluation mode in between parentheses.
Breakpoint commands, if any, are listed after that. A pending breakpoint is allowed to have a condition specified for it. The condition is not parsed for validity until a shared library is loaded that allows the pending breakpoint to resolve to a valid location.
info break
with a breakpoint
number n as argument lists only that breakpoint. The
convenience variable $_
and the default examining-address for
the x
command are set to the address of the last breakpoint
listed (see Examining Memory).
info break
displays a count of the number of times the breakpoint
has been hit. This is especially useful in conjunction with the
command. You can ignore a large number of breakpoint
hits, look at the breakpoint info to see how many times the breakpoint
was hit, and then run again, ignoring one less than that number. This
will get you quickly to the last hit of that breakpoint.
For a breakpoints with an enable count (xref) greater than 1,
info break
also displays that count.
GDB allows you to set any number of breakpoints at the same place in your program. There is nothing silly or meaningless about this. When the breakpoints are conditional, this is even useful (see Break Conditions).
It is possible that a breakpoint corresponds to several locations in your program. Examples of this situation are:
In all those cases, GDB will insert a breakpoint at all the relevant locations.
A breakpoint with multiple locations is displayed in the breakpoint table using several rows—one header row, followed by one row for each breakpoint location. The header row has '<MULTIPLE>' in the address column. The rows for individual locations contain the actual addresses for locations, and show the functions to which those locations belong. The number column for a location is of the form breakpoint-number.location-number.
For example:
Num Type Disp Enb Address What 1 breakpoint keep y <MULTIPLE> stop only if i==1 breakpoint already hit 1 time 1.1 y 0x080486a2 in void foo<int>() at t.cc:8 1.2 y 0x080486ca in void foo<double>() at t.cc:8
Each location can be individually enabled or disabled by passing
breakpoint-number.location-number as argument to the
and disable
commands. Note that you cannot
delete the individual locations from the list, you can only delete the
entire list of locations that belong to their parent breakpoint (with
the delete num command, where num is the number of
the parent breakpoint, 1 in the above example). Disabling or enabling
the parent breakpoint (see Disabling) affects all of the locations
that belong to that breakpoint.
It's quite common to have a breakpoint inside a shared library. Shared libraries can be loaded and unloaded explicitly, and possibly repeatedly, as the program is executed. To support this use case, GDB updates breakpoint locations whenever any shared library is loaded or unloaded. Typically, you would set a breakpoint in a shared library at the beginning of your debugging session, when the library is not loaded, and when the symbols from the library are not available. When you try to set breakpoint, GDB will ask you if you want to set a so called pending breakpoint—breakpoint whose address is not yet resolved.
After the program is run, whenever a new shared library is loaded, GDB reevaluates all the breakpoints. When a newly loaded shared library contains the symbol or line referred to by some pending breakpoint, that breakpoint is resolved and becomes an ordinary breakpoint. When a library is unloaded, all breakpoints that refer to its symbols or source lines become pending again.
This logic works for breakpoints with multiple locations, too. For example, if you have a breakpoint in a C++ template function, and a newly loaded shared library has an instantiation of that template, a new location is added to the list of locations for the breakpoint.
Except for having unresolved address, pending breakpoints do not differ from regular breakpoints. You can set conditions or commands, enable and disable them and perform other breakpoint operations.
GDB provides some additional commands for controlling what happens when the 'break' command cannot resolve breakpoint address specification to an address:
set breakpoint pending auto
This is the default behavior. When GDB cannot find the breakpoint location, it queries you whether a pending breakpoint should be created.
set breakpoint pending on
This indicates that an unrecognized breakpoint location should automatically result in a pending breakpoint being created.
set breakpoint pending off
This indicates that pending breakpoints are not to be created. Any unrecognized breakpoint location results in an error. This setting does not affect any pending breakpoints previously created.
show breakpoint pending
Show the current behavior setting for creating pending breakpoints.
The settings above only affect the break
command and its
variants. Once breakpoint is set, it will be automatically updated
as shared libraries are loaded and unloaded.
For some targets, GDB can automatically decide if hardware or
software breakpoints should be used, depending on whether the
breakpoint address is read-only or read-write. This applies to
breakpoints set with the break
command as well as to internal
breakpoints set by commands like next
and finish
. For
breakpoints set with hbreak
, GDB will always use hardware
You can control this automatic behaviour with the following commands::
set breakpoint auto-hw on
This is the default behavior. When GDB sets a breakpoint, it will try to use the target memory map to decide if software or hardware breakpoint must be used.
set breakpoint auto-hw off
This indicates GDB should not automatically select breakpoint type. If the target provides a memory map, GDB will warn when trying to set software breakpoint at a read-only address.
GDB normally implements breakpoints by replacing the program code at the breakpoint address with a special instruction, which, when executed, given control to the debugger. By default, the program code is so modified only when the program is resumed. As soon as the program stops, GDB restores the original instructions. This behaviour guards against leaving breakpoints inserted in the target should gdb abrubptly disconnect. However, with slow remote targets, inserting and removing breakpoint can reduce the performance. This behavior can be controlled with the following commands::
set breakpoint always-inserted off
All breakpoints, including newly added by the user, are inserted in the target only when the target is resumed. All breakpoints are removed from the target when it stops. This is the default mode.
set breakpoint always-inserted on
Causes all breakpoints to be inserted in the target at all times. If the user adds a new breakpoint, or changes an existing breakpoint, the breakpoints in the target are updated immediately. A breakpoint is removed from the target only when breakpoint itself is deleted.
GDB handles conditional breakpoints by evaluating these conditions when a breakpoint breaks. If the condition is true, then the process being debugged stops, otherwise the process is resumed.
If the target supports evaluating conditions on its end, GDB may download the breakpoint, together with its conditions, to it.
This feature can be controlled via the following commands:
set breakpoint condition-evaluation host
This option commands GDB to evaluate the breakpoint conditions on the host's side. Unconditional breakpoints are sent to the target which in turn receives the triggers and reports them back to GDB for condition evaluation. This is the standard evaluation mode.
set breakpoint condition-evaluation target
This option commands GDB to download breakpoint conditions to the target at the moment of their insertion. The target is responsible for evaluating the conditional expression and reporting breakpoint stop events back to GDB whenever the condition is true. Due to limitations of target-side evaluation, some conditions cannot be evaluated there, e.g., conditions that depend on local data that is only known to the host. Examples include conditional expressions involving convenience variables, complex types that cannot be handled by the agent expression parser and expressions that are too long to be sent over to the target, specially when the target is a remote system. In these cases, the conditions will be evaluated by GDB.
set breakpoint condition-evaluation auto
This is the default mode. If the target supports evaluating breakpoint conditions on its end, GDB will download breakpoint conditions to the target (limitations mentioned previously apply). If the target does not support breakpoint condition evaluation, then GDB will fallback to evaluating all these conditions on the host's side.
GDB itself sometimes sets breakpoints in your program for
special purposes, such as proper handling of longjmp
(in C
programs). These internal breakpoints are assigned negative numbers,
starting with -1
; 'info breakpoints' does not display them.
You can see these breakpoints with the GDB maintenance command
'maint info breakpoints' (see maint info breakpoints).
Next: Set Catchpoints, Previous: Set Breaks, Up: Breakpoints [Contents][Index]
You can use a watchpoint to stop execution whenever the value of an expression changes, without having to predict a particular place where this may happen. (This is sometimes called a data breakpoint.) The expression may be as simple as the value of a single variable, or as complex as many variables combined by operators. Examples include:
occupies 4 bytes).
You can set a watchpoint on an expression even if the expression can
not be evaluated yet. For instance, you can set a watchpoint on
'*global_ptr' before 'global_ptr' is initialized.
GDB will stop when your program sets 'global_ptr' and
the expression produces a valid value. If the expression becomes
valid in some other way than changing a variable (e.g. if the memory
pointed to by '*global_ptr' becomes readable as the result of a
call), GDB may not stop until the next time
the expression changes.
Depending on your system, watchpoints may be implemented in software or hardware. GDB does software watchpointing by single-stepping your program and testing the variable's value each time, which is hundreds of times slower than normal execution. (But this may still be worth it, to catch errors where you have no clue what part of your program is the culprit.)
On some systems, such as most PowerPC or x86-based targets, GDB includes support for hardware watchpoints, which do not slow down the running of your program.
watch [-l|-location] expr [thread thread-id] [mask maskvalue]
Set a watchpoint for an expression. GDB will break when the expression expr is written into by the program and its value changes. The simplest (and the most popular) use of this command is to watch the value of a single variable:
(gdb) watch foo
If the command includes a [thread thread-id]
argument, GDB breaks only when the thread identified by
thread-id changes the value of expr. If any other threads
change the value of expr, GDB will not break. Note
that watchpoints restricted to a single thread in this way only work
with Hardware Watchpoints.
Ordinarily a watchpoint respects the scope of variables in expr
(see below). The -location
argument tells GDB to
instead watch the memory referred to by expr. In this case,
GDB will evaluate expr, take the address of the result,
and watch the memory at that address. The type of the result is used
to determine the size of the watched memory. If the expression's
result does not have an address, then GDB will print an
The [mask maskvalue]
argument allows creation
of masked watchpoints, if the current architecture supports this
feature (e.g., PowerPC Embedded architecture, see PowerPC Embedded.) A masked watchpoint specifies a mask in addition
to an address to watch. The mask specifies that some bits of an address
(the bits which are reset in the mask) should be ignored when matching
the address accessed by the inferior against the watchpoint address.
Thus, a masked watchpoint watches many addresses simultaneously—those
addresses whose unmasked bits are identical to the unmasked bits in the
watchpoint address. The mask
argument implies -location
(gdb) watch foo mask 0xffff00ff (gdb) watch *0xdeadbeef mask 0xffffff00
rwatch [-l|-location] expr [thread thread-id] [mask maskvalue]
Set a watchpoint that will break when the value of expr is read by the program.
awatch [-l|-location] expr [thread thread-id] [mask maskvalue]
Set a watchpoint that will break when expr is either read from or written into by the program.
info watchpoints [n…]
This command prints a list of watchpoints, using the same format as
info break
(see Set Breaks).
If you watch for a change in a numerically entered address you need to dereference it, as the address itself is just a constant number which will never change. GDB refuses to create a watchpoint that watches a never-changing value:
(gdb) watch 0x600850 Cannot watch constant value 0x600850. (gdb) watch *(int *) 0x600850 Watchpoint 1: *(int *) 6293584
GDB sets a hardware watchpoint if possible. Hardware watchpoints execute very quickly, and the debugger reports a change in value at the exact instruction where the change occurs. If GDB cannot set a hardware watchpoint, it sets a software watchpoint, which executes more slowly and reports the change in value at the next statement, not the instruction, after the change occurs.
You can force GDB to use only software watchpoints with the
set can-use-hw-watchpoints 0 command. With this variable set to
zero, GDB will never try to use hardware watchpoints, even if
the underlying system supports them. (Note that hardware-assisted
watchpoints that were set before setting
to zero will still use the hardware
mechanism of watching expression values.)
set can-use-hw-watchpoints
Set whether or not to use hardware watchpoints.
show can-use-hw-watchpoints
Show the current mode of using hardware watchpoints.
For remote targets, you can restrict the number of hardware watchpoints GDB will use, see set remote hardware-breakpoint-limit.
When you issue the watch
command, GDB reports
Hardware watchpoint num: expr
if it was able to set a hardware watchpoint.
Currently, the awatch
and rwatch
commands can only set
hardware watchpoints, because accesses to data that don't change the
value of the watched expression cannot be detected without examining
every instruction as it is being executed, and GDB does not do
that currently. If GDB finds that it is unable to set a
hardware breakpoint with the awatch
or rwatch
command, it
will print a message like this:
Expression cannot be implemented with read/access watchpoint.
Sometimes, GDB cannot set a hardware watchpoint because the data type of the watched expression is wider than what a hardware watchpoint on the target machine can handle. For example, some systems can only watch regions that are up to 4 bytes wide; on such systems you cannot set hardware watchpoints for an expression that yields a double-precision floating-point number (which is typically 8 bytes wide). As a work-around, it might be possible to break the large region into a series of smaller ones and watch them with separate watchpoints.
If you set too many hardware watchpoints, GDB might be unable to insert all of them when you resume the execution of your program. Since the precise number of active watchpoints is unknown until such time as the program is about to be resumed, GDB might not be able to warn you about this when you set the watchpoints, and the warning will be printed only when the program is resumed:
Hardware watchpoint num: Could not insert watchpoint
If this happens, delete or disable some of the watchpoints.
Watching complex expressions that reference many variables can also exhaust the resources available for hardware-assisted watchpoints. That's because GDB needs to watch every variable in the expression with separately allocated resources.
If you call a function interactively using print
or call
any watchpoints you have set will be inactive until GDB reaches another
kind of breakpoint or the call completes.
GDB automatically deletes watchpoints that watch local
(automatic) variables, or expressions that involve such variables, when
they go out of scope, that is, when the execution leaves the block in
which these variables were defined. In particular, when the program
being debugged terminates, all local variables go out of scope,
and so only watchpoints that watch global variables remain set. If you
rerun the program, you will need to set all such watchpoints again. One
way of doing that would be to set a code breakpoint at the entry to the
function and when it breaks, set all the watchpoints.
In multi-threaded programs, watchpoints will detect changes to the watched expression from every thread.
Warning: In multi-threaded programs, software watchpoints have only limited usefulness. If GDB creates a software watchpoint, it can only watch the value of an expression in a single thread. If you are confident that the expression can only change due to the current thread's activity (and if you are also confident that no other thread can become current), then you can use software watchpoints as usual. However, GDB may not notice when a non-current thread's activity changes the expression. (Hardware watchpoints, in contrast, watch an expression in all threads.)
See set remote hardware-watchpoint-limit.
Next: Delete Breaks, Previous: Set Watchpoints, Up: Breakpoints [Contents][Index]
You can use catchpoints to cause the debugger to stop for certain
kinds of program events, such as C++ exceptions or the loading of a
shared library. Use the catch
command to set a catchpoint.
catch event
Stop when event occurs. The event can be any of the following:
throw [regexp]
rethrow [regexp]
catch [regexp]
The throwing, re-throwing, or catching of a C++ exception.
If regexp is given, then only exceptions whose type matches the regular expression will be caught.
The convenience variable $_exception
is available at an
exception-related catchpoint, on some systems. This holds the
exception being thrown.
There are currently some limitations to C++ exception handling in GDB:
variable rely on the presence of some SDT probes in libstdc++
If these probes are not present, then these features cannot be used.
These probes were first available in the GCC 4.8 release, but whether
or not they are available in your GCC also depends on how it was
convenience variable is only valid at the
instruction at which an exception-related catchpoint is set.
. You can use up
(see Selection) to get to your code.
set unwind-on-terminating-exception
An Ada exception being raised. If an exception name is specified
at the end of the command (eg catch exception Program_Error
the debugger will stop only when this specific exception is raised.
Otherwise, the debugger stops execution when any Ada exception is raised.
When inserting an exception catchpoint on a user-defined exception whose
name is identical to one of the exceptions defined by the language, the
fully qualified name must be used as the exception name. Otherwise,
GDB will assume that it should stop on the pre-defined exception
rather than the user-defined one. For instance, assuming an exception
called Constraint_Error
is defined in package Pck
, then
the command to use to catch such exceptions is catch exception
exception unhandled
An exception that was raised but is not handled by the program.
A failed Ada assertion.
A call to exec
syscall [name | number | group:groupname | g:groupname] …
A call to or return from a system call, a.k.a. syscall. A syscall is a mechanism for application programs to request a service from the operating system (OS) or one of the OS system services. GDB can catch some or all of the syscalls issued by the debuggee, and show the related information for each syscall. If no argument is specified, calls to and returns from all system calls will be caught.
name can be any system call name that is valid for the underlying OS. Just what syscalls are valid depends on the OS. On GNU and Unix systems, you can find the full list of valid syscall names on /usr/include/asm/unistd.h.
Normally, GDB knows in advance which syscalls are valid for each OS, so you can use the GDB command-line completion facilities (see command completion) to list the available choices.
You may also specify the system call numerically. A syscall's number is the value passed to the OS's syscall dispatcher to identify the requested service. When you specify the syscall by its name, GDB uses its database of syscalls to convert the name into the corresponding numeric code, but using the number directly may be useful if GDB's database does not have the complete list of syscalls on your system (e.g., because GDB lags behind the OS upgrades).
You may specify a group of related syscalls to be caught at once using
the group:
syntax (g:
is a shorter equivalent). For
instance, on some platforms GDB allows you to catch all
network related syscalls, by passing the argument group:network
to catch syscall
. Note that not all syscall groups are
available in every system. You can use the command completion
facilities (see command completion) to list the
syscall groups available on your environment.
The example below illustrates how this command works if you don't provide arguments to it:
(gdb) catch syscall Catchpoint 1 (syscall) (gdb) r Starting program: /tmp/catch-syscall Catchpoint 1 (call to syscall 'close'), \ 0xffffe424 in __kernel_vsyscall () (gdb) c Continuing. Catchpoint 1 (returned from syscall 'close'), \ 0xffffe424 in __kernel_vsyscall () (gdb)
Here is an example of catching a system call by name:
(gdb) catch syscall chroot Catchpoint 1 (syscall 'chroot' [61]) (gdb) r Starting program: /tmp/catch-syscall Catchpoint 1 (call to syscall 'chroot'), \ 0xffffe424 in __kernel_vsyscall () (gdb) c Continuing. Catchpoint 1 (returned from syscall 'chroot'), \ 0xffffe424 in __kernel_vsyscall () (gdb)
An example of specifying a system call numerically. In the case below, the syscall number has a corresponding entry in the XML file, so GDB finds its name and prints it:
(gdb) catch syscall 252 Catchpoint 1 (syscall(s) 'exit_group') (gdb) r Starting program: /tmp/catch-syscall Catchpoint 1 (call to syscall 'exit_group'), \ 0xffffe424 in __kernel_vsyscall () (gdb) c Continuing. Program exited normally. (gdb)
Here is an example of catching a syscall group:
(gdb) catch syscall group:process Catchpoint 1 (syscalls 'exit' [1] 'fork' [2] 'waitpid' [7] 'execve' [11] 'wait4' [114] 'clone' [120] 'vfork' [190] 'exit_group' [252] 'waitid' [284] 'unshare' [310]) (gdb) r Starting program: /tmp/catch-syscall Catchpoint 1 (call to syscall fork), 0x00007ffff7df4e27 in open64 () from /lib64/ld-linux-x86-64.so.2 (gdb) c Continuing.
However, there can be situations when there is no corresponding name in XML file for that syscall number. In this case, GDB prints a warning message saying that it was not able to find the syscall name, but the catchpoint will be set anyway. See the example below:
(gdb) catch syscall 764 warning: The number '764' does not represent a known syscall. Catchpoint 2 (syscall 764) (gdb)
If you configure GDB using the '--without-expat' option, it will not be able to display syscall names. Also, if your architecture does not have an XML file describing its system calls, you will not be able to see the syscall names. It is important to notice that these two features are used for accessing the syscall name database. In either case, you will see a warning like this:
(gdb) catch syscall warning: Could not open "syscalls/i386-linux.xml" warning: Could not load the syscall XML file 'syscalls/i386-linux.xml'. GDB will not be able to display syscall names. Catchpoint 1 (syscall) (gdb)
Of course, the file name will change depending on your architecture and system.
Still using the example above, you can also try to catch a syscall by its number. In this case, you would see something like:
(gdb) catch syscall 252 Catchpoint 1 (syscall(s) 252)
Again, in this case GDB would not be able to display syscall's names.
A call to fork
A call to vfork
load [regexp]
unload [regexp]
The loading or unloading of a shared library. If regexp is given, then the catchpoint will stop only if the regular expression matches one of the affected libraries.
signal [signal… | 'all']
The delivery of a signal.
With no arguments, this catchpoint will catch any signal that is not used internally by GDB, specifically, all signals except 'SIGTRAP' and 'SIGINT'.
With the argument 'all', all signals, including those used by GDB, will be caught. This argument cannot be used with other signal names.
Otherwise, the arguments are a list of signal names as given to
(see Signals). Only signals specified in this list
will be caught.
One reason that catch signal
can be more useful than
is that you can attach commands and conditions to the
When a signal is caught by a catchpoint, the signal's stop
settings, as specified by handle
, are ignored.
However, whether the signal is still delivered to the inferior depends
on the pass
setting; this can be changed in the catchpoint's
tcatch event
Set a catchpoint that is enabled only for one stop. The catchpoint is automatically deleted after the first time the event is caught.
Use the info break
command to list the current catchpoints.
Next: Disabling, Previous: Set Catchpoints, Up: Breakpoints [Contents][Index]
It is often necessary to eliminate a breakpoint, watchpoint, or catchpoint once it has done its job and you no longer want your program to stop there. This is called deleting the breakpoint. A breakpoint that has been deleted no longer exists; it is forgotten.
With the clear
command you can delete breakpoints according to
where they are in your program. With the delete
command you can
delete individual breakpoints, watchpoints, or catchpoints by specifying
their breakpoint numbers.
It is not necessary to delete a breakpoint to proceed past it. GDB automatically ignores breakpoints on the first instruction to be executed when you continue execution without changing the execution address.
Delete any breakpoints at the next instruction to be executed in the selected stack frame (see Selecting a Frame). When the innermost frame is selected, this is a good way to delete a breakpoint where your program just stopped.
clear location
Delete any breakpoints set at the specified location. See Specify Location, for the various forms of location; the most useful ones are listed below:
clear function
clear filename:function
Delete any breakpoints set at entry to the named function.
clear linenum
clear filename:linenum
Delete any breakpoints set at or within the code of the specified linenum of the specified filename.
delete [breakpoints] [range…]
Delete the breakpoints, watchpoints, or catchpoints of the breakpoint
ranges specified as arguments. If no argument is specified, delete all
breakpoints (GDB asks confirmation, unless you have set
confirm off
). You can abbreviate this command as d
Next: Conditions, Previous: Delete Breaks, Up: Breakpoints [Contents][Index]
Rather than deleting a breakpoint, watchpoint, or catchpoint, you might prefer to disable it. This makes the breakpoint inoperative as if it had been deleted, but remembers the information on the breakpoint so that you can enable it again later.
You disable and enable breakpoints, watchpoints, and catchpoints with
the enable
and disable
commands, optionally specifying
one or more breakpoint numbers as arguments. Use info break
print a list of all breakpoints, watchpoints, and catchpoints if you
do not know which numbers to use.
Disabling and enabling a breakpoint that has multiple locations affects all of its locations.
A breakpoint, watchpoint, or catchpoint can have any of several different states of enablement:
command starts out in this state.
command starts out in this state.
You can use the following commands to enable or disable breakpoints, watchpoints, and catchpoints:
disable [breakpoints] [range…]
Disable the specified breakpoints—or all breakpoints, if none are
listed. A disabled breakpoint has no effect but is not forgotten. All
options such as ignore-counts, conditions and commands are remembered in
case the breakpoint is enabled again later. You may abbreviate
as dis
enable [breakpoints] [range…]
Enable the specified breakpoints (or all defined breakpoints). They become effective once again in stopping your program.
enable [breakpoints] once range…
Enable the specified breakpoints temporarily. GDB disables any of these breakpoints immediately after stopping your program.
enable [breakpoints] count count range…
Enable the specified breakpoints temporarily. GDB records count with each of the specified breakpoints, and decrements a breakpoint's count when it is hit. When any count reaches 0, GDB disables that breakpoint. If a breakpoint has an ignore count (see Break Conditions), that will be decremented to 0 before count is affected.
enable [breakpoints] delete range…
Enable the specified breakpoints to work once, then die. GDB
deletes any of these breakpoints as soon as your program stops there.
Breakpoints set by the tbreak
command start out in this state.
Except for a breakpoint set with tbreak
(see Setting Breakpoints), breakpoints that you set are initially enabled;
subsequently, they become disabled or enabled only when you use one of
the commands above. (The command until
can set and delete a
breakpoint of its own, but it does not change the state of your other
breakpoints; see Continuing and
Next: Break Commands, Previous: Disabling, Up: Breakpoints [Contents][Index]
The simplest sort of breakpoint breaks every time your program reaches a specified place. You can also specify a condition for a breakpoint. A condition is just a Boolean expression in your programming language (see Expressions). A breakpoint with a condition evaluates the expression each time your program reaches it, and your program stops only if the condition is true.
This is the converse of using assertions for program validation; in that situation, you want to stop when the assertion is violated—that is, when the condition is false. In C, if you want to test an assertion expressed by the condition assert, you should set the condition '! assert' on the appropriate breakpoint.
Conditions are also accepted for watchpoints; you may not need them, since a watchpoint is inspecting the value of an expression anyhow—but it might be simpler, say, to just set a watchpoint on a variable name, and specify a condition that tests whether the new value is an interesting one.
Break conditions can have side effects, and may even call functions in your program. This can be useful, for example, to activate functions that log program progress, or to use your own print functions to format special data structures. The effects are completely predictable unless there is another enabled breakpoint at the same address. (In that case, GDB might see the other breakpoint first and stop your program without checking the condition of this one.) Note that breakpoint commands are usually more convenient and flexible than break conditions for the purpose of performing side effects when a breakpoint is reached (see Breakpoint Command Lists).
Breakpoint conditions can also be evaluated on the target's side if the target supports it. Instead of evaluating the conditions locally, GDB encodes the expression into an agent expression (see Agent Expressions) suitable for execution on the target, independently of GDB. Global variables become raw memory locations, locals become stack accesses, and so forth.
In this case, GDB will only be notified of a breakpoint trigger when its condition evaluates to true. This mechanism may provide faster response times depending on the performance characteristics of the target since it does not need to keep GDB informed about every breakpoint trigger, even those with false conditions.
Break conditions can be specified when a breakpoint is set, by using
'if' in the arguments to the break
command. See Setting Breakpoints. They can also be changed at any time
with the condition
You can also use the if
keyword with the watch
The catch
command does not recognize the if
is the only way to impose a further condition on a
condition bnum expression
Specify expression as the break condition for breakpoint,
watchpoint, or catchpoint number bnum. After you set a condition,
breakpoint bnum stops your program only if the value of
expression is true (nonzero, in C). When you use
, GDB checks expression immediately for
syntactic correctness, and to determine whether symbols in it have
referents in the context of your breakpoint. If expression uses
symbols not referenced in the context of the breakpoint, GDB
prints an error message:
No symbol "foo" in current context.
GDB does
not actually evaluate expression at the time the condition
command (or a command that sets a breakpoint with a condition, like
break if …
) is given, however. See Expressions.
condition bnum
Remove the condition from breakpoint number bnum. It becomes an ordinary unconditional breakpoint.
A special case of a breakpoint condition is to stop only when the breakpoint has been reached a certain number of times. This is so useful that there is a special way to do it, using the ignore count of the breakpoint. Every breakpoint has an ignore count, which is an integer. Most of the time, the ignore count is zero, and therefore has no effect. But if your program reaches a breakpoint whose ignore count is positive, then instead of stopping, it just decrements the ignore count by one and continues. As a result, if the ignore count value is n, the breakpoint does not stop the next n times your program reaches it.
ignore bnum count
Set the ignore count of breakpoint number bnum to count. The next count times the breakpoint is reached, your program's execution does not stop; other than to decrement the ignore count, GDB takes no action.
To make the breakpoint stop the next time it is reached, specify a count of zero.
When you use continue
to resume execution of your program from a
breakpoint, you can specify an ignore count directly as an argument to
, rather than using ignore
. See Continuing and Stepping.
If a breakpoint has a positive ignore count and a condition, the condition is not checked. Once the ignore count reaches zero, GDB resumes checking the condition.
You could achieve the effect of the ignore count with a condition such as '$foo-- <= 0' using a debugger convenience variable that is decremented each time. See Convenience Variables.
Ignore counts apply to breakpoints, watchpoints, and catchpoints.
Next: Dynamic Printf, Previous: Conditions, Up: Breakpoints [Contents][Index]
You can give any breakpoint (or watchpoint or catchpoint) a series of commands to execute when your program stops due to that breakpoint. For example, you might want to print the values of certain expressions, or enable other breakpoints.
commands [range…]
… command-list …
Specify a list of commands for the given breakpoints. The commands
themselves appear on the following lines. Type a line containing just
to terminate the commands.
To remove all commands from a breakpoint, type commands
follow it immediately with end
; that is, give no commands.
With no argument, commands
refers to the last breakpoint,
watchpoint, or catchpoint set (not to the breakpoint most recently
encountered). If the most recent breakpoints were set with a single
command, then the commands
will apply to all the breakpoints
set by that command. This applies to breakpoints set by
, and also applies when a single break
creates multiple breakpoints (see Ambiguous
Pressing RET as a means of repeating the last GDB command is disabled within a command-list.
You can use breakpoint commands to start your program up again. Simply
use the continue
command, or step
, or any other command
that resumes execution.
Any other commands in the command list, after a command that resumes
execution, are ignored. This is because any time you resume execution
(even with a simple next
or step
), you may encounter
another breakpoint—which could have its own command list, leading to
ambiguities about which list to execute.
If the first command you specify in a command list is silent
, the
usual message about stopping at a breakpoint is not printed. This may
be desirable for breakpoints that are to print a specific message and
then continue. If none of the remaining commands print anything, you
see no sign that the breakpoint was reached. silent
meaningful only at the beginning of a breakpoint command list.
The commands echo
, output
, and printf
allow you to
print precisely controlled output, and are often useful in silent
breakpoints. See Commands for Controlled Output.
For example, here is how you could use breakpoint commands to print the
value of x
at entry to foo
whenever x
is positive.
break foo if x>0 commands silent printf "x is %d\n",x cont end
One application for breakpoint commands is to compensate for one bug so
you can test for another. Put a breakpoint just after the erroneous line
of code, give it a condition to detect the case in which something
erroneous has been done, and give it commands to assign correct values
to any variables that need them. End with the continue
so that your program does not stop, and start with the silent
command so that no output is produced. Here is an example:
break 403 commands silent set x = y + 4 cont end
Next: Save Breakpoints, Previous: Break Commands, Up: Breakpoints [Contents][Index]
The dynamic printf command dprintf
combines a breakpoint with
formatted printing of your program's data to give you the effect of
inserting printf
calls into your program on-the-fly, without
having to recompile it.
In its most basic form, the output goes to the GDB console. However,
you can set the variable dprintf-style
for alternate handling.
For instance, you can ask to format the output by calling your
program's printf
function. This has the advantage that the
characters go to the program's output device, so they can recorded in
redirects to files and so forth.
If you are doing remote debugging with a stub or agent, you can also ask to have the printf handled by the remote agent. In addition to ensuring that the output goes to the remote program's device along with any other output the program might produce, you can also ask that the dprintf remain active even after disconnecting from the remote target. Using the stub/agent is also more efficient, as it can do everything without needing to communicate with GDB.
dprintf location,template,expression[,expression…]
Whenever execution reaches location, print the values of one or more expressions under the control of the string template. To print several values, separate them with commas.
set dprintf-style style
Set the dprintf output to be handled in one of several different styles enumerated below. A change of style affects all existing dynamic printfs immediately. (If you need individual control over the print commands, simply define normal breakpoints with explicitly-supplied command lists.)
Handle the output using the GDB printf
Handle the output by calling a function in your program (normally
Have the remote debugging agent (such as gdbserver
) handle
the output itself. This style is only available for agents that
support running commands on the target.
set dprintf-function function
Set the function to call if the dprintf style is call
. By
default its value is printf
. You may set it to any expression.
that GDB can evaluate to a function, as per the call
set dprintf-channel channel
Set a "channel" for dprintf. If set to a non-empty value,
GDB will evaluate it as an expression and pass the result as
a first argument to the dprintf-function
, in the manner of
and similar functions. Otherwise, the dprintf format
string will be the first argument, in the manner of printf
As an example, if you wanted dprintf
output to go to a logfile
that is a standard I/O stream assigned to the variable mylog
you could do the following:
(gdb) set dprintf-style call (gdb) set dprintf-function fprintf (gdb) set dprintf-channel mylog (gdb) dprintf 25,"at line 25, glob=%d\n",glob Dprintf 1 at 0x123456: file main.c, line 25. (gdb) info break 1 dprintf keep y 0x00123456 in main at main.c:25 call (void) fprintf (mylog,"at line 25, glob=%d\n",glob) continue (gdb)
Note that the info break
displays the dynamic printf commands
as normal breakpoint commands; you can thus easily see the effect of
the variable settings.
set disconnected-dprintf on
set disconnected-dprintf off
Choose whether dprintf
commands should continue to run if
GDB has disconnected from the target. This only applies
if the dprintf-style
is agent
show disconnected-dprintf off
Show the current choice for disconnected dprintf
GDB does not check the validity of function and channel, relying on you to supply values that are meaningful for the contexts in which they are being used. For instance, the function and channel may be the values of local variables, but if that is the case, then all enabled dynamic prints must be at locations within the scope of those locals. If evaluation fails, GDB will report an error.
Next: Static Probe Points, Previous: Dynamic Printf, Up: Breakpoints [Contents][Index]
To save breakpoint definitions to a file use the save breakpoints
save breakpoints [filename]
This command saves all current breakpoint definitions together with
their commands and ignore counts, into a file filename
suitable for use in a later debugging session. This includes all
types of breakpoints (breakpoints, watchpoints, catchpoints,
tracepoints). To read the saved breakpoint definitions, use the
command (see Command Files). Note that watchpoints
with expressions involving local variables may fail to be recreated
because it may not be possible to access the context where the
watchpoint is valid anymore. Because the saved breakpoint definitions
are simply a sequence of GDB commands that recreate the
breakpoints, you can edit the file in your favorite editing program,
and remove the breakpoint definitions you're not interested in, or
that can no longer be recreated.
Next: Error in Breakpoints, Previous: Save Breakpoints, Up: Breakpoints [Contents][Index]
GDB supports SDT probes in the code. SDT stands for Statically Defined Tracing, and the probes are designed to have a tiny runtime code and data footprint, and no dynamic relocations.
Currently, the following types of probes are supported on ELF-compatible systems:
SDT probes4. SystemTap
probes are usable
from assembly, C and C++ languages5.
USDT probes. DTrace
probes are usable from C and
C++ languages.
Some SystemTap
probes have an associated semaphore variable;
for instance, this happens automatically if you defined your probe
using a DTrace-style .d file. If your probe has a semaphore,
GDB will automatically enable it when you specify a
breakpoint using the '-probe-stap' notation. But, if you put a
breakpoint at a probe's location by some other method (e.g.,
break file:line
), then GDB will not automatically set
the semaphore. DTrace
probes do not support semaphores.
You can examine the available static static probes using info
, with optional arguments:
info probes [type] [provider [name [objfile]]]
If given, type is either stap
for listing
probes or dtrace
for listing DTrace
probes. If omitted all probes are listed regardless of their types.
If given, provider is a regular expression used to match against provider names when selecting which probes to list. If omitted, probes by all probes from all providers are listed.
If given, name is a regular expression to match against probe names when selecting which probes to list. If omitted, probe names are not considered when deciding whether to display them.
If given, objfile is a regular expression used to select which object files (executable or shared libraries) to examine. If not given, all object files are considered.
info probes all
List the available static probes, from all types.
Some probe points can be enabled and/or disabled. The effect of
enabling or disabling a probe depends on the type of probe being
handled. Some DTrace
probes can be enabled or
disabled, but SystemTap
probes cannot be disabled.
You can enable (or disable) one or more probes using the following commands, with optional arguments:
enable probes [provider [name [objfile]]]
If given, provider is a regular expression used to match against provider names when selecting which probes to enable. If omitted, all probes from all providers are enabled.
If given, name is a regular expression to match against probe names when selecting which probes to enable. If omitted, probe names are not considered when deciding whether to enable them.
If given, objfile is a regular expression used to select which object files (executable or shared libraries) to examine. If not given, all object files are considered.
disable probes [provider [name [objfile]]]
See the enable probes
command above for a description of the
optional arguments accepted by this command.
A probe may specify up to twelve arguments. These are available at the
point at which the probe is defined—that is, when the current PC is
at the probe's location. The arguments are available using the
convenience variables (see Convenience Vars)
. In SystemTap
probes each probe argument is an integer of the appropriate size;
types are not preserved. In DTrace
probes types are preserved
provided that they are recognized as such by GDB; otherwise
the value of the probe argument will be a long integer. The
convenience variable $_probe_argc
holds the number of arguments
at the current probe point.
These variables are always available, but attempts to access them at any location other than a probe point will cause GDB to give an error message.
Next: Breakpoint-related Warnings, Previous: Static Probe Points, Up: Breakpoints [Contents][Index]
If you request too many active hardware-assisted breakpoints and watchpoints, you will see this error message:
Stopped; cannot insert breakpoints. You may have requested too many hardware breakpoints and watchpoints.
This message is printed when you attempt to resume the program, since only then GDB knows exactly how many hardware breakpoints and watchpoints it needs to insert.
When this message is printed, you need to disable or remove some of the hardware-assisted breakpoints and watchpoints, and then continue.
Previous: Error in Breakpoints, Up: Breakpoints [Contents][Index]
Some processor architectures place constraints on the addresses at which breakpoints may be placed. For architectures thus constrained, GDB will attempt to adjust the breakpoint's address to comply with the constraints dictated by the architecture.
One example of such an architecture is the Fujitsu FR-V. The FR-V is a VLIW architecture in which a number of RISC-like instructions may be bundled together for parallel execution. The FR-V architecture constrains the location of a breakpoint instruction within such a bundle to the instruction with the lowest address. GDB honors this constraint by adjusting a breakpoint's address to the first in the bundle.
It is not uncommon for optimized code to have bundles which contain instructions from different source statements, thus it may happen that a breakpoint's address will be adjusted from one source statement to another. Since this adjustment may significantly alter GDB's breakpoint related behavior from what the user expects, a warning is printed when the breakpoint is first set and also when the breakpoint is hit.
A warning like the one below is printed when setting a breakpoint that's been subject to address adjustment:
warning: Breakpoint address adjusted from 0x00010414 to 0x00010410.
Such warnings are printed both for user settable and GDB's internal breakpoints. If you see one of these warnings, you should verify that a breakpoint set at the adjusted address will have the desired affect. If not, the breakpoint in question may be removed and other breakpoints may be set which will have the desired behavior. E.g., it may be sufficient to place the breakpoint at a later instruction. A conditional breakpoint may also be useful in some cases to prevent the breakpoint from triggering too often.
GDB will also issue a warning when stopping at one of these adjusted breakpoints:
warning: Breakpoint 1 address previously adjusted from 0x00010414 to 0x00010410.
When this warning is encountered, it may be too late to take remedial action except in cases where the breakpoint is hit earlier or more frequently than expected.
Next: Skipping Over Functions and Files, Previous: Breakpoints, Up: Stopping [Contents][Index]
Continuing means resuming program execution until your program
completes normally. In contrast, stepping means executing just
one more "step" of your program, where "step" may mean either one
line of source code, or one machine instruction (depending on what
particular command you use). Either when continuing or when stepping,
your program may stop even sooner, due to a breakpoint or a signal. (If
it stops due to a signal, you may want to use handle
, or use
'signal 0' to resume execution (see Signals),
or you may step into the signal's handler (see stepping and signal handlers).)
continue [ignore-count]
c [ignore-count]
fg [ignore-count]
Resume program execution, at the address where your program last stopped;
any breakpoints set at that address are bypassed. The optional argument
ignore-count allows you to specify a further number of times to
ignore a breakpoint at this location; its effect is like that of
(see Break Conditions).
The argument ignore-count is meaningful only when your program
stopped due to a breakpoint. At other times, the argument to
is ignored.
The synonyms c
and fg
(for foreground, as the
debugged program is deemed to be the foreground program) are provided
purely for convenience, and have exactly the same behavior as
To resume execution at a different place, you can use return
(see Returning from a Function) to go back to the
calling function; or jump
(see Continuing at a
Different Address) to go to an arbitrary location in your program.
A typical technique for using stepping is to set a breakpoint (see Breakpoints; Watchpoints; and Catchpoints) at the beginning of the function or the section of your program where a problem is believed to lie, run your program until it stops at that breakpoint, and then step through the suspect area, examining the variables that are interesting, until you see the problem happen.
Continue running your program until control reaches a different source
line, then stop it and return control to GDB. This command is
abbreviated s
Warning: If you use the
command while control is within a function that was compiled without debugging information, execution proceeds until control reaches a function that does have debugging information. Likewise, it will not step into a function which is compiled without debugging information. To step through functions without debugging information, use thestepi
command, described below.
The step
command only stops at the first instruction of a source
line. This prevents the multiple stops that could otherwise occur in
statements, for
loops, etc. step
to stop if a function that has debugging information is called within
the line. In other words, step
steps inside any functions
called within the line.
Also, the step
command only enters a function if there is line
number information for the function. Otherwise it acts like the
command. This avoids problems when using cc -gl
on MIPS machines. Previously, step
entered subroutines if there
was any debugging information about the routine.
step count
Continue running as in step
, but do so count times. If a
breakpoint is reached, or a signal not related to stepping occurs before
count steps, stepping stops right away.
next [count]
Continue to the next source line in the current (innermost) stack frame.
This is similar to step
, but function calls that appear within
the line of code are executed without stopping. Execution stops when
control reaches a different line of code at the original stack level
that was executing when you gave the next
command. This command
is abbreviated n
An argument count is a repeat count, as for step
The next
command only stops at the first instruction of a
source line. This prevents multiple stops that could otherwise occur in
statements, for
loops, etc.
set step-mode
set step-mode on
The set step-mode on
command causes the step
command to
stop at the first instruction of a function which contains no debug line
information rather than stepping over it.
This is useful in cases where you may be interested in inspecting the machine instructions of a function which has no symbolic info and do not want GDB to automatically skip over this function.
set step-mode off
Causes the step
command to step over any functions which contains no
debug information. This is the default.
show step-mode
Show whether GDB will stop in or step over functions without source line debug information.
Continue running until just after function in the selected stack frame
returns. Print the returned value (if any). This command can be
abbreviated as fin
Contrast this with the return
command (see Returning from a Function).
Continue running until a source line past the current line, in the
current stack frame, is reached. This command is used to avoid single
stepping through a loop more than once. It is like the next
command, except that when until
encounters a jump, it
automatically continues execution until the program counter is greater
than the address of the jump.
This means that when you reach the end of a loop after single stepping
though it, until
makes your program continue execution until it
exits the loop. In contrast, a next
command at the end of a loop
simply steps back to the beginning of the loop, which forces you to step
through the next iteration.
always stops your program if it attempts to exit the current
stack frame.
may produce somewhat counterintuitive results if the order
of machine code does not match the order of the source lines. For
example, in the following excerpt from a debugging session, the f
) command shows that execution is stopped at line
; yet when we use until
, we get to line 195
(gdb) f #0 main (argc=4, argv=0xf7fffae8) at m4.c:206 206 expand_input(); (gdb) until 195 for ( ; argc > 0; NEXTARG) {
This happened because, for execution efficiency, the compiler had
generated code for the loop closure test at the end, rather than the
start, of the loop—even though the test in a C for
-loop is
written before the body of the loop. The until
command appeared
to step back to the beginning of the loop when it advanced to this
expression; however, it has not really gone to an earlier
statement—not in terms of the actual machine code.
with no argument works by means of single
instruction stepping, and hence is slower than until
with an
until location
u location
Continue running your program until either the specified location is
reached, or the current stack frame returns. The location is any of
the forms described in Specify Location.
This form of the command uses temporary breakpoints, and
hence is quicker than until
without an argument. The specified
location is actually reached only if it is in the current frame. This
implies that until
can be used to skip over recursive function
invocations. For instance in the code below, if the current location is
line 96
, issuing until 99
will execute the program up to
line 99
in the same invocation of factorial, i.e., after the inner
invocations have returned.
94 int factorial (int value) 95 { 96 if (value > 1) { 97 value *= factorial (value - 1); 98 } 99 return (value); 100 }
advance location
Continue running the program up to the given location. An argument is
required, which should be of one of the forms described in
Specify Location.
Execution will also stop upon exit from the current stack
frame. This command is similar to until
, but advance
not skip over recursive function calls, and the target location doesn't
have to be in the same frame as the current one.
stepi arg
Execute one machine instruction, then stop and return to the debugger.
It is often useful to do 'display/i $pc' when stepping by machine instructions. This makes GDB automatically display the next instruction to be executed, each time your program stops. See Automatic Display.
An argument is a repeat count, as in step
nexti arg
Execute one machine instruction, but if it is a function call, proceed until the function returns.
An argument is a repeat count, as in next
By default, and if available, GDB makes use of
target-assisted range stepping. In other words, whenever you
use a stepping command (e.g., step
, next
tells the target to step the corresponding range of instruction
addresses instead of issuing multiple single-steps. This speeds up
line stepping, particularly for remote targets. Ideally, there should
be no reason you would want to turn range stepping off. However, it's
possible that a bug in the debug info, a bug in the remote stub (for
remote targets), or even a bug in GDB could make line
stepping behave incorrectly when target-assisted range stepping is
enabled. You can use the following command to turn off range stepping
if necessary:
set range-stepping
show range-stepping
Control whether range stepping is enabled.
If on
, and the target supports it, GDB tells the
target to step a range of addresses itself, instead of issuing
multiple single-steps. If off
, GDB always issues
single-steps, even if range stepping is supported by the target. The
default is on
Next: Signals, Previous: Continuing and Stepping, Up: Stopping [Contents][Index]
The program you are debugging may contain some functions which are
uninteresting to debug. The skip
command lets you tell GDB to
skip a function, all functions in a file or a particular function in
a particular file when stepping.
For example, consider the following C function:
101 int func() 102 { 103 foo(boring()); 104 bar(boring()); 105 }
Suppose you wish to step into the functions foo
and bar
, but you
are not interested in stepping through boring
. If you run step
at line 103, you'll enter boring()
, but if you run next
, you'll
step over both foo
and boring
One solution is to step
into boring
and use the finish
command to immediately exit it. But this can become tedious if boring
is called from many places.
A more flexible solution is to execute skip boring. This instructs
GDB never to step into boring
. Now when you execute
at line 103, you'll step over boring
and directly into
Functions may be skipped by providing either a function name, linespec
(see Specify Location), regular expression that matches the function's
name, file name or a glob
-style pattern that matches the file name.
On Posix systems the form of the regular expression is
"Extended Regular Expressions". See for example 'man 7 regex'
on GNU/Linux systems. On non-Posix systems the form of the regular
expression is whatever is provided by the regcomp
function of
the underlying system.
See for example 'man 7 glob' on GNU/Linux systems for a
description of glob
-style patterns.
skip [options]
The basic form of the skip
command takes zero or more options
that specify what to skip.
The options argument is any useful combination of the following:
-file file
-fi file
Functions in file will be skipped over when stepping.
-gfile file-glob-pattern
-gfi file-glob-pattern
Functions in files matching file-glob-pattern will be skipped over when stepping.
(gdb) skip -gfi utils/*.c
-function linespec
-fu linespec
Functions named by linespec or the function containing the line named by linespec will be skipped over when stepping. See Specify Location.
-rfunction regexp
-rfu regexp
Functions whose name matches regexp will be skipped over when stepping.
This form is useful for complex function names.
For example, there is generally no need to step into C++ std::string
constructors or destructors. Plus with C++ templates it can be hard to
write out the full name of the function, and often it doesn't matter what
the template arguments are. Specifying the function to be skipped as a
regular expression makes this easier.
(gdb) skip -rfu ^std::(allocator|basic_string)<.*>::~?\1 *\(
If you want to skip every templated C++ constructor and destructor
in the std
namespace you can do:
(gdb) skip -rfu ^std::([a-zA-z0-9_]+)<.*>::~?\1 *\(
If no options are specified, the function you're currently debugging will be skipped.
skip function [linespec]
After running this command, the function named by linespec or the function containing the line named by linespec will be skipped over when stepping. See Specify Location.
If you do not specify linespec, the function you're currently debugging will be skipped.
(If you have a function called file
that you want to skip, use
skip function file.)
skip file [filename]
After running this command, any function whose source lives in filename will be skipped over when stepping.
(gdb) skip file boring.c File boring.c will be skipped when stepping.
If you do not specify filename, functions whose source lives in the file you're currently debugging will be skipped.
Skips can be listed, deleted, disabled, and enabled, much like breakpoints. These are the commands for managing your list of skips:
info skip [range]
Print details about the specified skip(s). If range is not specified,
print a table with details about all functions and files marked for skipping.
info skip
prints the following information about each skip:
A number identifying this skip.
Enabled skips are marked with 'y'. Disabled skips are marked with 'n'.
If the file name is a 'glob' pattern this is 'y'. Otherwise it is 'n'.
The name or 'glob' pattern of the file to be skipped. If no file is specified this is '<none>'.
If the function name is a 'regular expression' this is 'y'. Otherwise it is 'n'.
The name or regular expression of the function to skip. If no function is specified this is '<none>'.
skip delete [range]
Delete the specified skip(s). If range is not specified, delete all skips.
skip enable [range]
Enable the specified skip(s). If range is not specified, enable all skips.
skip disable [range]
Disable the specified skip(s). If range is not specified, disable all skips.
Next: Thread Stops, Previous: Skipping Over Functions and Files, Up: Stopping [Contents][Index]
A signal is an asynchronous event that can happen in a program. The
operating system defines the possible kinds of signals, and gives each
kind a name and a number. For example, in Unix SIGINT
is the
signal a program gets when you type an interrupt character (often Ctrl-c);
is the signal a program gets from referencing a place in
memory far away from all the areas in use; SIGALRM
occurs when
the alarm clock timer goes off (which happens only if your program has
requested an alarm).
Some signals, including SIGALRM
, are a normal part of the
functioning of your program. Others, such as SIGSEGV
, indicate
errors; these signals are fatal (they kill your program immediately) if the
program has not specified in advance some other way to handle the signal.
does not indicate an error in your program, but it is normally
fatal so it can carry out the purpose of the interrupt: to kill the program.
GDB has the ability to detect any occurrence of a signal in your program. You can tell GDB in advance what to do for each kind of signal.
Normally, GDB is set up to let the non-erroneous signals like
be silently passed to your program
(so as not to interfere with their role in the program's functioning)
but to stop your program immediately whenever an error signal happens.
You can change these settings with the handle
info signals
info handle
Print a table of all the kinds of signals and how GDB has been told to handle each one. You can use this to see the signal numbers of all the defined types of signals.
info signals sig
Similar, but print information only about the specified signal number.
info handle
is an alias for info signals
catch signal [signal… | 'all']
Set a catchpoint for the indicated signals. See Set Catchpoints, for details about this command.
handle signal [keywords…]
Change the way GDB handles signal signal. The signal can be the number of a signal or its name (with or without the 'SIG' at the beginning); a list of signal numbers of the form 'low-high'; or the word 'all', meaning all the known signals. Optional arguments keywords, described below, say what change to make.
The keywords allowed by the handle
command can be abbreviated.
Their full names are:
GDB should not stop your program when this signal happens. It may still print a message telling you that the signal has come in.
GDB should stop your program when this signal happens. This implies
the print
keyword as well.
GDB should print a message when this signal happens.
GDB should not mention the occurrence of the signal at all. This
implies the nostop
keyword as well.
GDB should allow your program to see this signal; your program
can handle the signal, or else it may terminate if the signal is fatal
and not handled. pass
and noignore
are synonyms.
GDB should not allow your program to see this signal.
and ignore
are synonyms.
When a signal stops your program, the signal is not visible to the
program until you
continue. Your program sees the signal then, if pass
is in
effect for the signal in question at that time. In other words,
after GDB reports a signal, you can use the handle
command with pass
or nopass
to control whether your
program sees that signal when you continue.
The default is set to nostop
, noprint
, pass
non-erroneous signals such as SIGALRM
, and to stop
, print
, pass
for the
erroneous signals.
You can also use the signal
command to prevent your program from
seeing a signal, or cause it to see a signal it normally would not see,
or to give it any signal at any time. For example, if your program stopped
due to some sort of memory reference error, you might store correct
values into the erroneous variables and continue, hoping to see more
execution; but your program would probably terminate immediately as
a result of the fatal signal once it saw the signal. To prevent this,
you can continue with 'signal 0'. See Giving your
Program a Signal.
GDB optimizes for stepping the mainline code. If a signal
that has handle nostop
and handle pass
set arrives while
a stepping command (e.g., stepi
, step
, next
) is
in progress, GDB lets the signal handler run and then resumes
stepping the mainline code once the signal handler returns. In other
words, GDB steps over the signal handler. This prevents
signals that you've specified as not interesting (with handle
) from changing the focus of debugging unexpectedly. Note that
the signal handler itself may still hit a breakpoint, stop for another
signal that has handle stop
in effect, or for any other event
that normally results in stopping the stepping command sooner. Also
note that GDB still informs you that the program received a
signal if handle print
is set.
If you set handle pass
for a signal, and your program sets up a
handler for it, then issuing a stepping command, such as step
or stepi
, when your program is stopped due to the signal will
step into the signal handler (if the target supports that).
Likewise, if you use the queue-signal
command to queue a signal
to be delivered to the current thread when execution of the thread
resumes (see Giving your Program a Signal), then a
stepping command will step into the signal handler.
Here's an example, using stepi
to step to the first instruction
of SIGUSR1
's handler:
(gdb) handle SIGUSR1 Signal Stop Print Pass to program Description SIGUSR1 Yes Yes Yes User defined signal 1 (gdb) c Continuing. Program received signal SIGUSR1, User defined signal 1. main () sigusr1.c:28 28 p = 0; (gdb) si sigusr1_handler () at sigusr1.c:9 9 {
The same, but using queue-signal
instead of waiting for the
program to receive the signal first:
(gdb) n 28 p = 0; (gdb) queue-signal SIGUSR1 (gdb) si sigusr1_handler () at sigusr1.c:9 9 { (gdb)
On some targets, GDB can inspect extra signal information
associated with the intercepted signal, before it is actually
delivered to the program being debugged. This information is exported
by the convenience variable $_siginfo
, and consists of data
that is passed by the kernel to the signal handler at the time of the
receipt of a signal. The data type of the information itself is
target dependent. You can see the data type using the ptype
command. On Unix systems, it typically corresponds to the
standard siginfo_t
type, as defined in the signal.h
system header.
Here's an example, on a GNU/Linux system, printing the stray referenced address that raised a segmentation fault.
(gdb) continue Program received signal SIGSEGV, Segmentation fault. 0x0000000000400766 in main () 69 *(int *)p = 0; (gdb) ptype $_siginfo type = struct { int si_signo; int si_errno; int si_code; union { int _pad[28]; struct {...} _kill; struct {...} _timer; struct {...} _rt; struct {...} _sigchld; struct {...} _sigfault; struct {...} _sigpoll; } _sifields; } (gdb) ptype $_siginfo._sifields._sigfault type = struct { void *si_addr; } (gdb) p $_siginfo._sifields._sigfault.si_addr $1 = (void *) 0x7ffff7ff7000
Depending on target support, $_siginfo
may also be writable.
On some targets, a SIGSEGV
can be caused by a boundary
violation, i.e., accessing an address outside of the allowed range.
In those cases GDB may displays additional information,
depending on how GDB has been told to handle the signal.
With handle stop SIGSEGV
, GDB displays the violation
kind: "Upper" or "Lower", the memory address accessed and the
bounds, while with handle nostop SIGSEGV
no additional
information is displayed.
The usual output of a segfault is:
Program received signal SIGSEGV, Segmentation fault 0x0000000000400d7c in upper () at i386-mpx-sigsegv.c:68 68 value = *(p + len);
While a bound violation is presented as:
Program received signal SIGSEGV, Segmentation fault Upper bound violation while accessing address 0x7fffffffc3b3 Bounds: [lower = 0x7fffffffc390, upper = 0x7fffffffc3a3] 0x0000000000400d7c in upper () at i386-mpx-sigsegv.c:68 68 value = *(p + len);
GDB supports debugging programs with multiple threads (see Debugging Programs with Multiple Threads). There are two modes of controlling execution of your program within the debugger. In the default mode, referred to as all-stop mode, when any thread in your program stops (for example, at a breakpoint or while being stepped), all other threads in the program are also stopped by GDB. On some targets, GDB also supports non-stop mode, in which other threads can continue to run freely while you examine the stopped thread in the debugger.
• All-Stop Mode: | All threads stop when GDB takes control | |
• Non-Stop Mode: | Other threads continue to execute | |
• Background Execution: | Running your program asynchronously | |
• Thread-Specific Breakpoints: | Controlling breakpoints | |
• Interrupted System Calls: | GDB may interfere with system calls | |
• Observer Mode: | GDB does not alter program behavior |
Next: Non-Stop Mode, Up: Thread Stops [Contents][Index]
In all-stop mode, whenever your program stops under GDB for any reason, all threads of execution stop, not just the current thread. This allows you to examine the overall state of the program, including switching between threads, without worrying that things may change underfoot.
Conversely, whenever you restart the program, all threads start
executing. This is true even when single-stepping with commands
like step
or next
In particular, GDB cannot single-step all threads in lockstep. Since thread scheduling is up to your debugging target's operating system (not controlled by GDB), other threads may execute more than one statement while the current thread completes a single step. Moreover, in general other threads stop in the middle of a statement, rather than at a clean statement boundary, when the program stops.
You might even find your program stopped in another thread after continuing or even single-stepping. This happens whenever some other thread runs into a breakpoint, a signal, or an exception before the first thread completes whatever you requested.
Whenever GDB stops your program, due to a breakpoint or a signal, it automatically selects the thread where that breakpoint or signal happened. GDB alerts you to the context switch with a message such as '[Switching to Thread n]' to identify the thread.
On some OSes, you can modify GDB's default behavior by locking the OS scheduler to allow only a single thread to run.
set scheduler-locking mode
Set the scheduler locking mode. It applies to normal execution,
record mode, and replay mode. If it is off
, then there is no
locking and any thread may run at any time. If on
, then only
the current thread may run when the inferior is resumed. The
mode optimizes for single-stepping; it prevents other
threads from preempting the current thread while you are stepping, so
that the focus of debugging does not change unexpectedly. Other
threads never get a chance to run when you step, and they are
completely free to run when you use commands like 'continue',
'until', or 'finish'. However, unless another thread hits a
breakpoint during its timeslice, GDB does not change the
current thread away from the thread that you are debugging. The
mode behaves like off
in record mode and like
in replay mode.
show scheduler-locking
Display the current scheduler locking mode.
By default, when you issue one of the execution commands such as
, next
or step
, GDB allows only
threads of the current inferior to run. For example, if GDB
is attached to two inferiors, each with two threads, the
command resumes only the two threads of the current
inferior. This is useful, for example, when you debug a program that
forks and you want to hold the parent stopped (so that, for instance,
it doesn't run to exit), while you debug the child. In other
situations, you may not be interested in inspecting the current state
of any of the processes GDB is attached to, and you may want
to resume them all until some breakpoint is hit. In the latter case,
you can instruct GDB to allow all threads of all the
inferiors to run with the set schedule-multiple
set schedule-multiple
Set the mode for allowing threads of multiple processes to be resumed
when an execution command is issued. When on
, all threads of
all processes are allowed to run. When off
, only the threads
of the current process are resumed. The default is off
. The
mode takes precedence when set to on
or while you are stepping and set to step
show schedule-multiple
Display the current mode for resuming the execution of threads of multiple processes.
Next: Background Execution, Previous: All-Stop Mode, Up: Thread Stops [Contents][Index]
For some multi-threaded targets, GDB supports an optional mode of operation in which you can examine stopped program threads in the debugger while other threads continue to execute freely. This minimizes intrusion when debugging live systems, such as programs where some threads have real-time constraints or must continue to respond to external events. This is referred to as non-stop mode.
In non-stop mode, when a thread stops to report a debugging event,
only that thread is stopped; GDB does not stop other
threads as well, in contrast to the all-stop mode behavior. Additionally,
execution commands such as continue
and step
apply by default
only to the current thread in non-stop mode, rather than all threads as
in all-stop mode. This allows you to control threads explicitly in
ways that are not possible in all-stop mode — for example, stepping
one thread while allowing others to run freely, stepping
one thread while holding all others stopped, or stepping several threads
independently and simultaneously.
To enter non-stop mode, use this sequence of commands before you run or attach to your program:
# If using the CLI, pagination breaks non-stop. set pagination off # Finally, turn it on! set non-stop on
You can use these commands to manipulate the non-stop mode setting:
set non-stop on
Enable selection of non-stop mode.
set non-stop off
show non-stop
Show the current non-stop enablement setting.
Note these commands only reflect whether non-stop mode is enabled,
not whether the currently-executing program is being run in non-stop mode.
In particular, the set non-stop
preference is only consulted when
GDB starts or connects to the target program, and it is generally
not possible to switch modes once debugging has started. Furthermore,
since not all targets support non-stop mode, even when you have enabled
non-stop mode, GDB may still fall back to all-stop operation by
In non-stop mode, all execution commands apply only to the current thread
by default. That is, continue
only continues one thread.
To continue all threads, issue continue -a
or c -a
You can use GDB's background execution commands (see Background Execution) to run some threads in the background while you continue to examine or step others from GDB. The MI execution commands (see GDB/MI Program Execution) are always executed asynchronously in non-stop mode.
Suspending execution is done with the interrupt
command when
running in the background, or Ctrl-c during foreground execution.
In all-stop mode, this stops the whole process;
but in non-stop mode the interrupt applies only to the current thread.
To stop the whole program, use interrupt -a
Other execution commands do not currently support the -a
In non-stop mode, when a thread stops, GDB doesn't automatically make that thread current, as it does in all-stop mode. This is because the thread stop notifications are asynchronous with respect to GDB's command interpreter, and it would be confusing if GDB unexpectedly changed to a different thread just as you entered a command to operate on the previously current thread.
Next: Thread-Specific Breakpoints, Previous: Non-Stop Mode, Up: Thread Stops [Contents][Index]
GDB's execution commands have two variants: the normal foreground (synchronous) behavior, and a background (asynchronous) behavior. In foreground execution, GDB waits for the program to report that some thread has stopped before prompting for another command. In background execution, GDB immediately gives a command prompt so that you can issue other commands while your program runs.
If the target doesn't support async mode, GDB issues an error message if you attempt to use the background execution commands.
To specify background execution, add a &
to the command. For example,
the background form of the continue
command is continue&
, or
just c&
. The execution commands that accept background execution
See step.
See stepi.
See next.
See nexti.
See continue.
See finish.
See until.
Background execution is especially useful in conjunction with non-stop
mode for debugging programs with multiple threads; see Non-Stop Mode.
However, you can also use these commands in the normal all-stop mode with
the restriction that you cannot issue another execution command until the
previous one finishes. Examples of commands that are valid in all-stop
mode while the program is running include help
and info break
You can interrupt your program while it is running in the background by
using the interrupt
interrupt -a
Suspend execution of the running program. In all-stop mode,
stops the whole process, but in non-stop mode, it stops
only the current thread. To stop the whole program in non-stop mode,
use interrupt -a
Next: Interrupted System Calls, Previous: Background Execution, Up: Thread Stops [Contents][Index]
When your program has multiple threads (see Debugging Programs with Multiple Threads), you can choose whether to set breakpoints on all threads, or on a particular thread.
break location thread thread-id
break location thread thread-id if …
location specifies source lines; there are several ways of writing them (see Specify Location), but the effect is always to specify some source line.
Use the qualifier 'thread thread-id' with a breakpoint command to specify that you only want GDB to stop the program when a particular thread reaches this breakpoint. The thread-id specifier is one of the thread identifiers assigned by GDB, shown in the first column of the 'info threads' display.
If you do not specify 'thread thread-id' when you set a breakpoint, the breakpoint applies to all threads of your program.
You can use the thread
qualifier on conditional breakpoints as
well; in this case, place 'thread thread-id' before or
after the breakpoint condition, like this:
(gdb) break frik.c:13 thread 28 if bartab > lim
Thread-specific breakpoints are automatically deleted when GDB detects the corresponding thread is no longer in the thread list. For example:
(gdb) c Thread-specific breakpoint 3 deleted - thread 28 no longer in the thread list.
There are several ways for a thread to disappear, such as a regular
thread exit, but also when you detach from the process with the
command (see Debugging an Already-running
Process), or if GDB loses the remote connection
(see Remote Debugging), etc. Note that with some targets,
GDB is only able to detect a thread has exited when the user
explictly asks for the thread list with the info threads
Next: Observer Mode, Previous: Thread-Specific Breakpoints, Up: Thread Stops [Contents][Index]
There is an unfortunate side effect when using GDB to debug multi-threaded programs. If one thread stops for a breakpoint, or for some other reason, and another thread is blocked in a system call, then the system call may return prematurely. This is a consequence of the interaction between multiple threads and the signals that GDB uses to implement breakpoints and other events that stop execution.
To handle this problem, your program should check the return value of each system call and react appropriately. This is good programming style anyways.
For example, do not write code like this:
sleep (10);
The call to sleep
will return early if a different thread stops
at a breakpoint or for some other reason.
Instead, write this:
int unslept = 10; while (unslept > 0) unslept = sleep (unslept);
A system call is allowed to return early, so the system is still conforming to its specification. But GDB does cause your multi-threaded program to behave differently than it would without GDB.
Also, GDB uses internal breakpoints in the thread library to monitor certain events such as thread creation and thread destruction. When such an event happens, a system call in another thread may return prematurely, even though your program does not appear to stop.
Previous: Interrupted System Calls, Up: Thread Stops [Contents][Index]
If you want to build on non-stop mode and observe program behavior without any chance of disruption by GDB, you can set variables to disable all of the debugger's attempts to modify state, whether by writing memory, inserting breakpoints, etc. These operate at a low level, intercepting operations from all commands.
When all of these are set to off
, then GDB is said to
be observer mode. As a convenience, the variable
can be set to disable these, plus enable non-stop
Note that GDB will not prevent you from making nonsensical
combinations of these settings. For instance, if you have enabled
but disabled may-write-memory
then breakpoints that work by writing trap instructions into the code
stream will still not be able to be placed.
set observer on
set observer off
When set to on
, this disables all the permission variables
below (except for insert-fast-tracepoints
), plus enables
non-stop debugging. Setting this to off
switches back to
normal debugging, though remaining in non-stop mode.
show observer
Show whether observer mode is on or off.
set may-write-registers on
set may-write-registers off
This controls whether GDB will attempt to alter the values of
registers, such as with assignment expressions in print
, or the
command. It defaults to on
show may-write-registers
Show the current permission to write registers.
set may-write-memory on
set may-write-memory off
This controls whether GDB will attempt to alter the contents
of memory, such as with assignment expressions in print
. It
defaults to on
show may-write-memory
Show the current permission to write memory.
set may-insert-breakpoints on
set may-insert-breakpoints off
This controls whether GDB will attempt to insert breakpoints.
This affects all breakpoints, including internal breakpoints defined
by GDB. It defaults to on
show may-insert-breakpoints
Show the current permission to insert breakpoints.
set may-insert-tracepoints on
set may-insert-tracepoints off
This controls whether GDB will attempt to insert (regular)
tracepoints at the beginning of a tracing experiment. It affects only
non-fast tracepoints, fast tracepoints being under the control of
. It defaults to on
show may-insert-tracepoints
Show the current permission to insert tracepoints.
set may-insert-fast-tracepoints on
set may-insert-fast-tracepoints off
This controls whether GDB will attempt to insert fast
tracepoints at the beginning of a tracing experiment. It affects only
fast tracepoints, regular (non-fast) tracepoints being under the
control of may-insert-tracepoints
. It defaults to on
show may-insert-fast-tracepoints
Show the current permission to insert fast tracepoints.
set may-interrupt on
set may-interrupt off
This controls whether GDB will attempt to interrupt or stop
program execution. When this variable is off
, the
command will have no effect, nor will
Ctrl-c. It defaults to on
show may-interrupt
Show the current permission to interrupt or stop the program.
Next: Process Record and Replay, Previous: Stopping, Up: Top [Contents][Index]
When you are debugging a program, it is not unusual to realize that you have gone too far, and some event of interest has already happened. If the target environment supports it, GDB can allow you to "rewind" the program by running it backward.
A target environment that supports reverse execution should be able to "undo" the changes in machine state that have taken place as the program was executing normally. Variables, registers etc. should revert to their previous values. Obviously this requires a great deal of sophistication on the part of the target environment; not all target environments can support reverse execution.
When a program is executed in reverse, the instructions that have most recently been executed are "un-executed", in reverse order. The program counter runs backward, following the previous thread of execution in reverse. As each instruction is "un-executed", the values of memory and/or registers that were changed by that instruction are reverted to their previous states. After executing a piece of source code in reverse, all side effects of that code should be "undone", and all variables should be returned to their prior values6.
If you are debugging in a target environment that supports reverse execution, GDB provides the following commands.
reverse-continue [ignore-count]
rc [ignore-count]
Beginning at the point where your program last stopped, start executing in reverse. Reverse execution will stop for breakpoints and synchronous exceptions (signals), just like normal execution. Behavior of asynchronous signals depends on the target environment.
reverse-step [count]
Run the program backward until control reaches the start of a different source line; then stop it, and return control to GDB.
Like the step
command, reverse-step
will only stop
at the beginning of a source line. It "un-executes" the previously
executed source line. If the previous source line included calls to
debuggable functions, reverse-step
will step (backward) into
the called function, stopping at the beginning of the last
statement in the called function (typically a return statement).
Also, as with the step
command, if non-debuggable functions are
called, reverse-step
will run thru them backward without stopping.
reverse-stepi [count]
Reverse-execute one machine instruction. Note that the instruction
to be reverse-executed is not the one pointed to by the program
counter, but the instruction executed prior to that one. For instance,
if the last instruction was a jump, reverse-stepi
will take you
back from the destination of the jump to the jump instruction itself.
reverse-next [count]
Run backward to the beginning of the previous line executed in
the current (innermost) stack frame. If the line contains function
calls, they will be "un-executed" without stopping. Starting from
the first line of a function, reverse-next
will take you back
to the caller of that function, before the function was called,
just as the normal next
command would take you from the last
line of a function back to its return to its caller
reverse-nexti [count]
Like nexti
, reverse-nexti
executes a single instruction
in reverse, except that called functions are "un-executed" atomically.
That is, if the previously executed instruction was a return from
another function, reverse-nexti
will continue to execute
in reverse until the call to that function (from the current stack
frame) is reached.
Just as the finish
command takes you to the point where the
current function returns, reverse-finish
takes you to the point
where it was called. Instead of ending up at the end of the current
function invocation, you end up at the beginning.
set exec-direction
Set the direction of target execution.
set exec-direction reverse
GDB will perform all execution commands in reverse, until the
exec-direction mode is changed to "forward". Affected commands include
step, stepi, next, nexti, continue, and finish
. The return
command cannot be used in reverse mode.
set exec-direction forward
GDB will perform all execution commands in the normal fashion. This is the default.
Next: Stack, Previous: Reverse Execution, Up: Top [Contents][Index]
On some platforms, GDB provides a special process record and replay target that can record a log of the process execution, and replay it later with both forward and reverse execution commands.
When this target is in use, if the execution log includes the record for the next instruction, GDB will debug in replay mode. In the replay mode, the inferior does not really execute code instructions. Instead, all the events that normally happen during code execution are taken from the execution log. While code is not really executed in replay mode, the values of registers (including the program counter register) and the memory of the inferior are still changed as they normally would. Their contents are taken from the execution log.
If the record for the next instruction is not in the execution log, GDB will debug in record mode. In this mode, the inferior executes normally, and GDB records the execution log for future replay.
The process record and replay target supports reverse execution (see Reverse Execution), even if the platform on which the inferior runs does not. However, the reverse execution is limited in this case by the range of the instructions recorded in the execution log. In other words, reverse execution on platforms that don't support it directly can only be done in the replay mode.
When debugging in the reverse direction, GDB will work in replay mode as long as the execution log includes the record for the previous instruction; otherwise, it will work in record mode, if the platform supports reverse execution, or stop if not.
For architecture environments that support process record and replay, GDB provides the following commands:
record method
This command starts the process record and replay target. The
recording method can be specified as parameter. Without a parameter
the command uses the full
recording method. The following
recording methods are available:
Full record/replay recording using GDB's software record and replay implementation. This method allows replaying and reverse execution.
btrace format
Hardware-supported instruction recording. This method does not record
data. Further, the data is collected in a ring buffer so old data will
be overwritten when the buffer is full. It allows limited reverse
execution. Variables and registers are not available during reverse
execution. In remote debugging, recording continues on disconnect.
Recorded data can be inspected after reconnecting. The recording may
be stopped using record stop
The recording format can be specified as parameter. Without a parameter the command chooses the recording format. The following recording formats are available:
Use the Branch Trace Store (BTS) recording format. In this format, the processor stores a from/to record for each executed branch in the btrace ring buffer.
Use the Intel Processor Trace recording format. In this format, the processor stores the execution trace in a compressed form that is afterwards decoded by GDB.
The trace can be recorded with very low overhead. The compressed trace format also allows small trace buffers to already contain a big number of instructions compared to BTS.
Decoding the recorded execution trace, on the other hand, is more expensive than decoding BTS trace. This is mostly due to the increased number of instructions to process. You should increase the buffer-size with care.
Not all recording formats may be available on all processors.
The process record and replay target can only debug a process that is already running. Therefore, you need first to start the process with the run or start commands, and then start the recording with the record method command.
Displaced stepping (see displaced stepping) will be automatically disabled when process record and replay target is started. That's because the process record and replay target doesn't support displaced stepping.
If the inferior is in the non-stop mode (see Non-Stop Mode) or in
the asynchronous execution mode (see Background Execution), not
all recording methods are available. The full
recording method
does not support these two modes.
record stop
Stop the process record and replay target. When process record and replay target stops, the entire execution log will be deleted and the inferior will either be terminated, or will remain in its final state.
When you stop the process record and replay target in record mode (at the end of the execution log), the inferior will be stopped at the next instruction that would have been recorded. In other words, if you record for a while and then stop recording, the inferior process will be left in the same state as if the recording never happened.
On the other hand, if the process record and replay target is stopped while in replay mode (that is, not at the end of the execution log, but at some earlier point), the inferior process will become "live" at that earlier state, and it will then be possible to continue the usual "live" debugging of the process from that state.
When the inferior process exits, or GDB detaches from it, process record and replay target will automatically stop itself.
record goto
Go to a specific location in the execution log. There are several ways to specify the location to go to:
record goto begin
record goto start
Go to the beginning of the execution log.
record goto end
Go to the end of the execution log.
record goto n
Go to instruction number n in the execution log.
record save filename
Save the execution log to a file filename. Default filename is gdb_record.process_id, where process_id is the process ID of the inferior.
This command may not be available for all recording methods.
record restore filename
Restore the execution log from a file filename.
File must have been created with record save
set record full insn-number-max limit
set record full insn-number-max unlimited
Set the limit of instructions to be recorded for the full
recording method. Default value is 200000.
If limit is a positive number, then GDB will start
deleting instructions from the log once the number of the record
instructions becomes greater than limit. For every new recorded
instruction, GDB will delete the earliest recorded
instruction to keep the number of recorded instructions at the limit.
(Since deleting recorded instructions loses information, GDB
lets you control what happens when the limit is reached, by means of
the stop-at-limit
option, described below.)
If limit is unlimited
or zero, GDB will never
delete recorded instructions from the execution log. The number of
recorded instructions is limited only by the available memory.
show record full insn-number-max
Show the limit of instructions to be recorded with the full
recording method.
set record full stop-at-limit
Control the behavior of the full
recording method when the
number of recorded instructions reaches the limit. If ON (the
default), GDB will stop when the limit is reached for the
first time and ask you whether you want to stop the inferior or
continue running it and recording the execution log. If you decide
to continue recording, each new recorded instruction will cause the
oldest one to be deleted.
If this option is OFF, GDB will automatically delete the oldest record to make room for each new one, without asking.
show record full stop-at-limit
Show the current setting of stop-at-limit
set record full memory-query
Control the behavior when GDB is unable to record memory
changes caused by an instruction for the full
recording method.
If ON, GDB will query whether to stop the inferior in that
If this option is OFF (the default), GDB will automatically ignore the effect of such instructions on memory. Later, when GDB replays this execution log, it will mark the log of this instruction as not accessible, and it will not affect the replay results.
show record full memory-query
Show the current setting of memory-query
The btrace
record target does not trace data. As a
convenience, when replaying, GDB reads read-only memory off
the live program directly, assuming that the addresses of the
read-only areas don't change. This for example makes it possible to
disassemble code while replaying, but not to print variables.
In some cases, being able to inspect variables might be useful.
You can use the following command for that:
set record btrace replay-memory-access
Control the behavior of the btrace
recording method when
accessing memory during replay. If read-only
(the default),
GDB will only allow accesses to read-only memory.
If read-write
, GDB will allow accesses to read-only
and to read-write memory. Beware that the accessed memory corresponds
to the live target and not necessarily to the current replay
show record btrace replay-memory-access
Show the current setting of replay-memory-access
set record btrace bts buffer-size size
set record btrace bts buffer-size unlimited
Set the requested ring buffer size for branch tracing in BTS format. Default is 64KB.
If size is a positive number, then GDB will try to
allocate a buffer of at least size bytes for each new thread
that uses the btrace recording method and the BTS format.
The actually obtained buffer size may differ from the requested
size. Use the info record
command to see the actual
buffer size for each thread that uses the btrace recording method and
the BTS format.
If limit is unlimited
or zero, GDB will try to
allocate a buffer of 4MB.
Bigger buffers mean longer traces. On the other hand, GDB will also need longer to process the branch trace data before it can be used.
show record btrace bts buffer-size size
Show the current setting of the requested ring buffer size for branch tracing in BTS format.
set record btrace pt buffer-size size
set record btrace pt buffer-size unlimited
Set the requested ring buffer size for branch tracing in Intel Processor Trace format. Default is 16KB.
If size is a positive number, then GDB will try to
allocate a buffer of at least size bytes for each new thread
that uses the btrace recording method and the Intel Processor Trace
format. The actually obtained buffer size may differ from the
requested size. Use the info record
command to see the
actual buffer size for each thread.
If limit is unlimited
or zero, GDB will try to
allocate a buffer of 4MB.
Bigger buffers mean longer traces. On the other hand, GDB will also need longer to process the branch trace data before it can be used.
show record btrace pt buffer-size size
Show the current setting of the requested ring buffer size for branch tracing in Intel Processor Trace format.
info record
Show various statistics about the recording depending on the recording method:
For the full
recording method, it shows the state of process
record and its in-memory execution log buffer, including:
For the btrace
recording method, it shows:
For the bts
recording format, it also shows:
For the pt
recording format, it also shows:
record delete
When record target runs in replay mode ("in the past"), delete the subsequent execution log and begin to record a new execution log starting from the current address. This means you will abandon the previously recorded "future" and begin recording a new "future".
record instruction-history
Disassembles instructions from the recorded execution log. By
default, ten instructions are disassembled. This can be changed using
the set record instruction-history-size
command. Instructions
are printed in execution order.
It can also print mixed source+disassembly if you specify the the
or /s
modifier, and print the raw instructions in hex
as well as in symbolic form by specifying the /r
The current position marker is printed for the instruction at the
current program counter value. This instruction can appear multiple
times in the trace and the current position marker will be printed
every time. To omit the current position marker, specify the
To better align the printed instructions when the trace contains
instructions from more than one function, the function name may be
omitted by specifying the /f
Speculatively executed instructions are prefixed with '?'. This feature is not available for all recording formats.
There are several ways to specify what part of the execution log to disassemble:
record instruction-history insn
Disassembles ten instructions starting from instruction number insn.
record instruction-history insn, +/-n
Disassembles n instructions around instruction number
insn. If n is preceded with +
, disassembles
n instructions after instruction number insn. If
n is preceded with -
, disassembles n
instructions before instruction number insn.
record instruction-history
Disassembles ten more instructions after the last disassembly.
record instruction-history -
Disassembles ten more instructions before the last disassembly.
record instruction-history begin, end
Disassembles instructions beginning with instruction number begin until instruction number end. The instruction number end is included.
This command may not be available for all recording methods.
set record instruction-history-size size
set record instruction-history-size unlimited
Define how many instructions to disassemble in the record
command. The default value is 10.
A size of unlimited
means unlimited instructions.
show record instruction-history-size
Show how many instructions to disassemble in the record
record function-call-history
Prints the execution history at function granularity. It prints one
line for each sequence of instructions that belong to the same
function giving the name of that function, the source lines
for this instruction sequence (if the /l
modifier is
specified), and the instructions numbers that form the sequence (if
the /i
modifier is specified). The function names are indented
to reflect the call stack depth if the /c
modifier is
specified. The /l
, /i
, and /c
modifiers can be
given together.
(gdb) list 1, 10 1 void foo (void) 2 { 3 } 4 5 void bar (void) 6 { 7 ... 8 foo (); 9 ... 10 } (gdb) record function-call-history /ilc 1 bar inst 1,4 at foo.c:6,8 2 foo inst 5,10 at foo.c:2,3 3 bar inst 11,13 at foo.c:9,10
By default, ten lines are printed. This can be changed using the
set record function-call-history-size
command. Functions are
printed in execution order. There are several ways to specify what
to print:
record function-call-history func
Prints ten functions starting from function number func.
record function-call-history func, +/-n
Prints n functions around function number func. If
n is preceded with +
, prints n functions after
function number func. If n is preceded with -
prints n functions before function number func.
record function-call-history
Prints ten more functions after the last ten-line print.
record function-call-history -
Prints ten more functions before the last ten-line print.
record function-call-history begin, end
Prints functions beginning with function number begin until function number end. The function number end is included.
This command may not be available for all recording methods.
set record function-call-history-size size
set record function-call-history-size unlimited
Define how many lines to print in the
record function-call-history
command. The default value is 10.
A size of unlimited
means unlimited lines.
show record function-call-history-size
Show how many lines to print in the
record function-call-history
Next: Source, Previous: Process Record and Replay, Up: Top [Contents][Index]
When your program has stopped, the first thing you need to know is where it stopped and how it got there.
Each time your program performs a function call, information about the call is generated. That information includes the location of the call in your program, the arguments of the call, and the local variables of the function being called. The information is saved in a block of data called a stack frame. The stack frames are allocated in a region of memory called the call stack.
When your program stops, the GDB commands for examining the stack allow you to see all of this information.
One of the stack frames is selected by GDB and many GDB commands refer implicitly to the selected frame. In particular, whenever you ask GDB for the value of a variable in your program, the value is found in the selected frame. There are special GDB commands to select whichever frame you are interested in. See Selecting a Frame.
When your program stops, GDB automatically selects the
currently executing frame and describes it briefly, similar to the
command (see Information about a Frame).
• Frames: | Stack frames | |
• Backtrace: | Backtraces | |
• Selection: | Selecting a frame | |
• Frame Info: | Information on a frame | |
• Frame Filter Management: | Managing frame filters | |
The call stack is divided up into contiguous pieces called stack frames, or frames for short; each frame is the data associated with one call to one function. The frame contains the arguments given to the function, the function's local variables, and the address at which the function is executing.
When your program is started, the stack has only one frame, that of the
function main
. This is called the initial frame or the
outermost frame. Each time a function is called, a new frame is
made. Each time a function returns, the frame for that function invocation
is eliminated. If a function is recursive, there can be many frames for
the same function. The frame for the function in which execution is
actually occurring is called the innermost frame. This is the most
recently created of all the stack frames that still exist.
Inside your program, stack frames are identified by their addresses. A stack frame consists of many bytes, each of which has its own address; each kind of computer has a convention for choosing one byte whose address serves as the address of the frame. Usually this address is kept in a register called the frame pointer register (see $fp) while execution is going on in that frame.
GDB assigns numbers to all existing stack frames, starting with zero for the innermost frame, one for the frame that called it, and so on upward. These numbers do not really exist in your program; they are assigned by GDB to give you a way of designating stack frames in GDB commands.
Some compilers provide a way to compile functions so that they operate without stack frames. (For example, the GCC option
generates functions without a frame.) This is occasionally done with heavily used library functions to save the frame setup time. GDB has limited facilities for dealing with these function invocations. If the innermost function invocation has no stack frame, GDB nevertheless regards it as though it had a separate frame, which is numbered zero as usual, allowing correct tracing of the function call chain. However, GDB has no provision for frameless functions elsewhere in the stack.
A backtrace is a summary of how your program got where it is. It shows one line per frame, for many frames, starting with the currently executing frame (frame zero), followed by its caller (frame one), and on up the stack.
Print a backtrace of the entire stack: one line per frame for all frames in the stack.
You can stop the backtrace at any time by typing the system interrupt character, normally Ctrl-c.
backtrace n
bt n
Similar, but print only the innermost n frames.
backtrace -n
bt -n
Similar, but print only the outermost n frames.
backtrace full
bt full
bt full n
bt full -n
Print the values of the local variables also. As described above, n specifies the number of frames to print.
backtrace no-filters
bt no-filters
bt no-filters n
bt no-filters -n
bt no-filters full
bt no-filters full n
bt no-filters full -n
Do not run Python frame filters on this backtrace. See Frame Filter API, for more information. Additionally use disable frame-filter all to turn off all frame filters. This is only
relevant when GDB has been configured with Python
The names where
and info stack
(abbreviated info s
are additional aliases for backtrace
In a multi-threaded program, GDB by default shows the
backtrace only for the current thread. To display the backtrace for
several or all of the threads, use the command thread apply
(see thread apply). For example, if you type thread
apply all backtrace, GDB will display the backtrace for all
the threads; this is handy when you debug a core dump of a
multi-threaded program.
Each line in the backtrace shows the frame number and the function name.
The program counter value is also shown—unless you use set
print address off
. The backtrace also shows the source file name and
line number, as well as the arguments to the function. The program
counter value is omitted if it is at the beginning of the code for that
line number.
Here is an example of a backtrace. It was made with the command 'bt 3', so it shows the innermost three frames.
#0 m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8) at builtin.c:993 #1 0x6e38 in expand_macro (sym=0x2b600, data=...) at macro.c:242 #2 0x6840 in expand_token (obs=0x0, t=177664, td=0xf7fffb08) at macro.c:71 (More stack frames follow...)
The display for frame zero does not begin with a program counter
value, indicating that your program has stopped at the beginning of the
code for line 993
of builtin.c
The value of parameter data
in frame 1 has been replaced by
. By default, GDB prints the value of a parameter
only if it is a scalar (integer, pointer, enumeration, etc). See command
set print frame-arguments in Print Settings for more details
on how to configure the way function parameter values are printed.
If your program was compiled with optimizations, some compilers will optimize away arguments passed to functions if those arguments are never used after the call. Such optimizations generate code that passes arguments through registers, but doesn't store those arguments in the stack frame. GDB has no way of displaying such arguments in stack frames other than the innermost one. Here's what such a backtrace might look like:
#0 m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8) at builtin.c:993 #1 0x6e38 in expand_macro (sym=<optimized out>) at macro.c:242 #2 0x6840 in expand_token (obs=0x0, t=<optimized out>, td=0xf7fffb08) at macro.c:71 (More stack frames follow...)
The values of arguments that were not saved in their stack frames are shown as '<optimized out>'.
If you need to display the values of such optimized-out arguments, either deduce that from other variables whose values depend on the one you are interested in, or recompile without optimizations.
Most programs have a standard user entry point—a place where system
libraries and startup code transition into user code. For C this is
When GDB finds the entry function in a backtrace
it will terminate the backtrace, to avoid tracing into highly
system-specific (and generally uninteresting) code.
If you need to examine the startup code, or limit the number of levels in a backtrace, you can change this behavior:
set backtrace past-main
set backtrace past-main on
Backtraces will continue past the user entry point.
set backtrace past-main off
Backtraces will stop when they encounter the user entry point. This is the default.
show backtrace past-main
Display the current user entry point backtrace policy.
set backtrace past-entry
set backtrace past-entry on
Backtraces will continue past the internal entry point of an application.
This entry point is encoded by the linker when the application is built,
and is likely before the user entry point main
(or equivalent) is called.
set backtrace past-entry off
Backtraces will stop when they encounter the internal entry point of an application. This is the default.
show backtrace past-entry
Display the current internal entry point backtrace policy.
set backtrace limit n
set backtrace limit 0
set backtrace limit unlimited
Limit the backtrace to n levels. A value of unlimited
or zero means unlimited levels.
show backtrace limit
Display the current limit on backtrace levels.
You can control how file names are displayed.
set filename-display
set filename-display relative
Display file names relative to the compilation directory. This is the default.
set filename-display basename
Display only basename of a filename.
set filename-display absolute
Display an absolute filename.
show filename-display
Show the current way to display filenames.
Next: Frame Info, Previous: Backtrace, Up: Stack [Contents][Index]
Most commands for examining the stack and other data in your program work on whichever stack frame is selected at the moment. Here are the commands for selecting a stack frame; all of them finish by printing a brief description of the stack frame just selected.
frame n
f n
Select frame number n. Recall that frame zero is the innermost
(currently executing) frame, frame one is the frame that called the
innermost one, and so on. The highest-numbered frame is the one for
frame stack-addr [ pc-addr ]
f stack-addr [ pc-addr ]
Select the frame at address stack-addr. This is useful mainly if the chaining of stack frames has been damaged by a bug, making it impossible for GDB to assign numbers properly to all frames. In addition, this can be useful when your program has multiple stacks and switches between them. The optional pc-addr can also be given to specify the value of PC for the stack frame.
up n
Move n frames up the stack; n defaults to 1. For positive numbers n, this advances toward the outermost frame, to higher frame numbers, to frames that have existed longer.
down n
Move n frames down the stack; n defaults to 1. For
positive numbers n, this advances toward the innermost frame, to
lower frame numbers, to frames that were created more recently.
You may abbreviate down
as do
All of these commands end by printing two lines of output describing the frame. The first line shows the frame number, the function name, the arguments, and the source file and line number of execution in that frame. The second line shows the text of that source line.
For example:
(gdb) up #1 0x22f0 in main (argc=1, argv=0xf7fffbf4, env=0xf7fffbfc) at env.c:10 10 read_input_file (argv[i]);
After such a printout, the list
command with no arguments
prints ten lines centered on the point of execution in the frame.
You can also edit the program at the point of execution with your favorite
editing program by typing edit
See Printing Source Lines,
for details.
The select-frame
command is a variant of frame
that does
not display the new frame after selecting it. This command is
intended primarily for use in GDB command scripts, where the
output might be unnecessary and distracting.
up-silently n
down-silently n
These two commands are variants of up
and down
respectively; they differ in that they do their work silently, without
causing display of the new frame. They are intended primarily for use
in GDB command scripts, where the output might be unnecessary and
Next: Frame Filter Management, Previous: Selection, Up: Stack [Contents][Index]
There are several other commands to print information about the selected stack frame.
When used without any argument, this command does not change which
frame is selected, but prints a brief description of the currently
selected stack frame. It can be abbreviated f
. With an
argument, this command is used to select a stack frame.
See Selecting a Frame.
info frame
info f
This command prints a verbose description of the selected stack frame, including:
The verbose description is useful when something has gone wrong that has made the stack format fail to fit the usual conventions.
info frame addr
info f addr
Print a verbose description of the frame at address addr, without
selecting that frame. The selected frame remains unchanged by this
command. This requires the same kind of address (more than one for some
architectures) that you specify in the frame
See Selecting a Frame.
info args
Print the arguments of the selected frame, each on a separate line.
info locals
Print the local variables of the selected frame, each on a separate line. These are all variables (declared either static or automatic) accessible at the point of execution of the selected frame.
Previous: Frame Info, Up: Stack [Contents][Index]
Frame filters are Python based utilities to manage and decorate the output of frames. See Frame Filter API, for further information.
Managing frame filters is performed by several commands available within GDB, detailed here.
info frame-filter
Print a list of installed frame filters from all dictionaries, showing their name, priority and enabled status.
disable frame-filter filter-dictionary filter-name
Disable a frame filter in the dictionary matching
filter-dictionary and filter-name. The
filter-dictionary may be all
, global
, or the name of the object file where the frame filter
dictionary resides. When all
is specified, all frame filters
across all dictionaries are disabled. The filter-name is the name
of the frame filter and is used when all
is not the option for
filter-dictionary. A disabled frame-filter is not deleted, it
may be enabled again later.
enable frame-filter filter-dictionary filter-name
Enable a frame filter in the dictionary matching
filter-dictionary and filter-name. The
filter-dictionary may be all
, global
or the name of the object file where the frame filter
dictionary resides. When all
is specified, all frame filters across
all dictionaries are enabled. The filter-name is the name of the frame
filter and is used when all
is not the option for
(gdb) info frame-filter global frame-filters: Priority Enabled Name 1000 No PrimaryFunctionFilter 100 Yes Reverse progspace /build/test frame-filters: Priority Enabled Name 100 Yes ProgspaceFilter objfile /build/test frame-filters: Priority Enabled Name 999 Yes BuildProgra Filter (gdb) disable frame-filter /build/test BuildProgramFilter (gdb) info frame-filter global frame-filters: Priority Enabled Name 1000 No PrimaryFunctionFilter 100 Yes Reverse progspace /build/test frame-filters: Priority Enabled Name 100 Yes ProgspaceFilter objfile /build/test frame-filters: Priority Enabled Name 999 No BuildProgramFilter (gdb) enable frame-filter global PrimaryFunctionFilter (gdb) info frame-filter global frame-filters: Priority Enabled Name 1000 Yes PrimaryFunctionFilter 100 Yes Reverse progspace /build/test frame-filters: Priority Enabled Name 100 Yes ProgspaceFilter objfile /build/test frame-filters: Priority Enabled Name 999 No BuildProgramFilter
set frame-filter priority filter-dictionary filter-name priority
Set the priority of a frame filter in the dictionary matching
filter-dictionary, and the frame filter name matching
filter-name. The filter-dictionary may be global
or the name of the object file where the frame filter
dictionary resides. The priority is an integer.
show frame-filter priority filter-dictionary filter-name
Show the priority of a frame filter in the dictionary matching
filter-dictionary, and the frame filter name matching
filter-name. The filter-dictionary may be global
or the name of the object file where the frame filter
dictionary resides.
(gdb) info frame-filter global frame-filters: Priority Enabled Name 1000 Yes PrimaryFunctionFilter 100 Yes Reverse progspace /build/test frame-filters: Priority Enabled Name 100 Yes ProgspaceFilter objfile /build/test frame-filters: Priority Enabled Name 999 No BuildProgramFilter (gdb) set frame-filter priority global Reverse 50 (gdb) info frame-filter global frame-filters: Priority Enabled Name 1000 Yes PrimaryFunctionFilter 50 Yes Reverse progspace /build/test frame-filters: Priority Enabled Name 100 Yes ProgspaceFilter objfile /build/test frame-filters: Priority Enabled Name 999 No BuildProgramFilter
GDB can print parts of your program's source, since the debugging information recorded in the program tells GDB what source files were used to build it. When your program stops, GDB spontaneously prints the line where it stopped. Likewise, when you select a stack frame (see Selecting a Frame), GDB prints the line where execution in that frame has stopped. You can print other portions of source files by explicit command.
If you use GDB through its GNU Emacs interface, you may prefer to use Emacs facilities to view source; see Using GDB under GNU Emacs.
• List: | Printing source lines | |
• Specify Location: | How to specify code locations | |
• Edit: | Editing source files | |
• Search: | Searching source files | |
• Source Path: | Specifying source directories | |
• Machine Code: | Source and machine code |
Next: Specify Location, Up: Source [Contents][Index]
To print lines from a source file, use the list
(abbreviated l
). By default, ten lines are printed.
There are several ways to specify what part of the file you want to
print; see Specify Location, for the full list.
Here are the forms of the list
command most commonly used:
list linenum
Print lines centered around line number linenum in the current source file.
list function
Print lines centered around the beginning of function function.
Print more lines. If the last lines printed were printed with a
command, this prints lines following the last lines
printed; however, if the last line printed was a solitary line printed
as part of displaying a stack frame (see Examining the
Stack), this prints lines centered around that line.
list -
Print lines just before the lines last printed.
By default, GDB prints ten source lines with any of these forms of
the list
command. You can change this using set listsize
set listsize count
set listsize unlimited
Make the list
command display count source lines (unless
the list
argument explicitly specifies some other number).
Setting count to unlimited
or 0 means there's no limit.
show listsize
Display the number of lines that list
Repeating a list
command with RET discards the argument,
so it is equivalent to typing just list
. This is more useful
than listing the same lines again. An exception is made for an
argument of '-'; that argument is preserved in repetition so that
each repetition moves up in the source file.
In general, the list
command expects you to supply zero, one or two
locations. Locations specify source lines; there are several ways
of writing them (see Specify Location), but the effect is always
to specify some source line.
Here is a complete description of the possible arguments for list
list location
Print lines centered around the line specified by location.
list first,last
Print lines from first to last. Both arguments are
locations. When a list
command has two locations, and the
source file of the second location is omitted, this refers to
the same source file as the first location.
list ,last
Print lines ending with last.
list first,
Print lines starting with first.
list +
Print lines just after the lines last printed.
list -
Print lines just before the lines last printed.
As described in the preceding table.
• Linespec Locations: | Linespec locations | |
• Explicit Locations: | Explicit locations | |
• Address Locations: | Address locations |
Several GDB commands accept arguments that specify a location of your program's code. Since GDB is a source-level debugger, a location usually specifies some line in the source code. Locations may be specified using three different formats: linespec locations, explicit locations, or address locations.
Next: Explicit Locations, Up: Specify Location [Contents][Index]
A linespec is a colon-separated list of source location parameters such as file name, function name, etc. Here are all the different ways of specifying a linespec:
Specifies the line number linenum of the current source file.
Specifies the line offset lines before or after the current
line. For the list
command, the current line is the last one
printed; for the breakpoint commands, this is the line at which
execution stopped in the currently selected stack frame
(see Frames, for a description of stack frames.) When
used as the second of the two linespecs in a list
this specifies the line offset lines up or down from the first
Specifies the line linenum in the source file filename. If filename is a relative file name, then it will match any source file name with the same trailing components. For example, if filename is 'gcc/expr.c', then it will match source file name of /build/trunk/gcc/expr.c, but not /build/trunk/libcpp/expr.c or /build/trunk/gcc/x-expr.c.
Specifies the line that begins the body of the function function. For example, in C, this is the line with the open brace.
Specifies the line where label appears in function.
Specifies the line that begins the body of the function function in the file filename. You only need the file name with a function name to avoid ambiguity when there are identically named functions in different source files.
Specifies the line at which the label named label appears in the function corresponding to the currently selected stack frame. If there is no current selected stack frame (for instance, if the inferior is not running), then GDB will not search for a label.
-pstap|-probe-stap [objfile:[provider:]]name
The GNU/Linux tool SystemTap
provides a way for
applications to embed static probes. See Static Probe Points, for more
information on finding and using static probes. This form of linespec
specifies the location of such a static probe.
If objfile is given, only probes coming from that shared library or executable matching objfile as a regular expression are considered. If provider is given, then only probes from that provider are considered. If several probes match the spec, GDB will insert a breakpoint at each one of those probes.
Next: Address Locations, Previous: Linespec Locations, Up: Specify Location [Contents][Index]
Explicit locations allow the user to directly specify the source location's parameters using option-value pairs.
Explicit locations are useful when several functions, labels, or file names have the same name (base name for files) in the program's sources. In these cases, explicit locations point to the source line you meant more accurately and unambiguously. Also, using explicit locations might be faster in large programs.
For example, the linespec 'foo:bar' may refer to a function bar
defined in the file named foo or the label bar
in a function
named foo
. GDB must search either the file system or
the symbol table to know.
The list of valid explicit location options is summarized in the following table:
-source filename
The value specifies the source file name. To differentiate between
files with the same base name, prepend as many directories as is necessary
to uniquely identify the desired file, e.g., foo/bar/baz.c. Otherwise
GDB will use the first file it finds with the given base
name. This option requires the use of either -function
or -line
-function function
The value specifies the name of a function. Operations
on function locations unmodified by other options (such as -label
or -line
) refer to the line that begins the body of the function.
In C, for example, this is the line with the open brace.
-label label
The value specifies the name of a label. When the function name is not specified, the label is searched in the function of the currently selected stack frame.
-line number
The value specifies a line offset for the location. The offset may either
be absolute (-line 3
) or relative (-line +3
), depending on
the command. When specified without any other options, the line offset is
relative to the current line.
Explicit location options may be abbreviated by omitting any non-unique
trailing characters from the option name, e.g., break -s main.c -li 3
Previous: Explicit Locations, Up: Specify Location [Contents][Index]
Address locations indicate a specific program address. They have the generalized form *address.
For line-oriented commands, such as list
and edit
, this
specifies a source line that contains address. For break
other breakpoint-oriented commands, this can be used to set breakpoints in
parts of your program which do not have debugging information or
source files.
Here address may be any expression valid in the current working language (see working language) that specifies a code address. In addition, as a convenience, GDB extends the semantics of expressions used in locations to cover several situations that frequently occur during debugging. Here are the various forms of address:
Any expression valid in the current working language.
An address of a function or procedure derived from its name. In C,
C++, Java, Objective-C, Fortran, minimal, and assembly, this is
simply the function's name function (and actually a special case
of a valid expression). In Pascal and Modula-2, this is
. In Ada, this is function'Address
(although the Pascal form also works).
This form specifies the address of the function's first instruction, before the stack frame and arguments have been set up.
Like funcaddr above, but also specifies the name of the source file explicitly. This is useful if the name of the function does not specify the function unambiguously, e.g., if there are several functions with identical names in different source files.
Next: Search, Previous: Specify Location, Up: Source [Contents][Index]
To edit the lines in a source file, use the edit
The editing program of your choice
is invoked with the current line set to
the active line in the program.
Alternatively, there are several ways to specify what part of the file you
want to print if you want to see other parts of the program:
edit location
Edit the source file specified by location
. Editing starts at
that location, e.g., at the specified source line of the
specified file. See Specify Location, for all the possible forms
of the location argument; here are the forms of the edit
command most commonly used:
edit number
Edit the current source file with number as the active line number.
edit function
Edit the file containing function at the beginning of its definition.
You can customize GDB to use any editor you want
By default, it is /bin/ex, but you can change this
by setting the environment variable EDITOR
before using
GDB. For example, to configure GDB to use the
editor, you could use these commands with the sh
EDITOR=/usr/bin/vi export EDITOR gdb …
or in the csh
setenv EDITOR /usr/bin/vi gdb …
Next: Source Path, Previous: Edit, Up: Source [Contents][Index]
There are two commands for searching through the current source file for a regular expression.
forward-search regexp
search regexp
The command 'forward-search regexp' checks each line,
starting with the one following the last line listed, for a match for
regexp. It lists the line that is found. You can use the
synonym 'search regexp' or abbreviate the command name as
reverse-search regexp
The command 'reverse-search regexp' checks each line, starting
with the one before the last line listed and going backward, for a match
for regexp. It lists the line that is found. You can abbreviate
this command as rev
Next: Machine Code, Previous: Search, Up: Source [Contents][Index]
Executable programs sometimes do not record the directories of the source files from which they were compiled, just the names. Even when they do, the directories could be moved between the compilation and your debugging session. GDB has a list of directories to search for source files; this is called the source path. Each time GDB wants a source file, it tries all the directories in the list, in the order they are present in the list, until it finds a file with the desired name.
For example, suppose an executable references the file /usr/src/foo-1.0/lib/foo.c, and our source path is /mnt/cross. The file is first looked up literally; if this fails, /mnt/cross/usr/src/foo-1.0/lib/foo.c is tried; if this fails, /mnt/cross/foo.c is opened; if this fails, an error message is printed. GDB does not look up the parts of the source file name, such as /mnt/cross/src/foo-1.0/lib/foo.c. Likewise, the subdirectories of the source path are not searched: if the source path is /mnt/cross, and the binary refers to foo.c, GDB would not find it under /mnt/cross/usr/src/foo-1.0/lib.
Plain file names, relative file names with leading directories, file names containing dots, etc. are all treated as described above; for instance, if the source path is /mnt/cross, and the source file is recorded as ../lib/foo.c, GDB would first try ../lib/foo.c, then /mnt/cross/../lib/foo.c, and after that—/mnt/cross/foo.c.
Note that the executable search path is not used to locate the source files.
Whenever you reset or rearrange the source path, GDB clears out any information it has cached about where source files are found and where each line is in the file.
When you start GDB, its source path includes only 'cdir'
and 'cwd', in that order.
To add other directories, use the directory
The search path is used to find both program source files and GDB script files (read using the '-command' option and 'source' command).
In addition to the source path, GDB provides a set of commands that manage a list of source path substitution rules. A substitution rule specifies how to rewrite source directories stored in the program's debug information in case the sources were moved to a different directory between compilation and debugging. A rule is made of two strings, the first specifying what needs to be rewritten in the path, and the second specifying how it should be rewritten. In set substitute-path, we name these two parts from and to respectively. GDB does a simple string replacement of from with to at the start of the directory part of the source file name, and uses that result instead of the original file name to look up the sources.
Using the previous example, suppose the foo-1.0 tree has been
moved from /usr/src to /mnt/cross, then you can tell
GDB to replace /usr/src in all source path names with
/mnt/cross. The first lookup will then be
/mnt/cross/foo-1.0/lib/foo.c in place of the original location
of /usr/src/foo-1.0/lib/foo.c. To define a source path
substitution rule, use the set substitute-path
(see set substitute-path).
To avoid unexpected substitution results, a rule is applied only if the from part of the directory name ends at a directory separator. For instance, a rule substituting /usr/source into /mnt/cross will be applied to /usr/source/foo-1.0 but not to /usr/sourceware/foo-2.0. And because the substitution is applied only at the beginning of the directory name, this rule will not be applied to /root/usr/source/baz.c either.
In many cases, you can achieve the same result using the directory
command. However, set substitute-path
can be more efficient in
the case where the sources are organized in a complex tree with multiple
subdirectories. With the directory
command, you need to add each
subdirectory of your project. If you moved the entire tree while
preserving its internal organization, then set substitute-path
allows you to direct the debugger to all the sources with one single
set substitute-path
is also more than just a shortcut command.
The source path is only used if the file at the original location no
longer exists. On the other hand, set substitute-path
the debugger behavior to look at the rewritten location instead. So, if
for any reason a source file that is not relevant to your executable is
located at the original location, a substitution rule is the only
method available to point GDB at the new location.
You can configure a default source path substitution rule by configuring GDB with the '--with-relocated-sources=dir' option. The dir should be the name of a directory under GDB's configured prefix (set with '--prefix' or '--exec-prefix'), and directory names in debug information under dir will be adjusted automatically if the installed GDB is moved to a new location. This is useful if GDB, libraries or executables with debug information and corresponding source code are being moved together.
directory dirname …
dir dirname …
Add directory dirname to the front of the source path. Several directory names may be given to this command, separated by ':' (';' on MS-DOS and MS-Windows, where ':' usually appears as part of absolute file names) or whitespace. You may specify a directory that is already in the source path; this moves it forward, so GDB searches it sooner.
You can use the string '$cdir' to refer to the compilation directory (if one is recorded), and '$cwd' to refer to the current working directory. '$cwd' is not the same as '.'—the former tracks the current working directory as it changes during your GDB session, while the latter is immediately expanded to the current directory at the time you add an entry to the source path.
Reset the source path to its default value ('$cdir:$cwd' on Unix systems). This requires confirmation.
set directories path-list
Set the source path to path-list. '$cdir:$cwd' are added if missing.
show directories
Print the source path: show which directories it contains.
set substitute-path from to
Define a source path substitution rule, and add it at the end of the current list of existing substitution rules. If a rule with the same from was already defined, then the old rule is also deleted.
For example, if the file /foo/bar/baz.c was moved to /mnt/cross/baz.c, then the command
(gdb) set substitute-path /foo/bar /mnt/cross
will tell GDB to replace '/foo/bar' with '/mnt/cross', which will allow GDB to find the file baz.c even though it was moved.
In the case when more than one substitution rule have been defined, the rules are evaluated one by one in the order where they have been defined. The first one matching, if any, is selected to perform the substitution.
For instance, if we had entered the following commands:
(gdb) set substitute-path /usr/src/include /mnt/include (gdb) set substitute-path /usr/src /mnt/src
GDB would then rewrite /usr/src/include/defs.h into /mnt/include/defs.h by using the first rule. However, it would use the second rule to rewrite /usr/src/lib/foo.c into /mnt/src/lib/foo.c.
unset substitute-path [path]
If a path is specified, search the current list of substitution rules for a rule that would rewrite that path. Delete that rule if found. A warning is emitted by the debugger if no rule could be found.
If no path is specified, then all substitution rules are deleted.
show substitute-path [path]
If a path is specified, then print the source path substitution rule which would rewrite that path, if any.
If no path is specified, then print all existing source path substitution rules.
If your source path is cluttered with directories that are no longer of interest, GDB may sometimes cause confusion by finding the wrong versions of source. You can correct the situation as follows:
with no argument to reset the source path to its default value.
with suitable arguments to reinstall the
directories you want in the source path. You can add all the
directories in one command.
Previous: Source Path, Up: Source [Contents][Index]
You can use the command info line
to map source lines to program
addresses (and vice versa), and the command disassemble
to display
a range of addresses as machine instructions. You can use the command
set disassemble-next-line
to set whether to disassemble next
source line when execution stops. When run under GNU Emacs
mode, the info line
command causes the arrow to point to the
line specified. Also, info line
prints addresses in symbolic form as
well as hex.
info line location
Print the starting and ending addresses of the compiled code for source line location. You can specify source lines in any of the ways documented in Specify Location.
For example, we can use info line
to discover the location of
the object code for the first line of function
(gdb) info line m4_changequote Line 895 of "builtin.c" starts at pc 0x634c and ends at 0x6350.
We can also inquire (using *addr
as the form for
location) what source line covers a particular address:
(gdb) info line *0x63ff Line 926 of "builtin.c" starts at pc 0x63e4 and ends at 0x6404.
After info line
, the default address for the x
is changed to the starting address of the line, so that 'x/i' is
sufficient to begin examining the machine code (see Examining Memory). Also, this address is saved as the value of the
convenience variable $_
(see Convenience
disassemble /m
disassemble /s
disassemble /r
This specialized command dumps a range of memory as machine
instructions. It can also print mixed source+disassembly by specifying
the /m
or /s
modifier and print the raw instructions in hex
as well as in symbolic form by specifying the /r
The default memory range is the function surrounding the
program counter of the selected frame. A single argument to this
command is a program counter value; GDB dumps the function
surrounding this value. When two arguments are given, they should
be separated by a comma, possibly surrounded by whitespace. The
arguments specify a range of addresses to dump, in one of two forms:
the addresses from start (inclusive) to end (exclusive)
the addresses from start (inclusive) to
When 2 arguments are specified, the name of the function is also printed (since there could be several functions in the given range).
The argument(s) can be any expression yielding a numeric value, such as '0x32c4', '&main+10' or '$pc - 8'.
If the range of memory being disassembled contains current program counter,
the instruction at that location is shown with a =>
The following example shows the disassembly of a range of addresses of HP PA-RISC 2.0 code:
(gdb) disas 0x32c4, 0x32e4 Dump of assembler code from 0x32c4 to 0x32e4: 0x32c4 <main+204>: addil 0,dp 0x32c8 <main+208>: ldw 0x22c(sr0,r1),r26 0x32cc <main+212>: ldil 0x3000,r31 0x32d0 <main+216>: ble 0x3f8(sr4,r31) 0x32d4 <main+220>: ldo 0(r31),rp 0x32d8 <main+224>: addil -0x800,dp 0x32dc <main+228>: ldo 0x588(r1),r26 0x32e0 <main+232>: ldil 0x3000,r31 End of assembler dump.
Here is an example showing mixed source+assembly for Intel x86
with /m
or /s
, when the program is stopped just after
function prologue in a non-optimized function with no inline code.
(gdb) disas /m main Dump of assembler code for function main: 5 { 0x08048330 <+0>: push %ebp 0x08048331 <+1>: mov %esp,%ebp 0x08048333 <+3>: sub $0x8,%esp 0x08048336 <+6>: and $0xfffffff0,%esp 0x08048339 <+9>: sub $0x10,%esp 6 printf ("Hello.\n"); => 0x0804833c <+12>: movl $0x8048440,(%esp) 0x08048343 <+19>: call 0x8048284 <puts@plt> 7 return 0; 8 } 0x08048348 <+24>: mov $0x0,%eax 0x0804834d <+29>: leave 0x0804834e <+30>: ret End of assembler dump.
The /m
option is deprecated as its output is not useful when
there is either inlined code or re-ordered code.
The /s
option is the preferred choice.
Here is an example for AMD x86-64 showing the difference between
output and /s
This example has one inline function defined in a header file,
and the code is compiled with '-O2' optimization.
Note how the /m
output is missing the disassembly of
several instructions that are present in the /s
int foo (int a) { if (a < 0) return a * 2; if (a == 0) return 1; return a + 10; }
#include "foo.h" volatile int x, y; int main () { x = foo (y); return 0; }
(gdb) disas /m main Dump of assembler code for function main: 5 { 6 x = foo (y); 0x0000000000400400 <+0>: mov 0x200c2e(%rip),%eax # 0x601034 <y> 0x0000000000400417 <+23>: mov %eax,0x200c13(%rip) # 0x601030 <x> 7 return 0; 8 } 0x000000000040041d <+29>: xor %eax,%eax 0x000000000040041f <+31>: retq 0x0000000000400420 <+32>: add %eax,%eax 0x0000000000400422 <+34>: jmp 0x400417 <main+23> End of assembler dump. (gdb) disas /s main Dump of assembler code for function main: foo.c: 5 { 6 x = foo (y); 0x0000000000400400 <+0>: mov 0x200c2e(%rip),%eax # 0x601034 <y> foo.h: 4 if (a < 0) 0x0000000000400406 <+6>: test %eax,%eax 0x0000000000400408 <+8>: js 0x400420 <main+32> 6 if (a == 0) 7 return 1; 8 return a + 10; 0x000000000040040a <+10>: lea 0xa(%rax),%edx 0x000000000040040d <+13>: test %eax,%eax 0x000000000040040f <+15>: mov $0x1,%eax 0x0000000000400414 <+20>: cmovne %edx,%eax foo.c: 6 x = foo (y); 0x0000000000400417 <+23>: mov %eax,0x200c13(%rip) # 0x601030 <x> 7 return 0; 8 } 0x000000000040041d <+29>: xor %eax,%eax 0x000000000040041f <+31>: retq foo.h: 5 return a * 2; 0x0000000000400420 <+32>: add %eax,%eax 0x0000000000400422 <+34>: jmp 0x400417 <main+23> End of assembler dump.
Here is another example showing raw instructions in hex for AMD x86-64,
(gdb) disas /r 0x400281,+10 Dump of assembler code from 0x400281 to 0x40028b: 0x0000000000400281: 38 36 cmp %dh,(%rsi) 0x0000000000400283: 2d 36 34 2e 73 sub $0x732e3436,%eax 0x0000000000400288: 6f outsl %ds:(%rsi),(%dx) 0x0000000000400289: 2e 32 00 xor %cs:(%rax),%al End of assembler dump.
Addresses cannot be specified as a location (see Specify Location).
So, for example, if you want to disassemble function bar
in file foo.c, you must type 'disassemble 'foo.c'::bar'
and not 'disassemble foo.c:bar'.
Some architectures have more than one commonly-used set of instruction mnemonics or other syntax.
For programs that were dynamically linked and use shared libraries, instructions that call functions or branch to locations in the shared libraries might show a seemingly bogus location—it's actually a location of the relocation table. On some architectures, GDB might be able to resolve these to actual function names.
set disassembly-flavor instruction-set
Select the instruction set to use when disassembling the
program via the disassemble
or x/i
Currently this command is only defined for the Intel x86 family. You
can set instruction-set to either intel
or att
The default is att
, the AT&T flavor used by default by Unix
assemblers for x86-based targets.
show disassembly-flavor
Show the current setting of the disassembly flavor.
set disassemble-next-line
show disassemble-next-line
Control whether or not GDB will disassemble the next source line or instruction when execution stops. If ON, GDB will display disassembly of the next source line when execution of the program being debugged stops. This is in addition to displaying the source line itself, which GDB always does if possible. If the next source line cannot be displayed for some reason (e.g., if GDB cannot find the source file, or there's no line info in the debug info), GDB will display disassembly of the next instruction instead of showing the next source line. If AUTO, GDB will display disassembly of next instruction only if the source line cannot be displayed. This setting causes GDB to display some feedback when you step through a function with no line info or whose source file is unavailable. The default is OFF, which means never display the disassembly of the next line or instruction.
Next: Optimized Code, Previous: Source, Up: Top [Contents][Index]
The usual way to examine data in your program is with the print
command (abbreviated p
), or its synonym inspect
. It
evaluates and prints the value of an expression of the language your
program is written in (see Using GDB with
Different Languages). It may also print the expression using a
Python-based pretty-printer (see Pretty Printing).
print expr
print /f expr
expr is an expression (in the source language). By default the value of expr is printed in a format appropriate to its data type; you can choose a different format by specifying '/f', where f is a letter specifying the format; see Output Formats.
print /f
If you omit expr, GDB displays the last value again (from the value history; see Value History). This allows you to conveniently inspect the same value in an alternative format.
A more low-level way of examining data is with the x
It examines data in memory at a specified address and prints it in a
specified format. See Examining Memory.
If you are interested in information about types, or about how the
fields of a struct or a class are declared, use the ptype exp
command rather than print
. See Examining the Symbol
Another way of examining values of expressions and type information is
through the Python extension command explore
(available only if
the GDB build is configured with --with-python
). It
offers an interactive way to start at the highest level (or, the most
abstract level) of the data type of an expression (or, the data type
itself) and explore all the way down to leaf scalar values/fields
embedded in the higher level data types.
explore arg
arg is either an expression (in the source language), or a type visible in the current context of the program being debugged.
The working of the explore
command can be illustrated with an
example. If a data type struct ComplexStruct
is defined in your
C program as
struct SimpleStruct { int i; double d; }; struct ComplexStruct { struct SimpleStruct *ss_p; int arr[10]; };
followed by variable declarations as
struct SimpleStruct ss = { 10, 1.11 }; struct ComplexStruct cs = { &ss, { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 } };
then, the value of the variable cs
can be explored using the
command as follows.
(gdb) explore cs The value of `cs' is a struct/class of type `struct ComplexStruct' with the following fields: ss_p = <Enter 0 to explore this field of type `struct SimpleStruct *'> arr = <Enter 1 to explore this field of type `int [10]'> Enter the field number of choice:
Since the fields of cs
are not scalar values, you are being
prompted to chose the field you want to explore. Let's say you choose
the field ss_p
by entering 0
. Then, since this field is a
pointer, you will be asked if it is pointing to a single value. From
the declaration of cs
above, it is indeed pointing to a single
value, hence you enter y
. If you enter n
, then you will
be asked if it were pointing to an array of values, in which case this
field will be explored as if it were an array.
`cs.ss_p' is a pointer to a value of type `struct SimpleStruct' Continue exploring it as a pointer to a single value [y/n]: y The value of `*(cs.ss_p)' is a struct/class of type `struct SimpleStruct' with the following fields: i = 10 .. (Value of type `int') d = 1.1100000000000001 .. (Value of type `double') Press enter to return to parent value:
If the field arr
of cs
was chosen for exploration by
entering 1
earlier, then since it is as array, you will be
prompted to enter the index of the element in the array that you want
to explore.
`cs.arr' is an array of `int'. Enter the index of the element you want to explore in `cs.arr': 5 `(cs.arr)[5]' is a scalar value of type `int'. (cs.arr)[5] = 4 Press enter to return to parent value:
In general, at any stage of exploration, you can go deeper towards the leaf values by responding to the prompts appropriately, or hit the return key to return to the enclosing data structure (the higher level data structure).
Similar to exploring values, you can use the explore
command to
explore types. Instead of specifying a value (which is typically a
variable name or an expression valid in the current context of the
program being debugged), you specify a type name. If you consider the
same example as above, your can explore the type
struct ComplexStruct
by passing the argument
struct ComplexStruct
to the explore
(gdb) explore struct ComplexStruct
By responding to the prompts appropriately in the subsequent interactive
session, you can explore the type struct ComplexStruct
in a
manner similar to how the value cs
was explored in the above
The explore
command also has two sub-commands,
explore value
and explore type
. The former sub-command is
a way to explicitly specify that value exploration of the argument is
being invoked, while the latter is a way to explicitly specify that type
exploration of the argument is being invoked.
explore value expr
This sub-command of explore
explores the value of the
expression expr (if expr is an expression valid in the
current context of the program being debugged). The behavior of this
command is identical to that of the behavior of the explore
command being passed the argument expr.
explore type arg
This sub-command of explore
explores the type of arg (if
arg is a type visible in the current context of program being
debugged), or the type of the value/expression arg (if arg
is an expression valid in the current context of the program being
debugged). If arg is a type, then the behavior of this command is
identical to that of the explore
command being passed the
argument arg. If arg is an expression, then the behavior of
this command will be identical to that of the explore
being passed the type of arg as the argument.
• Expressions: | Expressions | |
• Ambiguous Expressions: | Ambiguous Expressions | |
• Variables: | Program variables | |
• Arrays: | Artificial arrays | |
• Output Formats: | Output formats | |
• Memory: | Examining memory | |
• Auto Display: | Automatic display | |
• Print Settings: | Print settings | |
• Pretty Printing: | Python pretty printing | |
• Value History: | Value history | |
• Convenience Vars: | Convenience variables | |
• Convenience Funs: | Convenience functions | |
• Registers: | Registers | |
• Floating Point Hardware: | Floating point hardware | |
• Vector Unit: | Vector Unit | |
• OS Information: | Auxiliary data provided by operating system | |
• Memory Region Attributes: | Memory region attributes | |
• Dump/Restore Files: | Copy between memory and a file | |
• Core File Generation: | Cause a program dump its core | |
• Character Sets: | Debugging programs that use a different character set than GDB does | |
• Caching Target Data: | Data caching for targets | |
• Searching Memory: | Searching memory for a sequence of bytes | |
• Value Sizes: | Managing memory allocated for values |
Next: Ambiguous Expressions, Up: Data [Contents][Index]
and many other GDB commands accept an expression and
compute its value. Any kind of constant, variable or operator defined
by the programming language you are using is valid in an expression in
GDB. This includes conditional expressions, function calls,
casts, and string constants. It also includes preprocessor macros, if
you compiled your program to include this information; see
GDB supports array constants in expressions input by
the user. The syntax is {element, element…}. For example,
you can use the command print {1, 2, 3}
to create an array
of three integers. If you pass an array to a function or assign it
to a program variable, GDB copies the array to memory that
is malloc
ed in the target program.
Because C is so widespread, most of the expressions shown in examples in this manual are in C. See Using GDB with Different Languages, for information on how to use expressions in other languages.
In this section, we discuss operators that you can use in GDB expressions regardless of your programming language.
Casts are supported in all languages, not just in C, because it is so useful to cast a number into a pointer in order to examine a structure at that address in memory.
GDB supports these operators, in addition to those common to programming languages:
'@' is a binary operator for treating parts of memory as arrays. See Artificial Arrays, for more information.
'::' allows you to specify a variable in terms of the file or function where it is defined. See Program Variables.
{type} addr
Refers to an object of type type stored at address addr in memory. The address addr may be any expression whose value is an integer or pointer (but parentheses are required around binary operators, just as in a cast). This construct is allowed regardless of what kind of data is normally supposed to reside at addr.
Next: Variables, Previous: Expressions, Up: Data [Contents][Index]
Expressions can sometimes contain some ambiguous elements. For instance, some programming languages (notably Ada, C++ and Objective-C) permit a single function name to be defined several times, for application in different contexts. This is called overloading. Another example involving Ada is generics. A generic package is similar to C++ templates and is typically instantiated several times, resulting in the same function name being defined in different contexts.
In some cases and depending on the language, it is possible to adjust the expression to remove the ambiguity. For instance in C++, you can specify the signature of the function you want to break on, as in break function(types). In Ada, using the fully qualified name of your function often makes the expression unambiguous as well.
When an ambiguity that needs to be resolved is detected, the debugger has the capability to display a menu of numbered choices for each possibility, and then waits for the selection with the prompt '>'. The first option is always '[0] cancel', and typing 0 RET aborts the current command. If the command in which the expression was used allows more than one choice to be selected, the next option in the menu is '[1] all', and typing 1 RET selects all possible choices.
For example, the following session excerpt shows an attempt to set a
breakpoint at the overloaded symbol String::after
We choose three particular definitions of that function name:
(gdb) b String::after [0] cancel [1] all [2] file:String.cc; line number:867 [3] file:String.cc; line number:860 [4] file:String.cc; line number:875 [5] file:String.cc; line number:853 [6] file:String.cc; line number:846 [7] file:String.cc; line number:735 > 2 4 6 Breakpoint 1 at 0xb26c: file String.cc, line 867. Breakpoint 2 at 0xb344: file String.cc, line 875. Breakpoint 3 at 0xafcc: file String.cc, line 846. Multiple breakpoints were set. Use the "delete" command to delete unwanted breakpoints. (gdb)
set multiple-symbols mode
This option allows you to adjust the debugger behavior when an expression is ambiguous.
By default, mode is set to all
. If the command with which
the expression is used allows more than one choice, then GDB
automatically selects all possible choices. For instance, inserting
a breakpoint on a function using an ambiguous name results in a breakpoint
inserted on each possible match. However, if a unique choice must be made,
then GDB uses the menu to help you disambiguate the expression.
For instance, printing the address of an overloaded function will result
in the use of the menu.
When mode is set to ask
, the debugger always uses the menu
when an ambiguity is detected.
Finally, when mode is set to cancel
, the debugger reports
an error due to the ambiguity and the command is aborted.
show multiple-symbols
Show the current value of the multiple-symbols
Next: Arrays, Previous: Ambiguous Expressions, Up: Data [Contents][Index]
The most common kind of expression to use is the name of a variable in your program.
Variables in expressions are understood in the selected stack frame (see Selecting a Frame); they must be either:
This means that in the function
foo (a) int a; { bar (a); { int b = test (); bar (b); } }
you can examine and use the variable a
whenever your program is
executing within the function foo
, but you can only use or
examine the variable b
while your program is executing inside
the block where b
is declared.
There is an exception: you can refer to a variable or function whose
scope is a single source file even if the current execution point is not
in this file. But it is possible to have more than one such variable or
function with the same name (in different source files). If that
happens, referring to that name has unpredictable effects. If you wish,
you can specify a static variable in a particular function or file by
using the colon-colon (::
) notation:
file::variable function::variable
Here file or function is the name of the context for the
static variable. In the case of file names, you can use quotes to
make sure GDB parses the file name as a single word—for example,
to print a global value of x
defined in f2.c:
(gdb) p 'f2.c'::x
The ::
notation is normally used for referring to
static variables, since you typically disambiguate uses of local variables
in functions by selecting the appropriate frame and using the
simple name of the variable. However, you may also use this notation
to refer to local variables in frames enclosing the selected frame:
void foo (int a) { if (a < 10) bar (a); else process (a); /* Stop here */ } int bar (int a) { foo (a + 5); }
For example, if there is a breakpoint at the commented line,
here is what you might see
when the program stops after executing the call bar(0)
(gdb) p a $1 = 10 (gdb) p bar::a $2 = 5 (gdb) up 2 #2 0x080483d0 in foo (a=5) at foobar.c:12 (gdb) p a $3 = 5 (gdb) p bar::a $4 = 0
These uses of '::' are very rarely in conflict with the very similar use of the same notation in C++. When they are in conflict, the C++ meaning takes precedence; however, this can be overridden by quoting the file or function name with single quotes.
For example, suppose the program is stopped in a method of a class
that has a field named includefile
, and there is also an
include file named includefile that defines a variable,
(gdb) p includefile $1 = 23 (gdb) p includefile::some_global A syntax error in expression, near `'. (gdb) p 'includefile'::some_global $2 = 27
Warning: Occasionally, a local variable may appear to have the wrong value at certain points in a function—just after entry to a new scope, and just before exit.
You may see this problem when you are stepping by machine instructions. This is because, on most machines, it takes more than one instruction to set up a stack frame (including local variable definitions); if you are stepping by machine instructions, variables may appear to have the wrong values until the stack frame is completely built. On exit, it usually also takes more than one machine instruction to destroy a stack frame; after you begin stepping through that group of instructions, local variable definitions may be gone.
This may also happen when the compiler does significant optimizations. To be sure of always seeing accurate values, turn off all optimization when compiling.
Another possible effect of compiler optimizations is to optimize unused variables out of existence, or assign variables to registers (as opposed to memory addresses). Depending on the support for such cases offered by the debug info format used by the compiler, GDB might not be able to display values for such local variables. If that happens, GDB will print a message like this:
No symbol "foo" in current context.
To solve such problems, either recompile without optimizations, or use a different debug info format, if the compiler supports several such formats. See Compilation, for more information on choosing compiler options. See C and C++, for more information about debug info formats that are best suited to C++ programs.
If you ask to print an object whose contents are unknown to GDB, e.g., because its data type is not completely specified by the debug information, GDB will say '<incomplete type>'. See incomplete type, for more about this.
If you append @entry string to a function parameter name you get its value at the time the function got called. If the value is not available an error message is printed. Entry values are available only with some compilers. Entry values are normally also printed at the function parameter list according to set print entry-values.
Breakpoint 1, d (i=30) at gdb.base/entry-value.c:29 29 i++; (gdb) next 30 e (i); (gdb) print i $1 = 31 (gdb) print i@entry $2 = 30
Strings are identified as arrays of char
values without specified
signedness. Arrays of either signed char
or unsigned char
printed as arrays of 1 byte sized integers. -fsigned-char
GCC options have no effect as GDB
defines literal string type "char"
as char
without a sign.
For program code
char var0[] = "A"; signed char var1[] = "A";
You get during debugging
(gdb) print var0 $1 = "A" (gdb) print var1 $2 = {65 'A', 0 '\0'}
Next: Output Formats, Previous: Variables, Up: Data [Contents][Index]
It is often useful to print out several successive objects of the same type in memory; a section of an array, or an array of dynamically determined size for which only a pointer exists in the program.
You can do this by referring to a contiguous span of memory as an artificial array, using the binary operator '@'. The left operand of '@' should be the first element of the desired array and be an individual object. The right operand should be the desired length of the array. The result is an array value whose elements are all of the type of the left argument. The first element is actually the left argument; the second element comes from bytes of memory immediately following those that hold the first element, and so on. Here is an example. If a program says
int *array = (int *) malloc (len * sizeof (int));
you can print the contents of array
p *array@len
The left operand of '@' must reside in memory. Array values made with '@' in this way behave just like other arrays in terms of subscripting, and are coerced to pointers when used in expressions. Artificial arrays most often appear in expressions via the value history (see Value History), after printing one out.
Another way to create an artificial array is to use a cast. This re-interprets a value as if it were an array. The value need not be in memory:
(gdb) p/x (short[2])0x12345678 $1 = {0x1234, 0x5678}
As a convenience, if you leave the array length out (as in '(type[])value') GDB calculates the size to fill the value (as 'sizeof(value)/sizeof(type)':
(gdb) p/x (short[])0x12345678 $2 = {0x1234, 0x5678}
Sometimes the artificial array mechanism is not quite enough; in
moderately complex data structures, the elements of interest may not
actually be adjacent—for example, if you are interested in the values
of pointers in an array. One useful work-around in this situation is
to use a convenience variable (see Convenience
Variables) as a counter in an expression that prints the first
interesting value, and then repeat that expression via RET. For
instance, suppose you have an array dtab
of pointers to
structures, and you are interested in the values of a field fv
in each structure. Here is an example of what you might type:
set $i = 0 p dtab[$i++]->fv RET RET …
By default, GDB prints a value according to its data type. Sometimes this is not what you want. For example, you might want to print a number in hex, or a pointer in decimal. Or you might want to view data in memory at a certain address as a character string or as an instruction. To do these things, specify an output format when you print a value.
The simplest use of output formats is to say how to print a value
already computed. This is done by starting the arguments of the
command with a slash and a format letter. The format
letters supported are:
Regard the bits of the value as an integer, and print the integer in hexadecimal.
Print as integer in signed decimal.
Print as integer in unsigned decimal.
Print as integer in octal.
Print as integer in binary. The letter 't' stands for "two". 10
Print as an address, both absolute in hexadecimal and as an offset from the nearest preceding symbol. You can use this format used to discover where (in what function) an unknown address is located:
(gdb) p/a 0x54320 $3 = 0x54320 <_initialize_vx+396>
The command info symbol 0x54320
yields similar results.
See info symbol.
Regard as an integer and print it as a character constant. This prints both the numerical value and its character representation. The character representation is replaced with the octal escape '\nnn' for characters outside the 7-bit ASCII range.
Without this format, GDB displays char
unsigned char
, and signed char
data as character
constants. Single-byte members of vectors are displayed as integer
Regard the bits of the value as a floating point number and print using typical floating point syntax.
Regard as a string, if possible. With this format, pointers to single-byte data are displayed as null-terminated strings and arrays of single-byte data are displayed as fixed-length strings. Other values are displayed in their natural types.
Without this format, GDB displays pointers to and arrays of
, unsigned char
, and signed char
strings. Single-byte members of a vector are displayed as an integer
Like 'x' formatting, the value is treated as an integer and printed as hexadecimal, but leading zeros are printed to pad the value to the size of the integer type.
Print using the 'raw' formatting. By default, GDB will use a Python-based pretty-printer, if one is available (see Pretty Printing). This typically results in a higher-level display of the value's contents. The 'r' format bypasses any Python pretty-printer which might exist.
For example, to print the program counter in hex (see Registers), type
p/x $pc
Note that no space is required before the slash; this is because command names in GDB cannot contain a slash.
To reprint the last value in the value history with a different format,
you can use the print
command with just a format and no
expression. For example, 'p/x' reprints the last value in hex.
Next: Auto Display, Previous: Output Formats, Up: Data [Contents][Index]
You can use the command x
(for "examine") to examine memory in
any of several formats, independently of your program's data types.
x/nfu addr
x addr
Use the x
command to examine memory.
n, f, and u are all optional parameters that specify how much memory to display and how to format it; addr is an expression giving the address where you want to start displaying memory. If you use defaults for nfu, you need not type the slash '/'. Several commands set convenient defaults for addr.
The repeat count is a decimal integer; the default is 1. It specifies how much memory (counting by units u) to display. If a negative number is specified, memory is examined backward from addr.
The display format is one of the formats used by print
('x', 'd', 'u', 'o', 't', 'a', 'c',
'f', 's'), and in addition 'i' (for machine instructions).
The default is 'x' (hexadecimal) initially. The default changes
each time you use either x
or print
The unit size is any of
Halfwords (two bytes).
Words (four bytes). This is the initial default.
Giant words (eight bytes).
Each time you specify a unit size with x
, that size becomes the
default unit the next time you use x
. For the 'i' format,
the unit size is ignored and is normally not written. For the 's' format,
the unit size defaults to 'b', unless it is explicitly given.
Use x /hs to display 16-bit char strings and x /ws to display
32-bit strings. The next use of x /s will again display 8-bit strings.
Note that the results depend on the programming language of the
current compilation unit. If the language is C, the 's'
modifier will use the UTF-16 encoding while 'w' will use
UTF-32. The encoding is set by the programming language and cannot
be altered.
addr is the address where you want GDB to begin displaying
memory. The expression need not have a pointer value (though it may);
it is always interpreted as an integer address of a byte of memory.
See Expressions, for more information on expressions. The default for
addr is usually just after the last address examined—but several
other commands also set the default address: info breakpoints
the address of the last breakpoint listed), info line
(to the
starting address of a line), and print
(if you use it to display
a value from memory).
For example, 'x/3uh 0x54320' is a request to display three halfwords
) of memory, formatted as unsigned decimal integers ('u'),
starting at address 0x54320
. 'x/4xw $sp' prints the four
words ('w') of memory above the stack pointer (here, '$sp';
see Registers) in hexadecimal ('x').
You can also specify a negative repeat count to examine memory backward
from the given address. For example, 'x/-3uh 0x54320' prints three
halfwords (h
) at 0x54314
, 0x54328
, and 0x5431c
Since the letters indicating unit sizes are all distinct from the letters specifying output formats, you do not have to remember whether unit size or format comes first; either order works. The output specifications '4xw' and '4wx' mean exactly the same thing. (However, the count n must come first; 'wx4' does not work.)
Even though the unit size u is ignored for the formats 's'
and 'i', you might still want to use a count n; for example,
'3i' specifies that you want to see three machine instructions,
including any operands. For convenience, especially when used with
the display
command, the 'i' format also prints branch delay
slot instructions, if any, beyond the count specified, which immediately
follow the last instruction that is within the count. The command
gives an alternative way of inspecting machine
instructions; see Source and Machine Code.
If a negative repeat count is specified for the formats 's' or 'i', the command displays null-terminated strings or instructions before the given address as many as the absolute value of the given number. For the 'i' format, we use line number information in the debug info to accurately locate instruction boundaries while disassembling backward. If line info is not available, the command stops examining memory with an error message.
All the defaults for the arguments to x
are designed to make it
easy to continue scanning memory with minimal specifications each time
you use x
. For example, after you have inspected three machine
instructions with 'x/3i addr', you can inspect the next seven
with just 'x/7'. If you use RET to repeat the x
the repeat count n is used again; the other arguments default as
for successive uses of x
When examining machine instructions, the instruction at current program
counter is shown with a =>
marker. For example:
(gdb) x/5i $pc-6 0x804837f <main+11>: mov %esp,%ebp 0x8048381 <main+13>: push %ecx 0x8048382 <main+14>: sub $0x4,%esp => 0x8048385 <main+17>: movl $0x8048460,(%esp) 0x804838c <main+24>: call 0x80482d4 <puts@plt>
The addresses and contents printed by the x
command are not saved
in the value history because there is often too much of them and they
would get in the way. Instead, GDB makes these values available for
subsequent use in expressions as values of the convenience variables
and $__
. After an x
command, the last address
examined is available for use in expressions in the convenience variable
. The contents of that address, as examined, are available in
the convenience variable $__
If the x
command has a repeat count, the address and contents saved
are from the last memory unit printed; this is not the same as the last
address printed if several units were printed on the last line of output.
Most targets have an addressable memory unit size of 8 bits. This means that to each memory address are associated 8 bits of data. Some targets, however, have other addressable memory unit sizes. Within GDB and this document, the term addressable memory unit (or memory unit for short) is used when explicitly referring to a chunk of data of that size. The word byte is used to refer to a chunk of data of 8 bits, regardless of the addressable memory unit size of the target. For most systems, addressable memory unit is a synonym of byte.
When you are debugging a program running on a remote target machine
(see Remote Debugging), you may wish to verify the program's image
in the remote machine's memory against the executable file you
downloaded to the target. Or, on any target, you may want to check
whether the program has corrupted its own read-only sections. The
command is provided for such situations.
compare-sections [section-name|-r
Compare the data of a loadable section section-name in the
executable file of the program being debugged with the same section in
the target machine's memory, and report any mismatches. With no
arguments, compares all loadable sections. With an argument of
, compares all loadable read-only sections.
Note: for remote targets, this command can be accelerated if the target supports computing the CRC checksum of a block of memory (see qCRC packet).
Next: Print Settings, Previous: Memory, Up: Data [Contents][Index]
If you find that you want to print the value of an expression frequently (to see how it changes), you might want to add it to the automatic display list so that GDB prints its value each time your program stops. Each expression added to the list is given a number to identify it; to remove an expression from the list, you specify that number. The automatic display looks like this:
2: foo = 38 3: bar[5] = (struct hack *) 0x3804
This display shows item numbers, expressions and their current values. As with
displays you request manually using x
or print
, you can
specify the output format you prefer; in fact, display
whether to use print
or x
depending your format
specification—it uses x
if you specify either the 'i'
or 's' format, or a unit size; otherwise it uses print
display expr
Add the expression expr to the list of expressions to display each time your program stops. See Expressions.
does not repeat if you press RET again after using it.
display/fmt expr
For fmt specifying only a display format and not a size or count, add the expression expr to the auto-display list but arrange to display it each time in the specified format fmt. See Output Formats.
display/fmt addr
For fmt 'i' or 's', or including a unit-size or a number of units, add the expression addr as a memory address to be examined each time your program stops. Examining means in effect doing 'x/fmt addr'. See Examining Memory.
For example, 'display/i $pc' can be helpful, to see the machine instruction about to be executed each time execution stops ('$pc' is a common name for the program counter; see Registers).
undisplay dnums…
delete display dnums…
Remove items from the list of expressions to display. Specify the
numbers of the displays that you want affected with the command
argument dnums. It can be a single display number, one of the
numbers shown in the first field of the 'info display' display;
or it could be a range of display numbers, as in 2-4
does not repeat if you press RET after using it.
(Otherwise you would just get the error 'No display number …'.)
disable display dnums…
Disable the display of item numbers dnums. A disabled display
item is not printed automatically, but is not forgotten. It may be
enabled again later. Specify the numbers of the displays that you
want affected with the command argument dnums. It can be a
single display number, one of the numbers shown in the first field of
the 'info display' display; or it could be a range of display
numbers, as in 2-4
enable display dnums…
Enable display of item numbers dnums. It becomes effective once
again in auto display of its expression, until you specify otherwise.
Specify the numbers of the displays that you want affected with the
command argument dnums. It can be a single display number, one
of the numbers shown in the first field of the 'info display'
display; or it could be a range of display numbers, as in 2-4
Display the current values of the expressions on the list, just as is done when your program stops.
info display
Print the list of expressions previously set up to display automatically, each one with its item number, but without showing the values. This includes disabled expressions, which are marked as such. It also includes expressions which would not be displayed right now because they refer to automatic variables not currently available.
If a display expression refers to local variables, then it does not make
sense outside the lexical context for which it was set up. Such an
expression is disabled when execution enters a context where one of its
variables is not defined. For example, if you give the command
display last_char
while inside a function with an argument
, GDB displays this argument while your program
continues to stop inside that function. When it stops elsewhere—where
there is no variable last_char
—the display is disabled
automatically. The next time your program stops where last_char
is meaningful, you can enable the display expression once again.
Next: Pretty Printing, Previous: Auto Display, Up: Data [Contents][Index]
GDB provides the following ways to control how arrays, structures, and symbols are printed.
These settings are useful for debugging programs in any language:
set print address
set print address on
GDB prints memory addresses showing the location of stack
traces, structure values, pointer values, breakpoints, and so forth,
even when it also displays the contents of those addresses. The default
is on
. For example, this is what a stack frame display looks like with
set print address on
(gdb) f #0 set_quotes (lq=0x34c78 "<<", rq=0x34c88 ">>") at input.c:530 530 if (lquote != def_lquote)
set print address off
Do not print addresses when displaying their contents. For example,
this is the same stack frame displayed with set print address off
(gdb) set print addr off (gdb) f #0 set_quotes (lq="<<", rq=">>") at input.c:530 530 if (lquote != def_lquote)
You can use 'set print address off' to eliminate all machine
dependent displays from the GDB interface. For example, with
print address off
, you should get the same text for backtraces on
all machines—whether or not they involve pointer arguments.
show print address
Show whether or not addresses are to be printed.
When GDB prints a symbolic address, it normally prints the
closest earlier symbol plus an offset. If that symbol does not uniquely
identify the address (for example, it is a name whose scope is a single
source file), you may need to clarify. One way to do this is with
info line
, for example 'info line *0x4537'. Alternately,
you can set GDB to print the source file and line number when
it prints a symbolic address:
set print symbol-filename on
Tell GDB to print the source file name and line number of a symbol in the symbolic form of an address.
set print symbol-filename off
Do not print source file name and line number of a symbol. This is the default.
show print symbol-filename
Show whether or not GDB will print the source file name and line number of a symbol in the symbolic form of an address.
Another situation where it is helpful to show symbol filenames and line numbers is when disassembling code; GDB shows you the line number and source file that corresponds to each instruction.
Also, you may wish to see the symbolic form only if the address being printed is reasonably close to the closest earlier symbol:
set print max-symbolic-offset max-offset
set print max-symbolic-offset unlimited
Tell GDB to only display the symbolic form of an address if the
offset between the closest earlier symbol and the address is less than
max-offset. The default is unlimited
, which tells GDB
to always print the symbolic form of an address if any symbol precedes
it. Zero is equivalent to unlimited
show print max-symbolic-offset
Ask how large the maximum offset is that GDB prints in a symbolic address.
If you have a pointer and you are not sure where it points, try
'set print symbol-filename on'. Then you can determine the name
and source file location of the variable where it points, using
'p/a pointer'. This interprets the address in symbolic form.
For example, here GDB shows that a variable ptt
at another variable t
, defined in hi2.c:
(gdb) set print symbol-filename on (gdb) p/a ptt $4 = 0xe008 <t in hi2.c>
Warning: For pointers that point to a local variable, 'p/a' does not show the symbol name and filename of the referent, even with the appropriate
set print
options turned on.
You can also enable '/a'-like formatting all the time using 'set print symbol on':
set print symbol on
Tell GDB to print the symbol corresponding to an address, if one exists.
set print symbol off
Tell GDB not to print the symbol corresponding to an address. In this mode, GDB will still print the symbol corresponding to pointers to functions. This is the default.
show print symbol
Show whether GDB will display the symbol corresponding to an address.
Other settings control how different kinds of objects are printed:
set print array
set print array on
Pretty print arrays. This format is more convenient to read, but uses more space. The default is off.
set print array off
Return to compressed format for arrays.
show print array
Show whether compressed or pretty format is selected for displaying arrays.
set print array-indexes
set print array-indexes on
Print the index of each element when displaying arrays. May be more convenient to locate a given element in the array or quickly find the index of a given element in that printed array. The default is off.
set print array-indexes off
Stop printing element indexes when displaying arrays.
show print array-indexes
Show whether the index of each element is printed when displaying arrays.
set print elements number-of-elements
set print elements unlimited
Set a limit on how many elements of an array GDB will print.
If GDB is printing a large array, it stops printing after it has
printed the number of elements set by the set print elements
This limit also applies to the display of strings.
When GDB starts, this limit is set to 200.
Setting number-of-elements to unlimited
or zero means
that the number of elements to print is unlimited.
show print elements
Display the number of elements of a large array that GDB will print. If the number is 0, then the printing is unlimited.
set print frame-arguments value
This command allows to control how the values of arguments are printed when the debugger prints a frame (see Frames). The possible values are:
The values of all arguments are printed.
Print the value of an argument only if it is a scalar. The value of more
complex arguments such as arrays, structures, unions, etc, is replaced
by …
. This is the default. Here is an example where
only scalar arguments are shown:
#1 0x08048361 in call_me (i=3, s=…, ss=0xbf8d508c, u=…, e=green) at frame-args.c:23
None of the argument values are printed. Instead, the value of each argument
is replaced by …
. In this case, the example above now becomes:
#1 0x08048361 in call_me (i=…, s=…, ss=…, u=…, e=…) at frame-args.c:23
By default, only scalar arguments are printed. This command can be used
to configure the debugger to print the value of all arguments, regardless
of their type. However, it is often advantageous to not print the value
of more complex parameters. For instance, it reduces the amount of
information printed in each frame, making the backtrace more readable.
Also, it improves performance when displaying Ada frames, because
the computation of large arguments can sometimes be CPU-intensive,
especially in large applications. Setting print frame-arguments
to scalars
(the default) or none
avoids this computation,
thus speeding up the display of each Ada frame.
show print frame-arguments
Show how the value of arguments should be displayed when printing a frame.
set print raw frame-arguments on
Print frame arguments in raw, non pretty-printed, form.
set print raw frame-arguments off
Print frame arguments in pretty-printed form, if there is a pretty-printer for the value (see Pretty Printing), otherwise print the value in raw form. This is the default.
show print raw frame-arguments
Show whether to print frame arguments in raw form.
set print entry-values value
Set printing of frame argument values at function entry. In some cases GDB can determine the value of function argument which was passed by the function caller, even if the value was modified inside the called function and therefore is different. With optimized code, the current value could be unavailable, but the entry value may still be known.
The default value is default
(see below for its description). Older
GDB behaved as with the setting no
. Compilers not supporting
this feature will behave in the default
setting the same way as with the
This functionality is currently supported only by DWARF 2 debugging format and the compiler has to produce 'DW_TAG_GNU_call_site' tags. With GCC, you need to specify -O -g during compilation, to get this information.
The value parameter can be one of the following:
Print only actual parameter values, never print values from function entry point.
#0 equal (val=5) #0 different (val=6) #0 lost (val=<optimized out>) #0 born (val=10) #0 invalid (val=<optimized out>)
Print only parameter values from function entry point. The actual parameter values are never printed.
#0 equal (val@entry=5) #0 different (val@entry=5) #0 lost (val@entry=5) #0 born (val@entry=<optimized out>) #0 invalid (val@entry=<optimized out>)
Print only parameter values from function entry point. If value from function entry point is not known while the actual value is known, print the actual value for such parameter.
#0 equal (val@entry=5) #0 different (val@entry=5) #0 lost (val@entry=5) #0 born (val=10) #0 invalid (val@entry=<optimized out>)
Print actual parameter values. If actual parameter value is not known while value from function entry point is known, print the entry point value for such parameter.
#0 equal (val=5) #0 different (val=6) #0 lost (val@entry=5) #0 born (val=10) #0 invalid (val=<optimized out>)
Always print both the actual parameter value and its value from function entry point, even if values of one or both are not available due to compiler optimizations.
#0 equal (val=5, val@entry=5) #0 different (val=6, val@entry=5) #0 lost (val=<optimized out>, val@entry=5) #0 born (val=10, val@entry=<optimized out>) #0 invalid (val=<optimized out>, val@entry=<optimized out>)
Print the actual parameter value if it is known and also its value from
function entry point if it is known. If neither is known, print for the actual
value <optimized out>
. If not in MI mode (see GDB/MI) and if both
values are known and identical, print the shortened
#0 equal (val=val@entry=5) #0 different (val=6, val@entry=5) #0 lost (val@entry=5) #0 born (val=10) #0 invalid (val=<optimized out>)
Always print the actual parameter value. Print also its value from function
entry point, but only if it is known. If not in MI mode (see GDB/MI) and
if both values are known and identical, print the shortened
#0 equal (val=val@entry=5) #0 different (val=6, val@entry=5) #0 lost (val=<optimized out>, val@entry=5) #0 born (val=10) #0 invalid (val=<optimized out>)
For analysis messages on possible failures of frame argument values at function entry resolution see set debug entry-values.
show print entry-values
Show the method being used for printing of frame argument values at function entry.
set print repeats number-of-repeats
set print repeats unlimited
Set the threshold for suppressing display of repeated array
elements. When the number of consecutive identical elements of an
array exceeds the threshold, GDB prints the string
"<repeats n times>"
, where n is the number of
identical repetitions, instead of displaying the identical elements
themselves. Setting the threshold to unlimited
or zero will
cause all elements to be individually printed. The default threshold
is 10.
show print repeats
Display the current threshold for printing repeated identical elements.
set print null-stop
Cause GDB to stop printing the characters of an array when the first NULL is encountered. This is useful when large arrays actually contain only short strings. The default is off.
show print null-stop
Show whether GDB stops printing an array on the first NULL character.
set print pretty on
Cause GDB to print structures in an indented format with one member per line, like this:
$1 = { next = 0x0, flags = { sweet = 1, sour = 1 }, meat = 0x54 "Pork" }
set print pretty off
Cause GDB to print structures in a compact format, like this:
$1 = {next = 0x0, flags = {sweet = 1, sour = 1}, \ meat = 0x54 "Pork"}
This is the default format.
show print pretty
Show which format GDB is using to print structures.
set print sevenbit-strings on
Print using only seven-bit characters; if this option is set,
GDB displays any eight-bit characters (in strings or
character values) using the notation \
nnn. This setting is
best if you are working in English (ASCII) and you use the
high-order bit of characters as a marker or "meta" bit.
set print sevenbit-strings off
Print full eight-bit characters. This allows the use of more international character sets, and is the default.
show print sevenbit-strings
Show whether or not GDB is printing only seven-bit characters.
set print union on
Tell GDB to print unions which are contained in structures and other unions. This is the default setting.
set print union off
Tell GDB not to print unions which are contained in
structures and other unions. GDB will print "{...}"
show print union
Ask GDB whether or not it will print unions which are contained in structures and other unions.
For example, given the declarations
typedef enum {Tree, Bug} Species; typedef enum {Big_tree, Acorn, Seedling} Tree_forms; typedef enum {Caterpillar, Cocoon, Butterfly} Bug_forms; struct thing { Species it; union { Tree_forms tree; Bug_forms bug; } form; }; struct thing foo = {Tree, {Acorn}};
with set print union on
in effect 'p foo' would print
$1 = {it = Tree, form = {tree = Acorn, bug = Cocoon}}
and with set print union off
in effect it would print
$1 = {it = Tree, form = {...}}
set print union
affects programs written in C-like languages
and in Pascal.
These settings are of interest when debugging C++ programs:
set print demangle
set print demangle on
Print C++ names in their source form rather than in the encoded ("mangled") form passed to the assembler and linker for type-safe linkage. The default is on.
show print demangle
Show whether C++ names are printed in mangled or demangled form.
set print asm-demangle
set print asm-demangle on
Print C++ names in their source form rather than their mangled form, even in assembler code printouts such as instruction disassemblies. The default is off.
show print asm-demangle
Show whether C++ names in assembly listings are printed in mangled or demangled form.
set demangle-style style
Choose among several encoding schemes used by different compilers to represent C++ names. The choices for style are currently:
Allow GDB to choose a decoding style by inspecting your program. This is the default.
Decode based on the GNU C++ compiler (g++
) encoding algorithm.
Decode based on the HP ANSI C++ (aCC
) encoding algorithm.
Decode based on the Lucid C++ compiler (lcc
) encoding algorithm.
Decode using the algorithm in the C++ Annotated Reference Manual.
Warning: this setting alone is not sufficient to allow
debugging cfront
-generated executables. GDB would
require further enhancement to permit that.
If you omit style, you will see a list of possible formats.
show demangle-style
Display the encoding style currently in use for decoding C++ symbols.
set print object
set print object on
When displaying a pointer to an object, identify the actual (derived) type of the object rather than the declared type, using the virtual function table. Note that the virtual function table is required—this feature can only work for objects that have run-time type identification; a single virtual method in the object's declared type is sufficient. Note that this setting is also taken into account when working with variable objects via MI (see GDB/MI).
set print object off
Display only the declared type of objects, without reference to the virtual function table. This is the default setting.
show print object
Show whether actual, or declared, object types are displayed.
set print static-members
set print static-members on
Print static members when displaying a C++ object. The default is on.
set print static-members off
Do not print static members when displaying a C++ object.
show print static-members
Show whether C++ static members are printed or not.
set print pascal_static-members
set print pascal_static-members on
Print static members when displaying a Pascal object. The default is on.
set print pascal_static-members off
Do not print static members when displaying a Pascal object.
show print pascal_static-members
Show whether Pascal static members are printed or not.
set print vtbl
set print vtbl on
Pretty print C++ virtual function tables. The default is off.
(The vtbl
commands do not work on programs compiled with the HP
ANSI C++ compiler (aCC
set print vtbl off
Do not pretty print C++ virtual function tables.
show print vtbl
Show whether C++ virtual function tables are pretty printed, or not.
Next: Value History, Previous: Print Settings, Up: Data [Contents][Index]
GDB provides a mechanism to allow pretty-printing of values using Python code. It greatly simplifies the display of complex objects. This mechanism works for both MI and the CLI.
• Pretty-Printer Introduction: | Introduction to pretty-printers | |
• Pretty-Printer Example: | An example pretty-printer | |
• Pretty-Printer Commands: | Pretty-printer commands |
Next: Pretty-Printer Example, Up: Pretty Printing [Contents][Index]
When GDB prints a value, it first sees if there is a pretty-printer registered for the value. If there is then GDB invokes the pretty-printer to print the value. Otherwise the value is printed normally.
Pretty-printers are normally named. This makes them easy to manage. The 'info pretty-printer' command will list all the installed pretty-printers with their names. If a pretty-printer can handle multiple data types, then its subprinters are the printers for the individual data types. Each such subprinter has its own name. The format of the name is printer-name;subprinter-name.
Pretty-printers are installed by registering them with GDB. Typically they are automatically loaded and registered when the corresponding debug information is loaded, thus making them available without having to do anything special.
There are three places where a pretty-printer can be registered.
See Selecting Pretty-Printers, for further information on how pretty-printers are selected,
See Writing a Pretty-Printer, for implementing pretty printers for new types.
Next: Pretty-Printer Commands, Previous: Pretty-Printer Introduction, Up: Pretty Printing [Contents][Index]
Here is how a C++ std::string
looks without a pretty-printer:
(gdb) print s $1 = { static npos = 4294967295, _M_dataplus = { <std::allocator<char>> = { <__gnu_cxx::new_allocator<char>> = { <No data fields>}, <No data fields> }, members of std::basic_string<char, std::char_traits<char>, std::allocator<char> >::_Alloc_hider: _M_p = 0x804a014 "abcd" } }
With a pretty-printer for std::string
only the contents are printed:
(gdb) print s $2 = "abcd"
Previous: Pretty-Printer Example, Up: Pretty Printing [Contents][Index]
info pretty-printer [object-regexp [name-regexp]]
Print the list of installed pretty-printers. This includes disabled pretty-printers, which are marked as such.
object-regexp is a regular expression matching the objects
whose pretty-printers to list.
Objects can be global
, the program space's file
(see Progspaces In Python),
and the object files within that program space (see Objfiles In Python).
See Selecting Pretty-Printers, for details on how GDB
looks up a printer from these three objects.
name-regexp is a regular expression matching the name of the printers to list.
disable pretty-printer [object-regexp [name-regexp]]
Disable pretty-printers matching object-regexp and name-regexp. A disabled pretty-printer is not forgotten, it may be enabled again later.
enable pretty-printer [object-regexp [name-regexp]]
Enable pretty-printers matching object-regexp and name-regexp.
Suppose we have three pretty-printers installed: one from library1.so
named foo
that prints objects of type foo
, and
another from library2.so named bar
that prints two types of objects,
and bar2
(gdb) info pretty-printer library1.so: foo library2.so: bar bar1 bar2 (gdb) info pretty-printer library2 library2.so: bar bar1 bar2 (gdb) disable pretty-printer library1 1 printer disabled 2 of 3 printers enabled (gdb) info pretty-printer library1.so: foo [disabled] library2.so: bar bar1 bar2 (gdb) disable pretty-printer library2 bar:bar1 1 printer disabled 1 of 3 printers enabled (gdb) info pretty-printer library2 library1.so: foo [disabled] library2.so: bar bar1 [disabled] bar2 (gdb) disable pretty-printer library2 bar 1 printer disabled 0 of 3 printers enabled (gdb) info pretty-printer library2 library1.so: foo [disabled] library2.so: bar [disabled] bar1 [disabled] bar2
Note that for bar
the entire printer can be disabled,
as can each individual subprinter.
Next: Convenience Vars, Previous: Pretty Printing, Up: Data [Contents][Index]
Values printed by the print
command are saved in the GDB
value history. This allows you to refer to them in other expressions.
Values are kept until the symbol table is re-read or discarded
(for example with the file
or symbol-file
When the symbol table changes, the value history is discarded,
since the values may contain pointers back to the types defined in the
symbol table.
The values printed are given history numbers by which you can
refer to them. These are successive integers starting with one.
shows you the history number assigned to a value by
printing '$num = ' before the value; here num is the
history number.
To refer to any previous value, use '$' followed by the value's
history number. The way print
labels its output is designed to
remind you of this. Just $
refers to the most recent value in
the history, and $$
refers to the value before that.
refers to the nth value from the end; $$2
is the value just prior to $$
, $$1
is equivalent to
, and $$0
is equivalent to $
For example, suppose you have just printed a pointer to a structure and want to see the contents of the structure. It suffices to type
p *$
If you have a chain of structures where the component next
to the next one, you can print the contents of the next one with this:
p *$.next
You can print successive links in the chain by repeating this command—which you can do by just typing RET.
Note that the history records values, not expressions. If the value of
is 4 and you type these commands:
print x set x=5
then the value recorded in the value history by the print
remains 4 even though the value of x
has changed.
show values
Print the last ten values in the value history, with their item numbers.
This is like 'p $$9' repeated ten times, except that show
does not change the history.
show values n
Print ten history values centered on history item number n.
show values +
Print ten history values just after the values last printed. If no more
values are available, show values +
produces no display.
Pressing RET to repeat show values n
has exactly the
same effect as 'show values +'.
Next: Convenience Funs, Previous: Value History, Up: Data [Contents][Index]
GDB provides convenience variables that you can use within GDB to hold on to a value and refer to it later. These variables exist entirely within GDB; they are not part of your program, and setting a convenience variable has no direct effect on further execution of your program. That is why you can use them freely.
Convenience variables are prefixed with '$'. Any name preceded by '$' can be used for a convenience variable, unless it is one of the predefined machine-specific register names (see Registers). (Value history references, in contrast, are numbers preceded by '$'. See Value History.)
You can save a value in a convenience variable with an assignment expression, just as you would set a variable in your program. For example:
set $foo = *object_ptr
would save in $foo
the value contained in the object pointed to by
Using a convenience variable for the first time creates it, but its
value is void
until you assign a new value. You can alter the
value with another assignment at any time.
Convenience variables have no fixed types. You can assign a convenience variable any type of value, including structures and arrays, even if that variable already has a value of a different type. The convenience variable, when used as an expression, has the type of its current value.
show convenience
Print a list of convenience variables used so far, and their values,
as well as a list of the convenience functions.
Abbreviated show conv
init-if-undefined $variable = expression
Set a convenience variable if it has not already been set. This is useful for user-defined commands that keep some state. It is similar, in concept, to using local static variables with initializers in C (except that convenience variables are global). It can also be used to allow users to override default values used in a command script.
If the variable is already defined then the expression is not evaluated so any side-effects do not occur.
One of the ways to use a convenience variable is as a counter to be incremented or a pointer to be advanced. For example, to print a field from successive elements of an array of structures:
set $i = 0 print bar[$i++]->contents
Repeat that command by typing RET.
Some convenience variables are created automatically by GDB and given values likely to be useful.
The variable $_
is automatically set by the x
command to
the last address examined (see Examining Memory). Other
commands which provide a default address for x
to examine also
set $_
to that address; these commands include info line
and info breakpoint
. The type of $_
is void *
except when set by the x
command, in which case it is a pointer
to the type of $__
The variable $__
is automatically set by the x
to the value found in the last address examined. Its type is chosen
to match the format in which the data was printed.
When the program being debugged terminates normally, GDB
automatically sets this variable to the exit code of the program, and
resets $_exitsignal
to void
When the program being debugged dies due to an uncaught signal,
GDB automatically sets this variable to that signal's number,
and resets $_exitcode
to void
To distinguish between whether the program being debugged has exited
(i.e., $_exitcode
is not void
) or signalled (i.e.,
is not void
), the convenience function
can be used (see Convenience
Functions). For example, considering the following source code:
#include <signal.h> int main (int argc, char *argv[]) { raise (SIGALRM); return 0; }
A valid way of telling whether the program being debugged has exited or signalled would be:
(gdb) define has_exited_or_signalled Type commands for definition of ``has_exited_or_signalled''. End with a line saying just ``end''. >if $_isvoid ($_exitsignal) >echo The program has exited\n >else >echo The program has signalled\n >end >end (gdb) run Starting program: Program terminated with signal SIGALRM, Alarm clock. The program no longer exists. (gdb) has_exited_or_signalled The program has signalled
As can be seen, GDB correctly informs that the program being
debugged has signalled, since it calls raise
and raises a
signal. If the program being debugged had not called
, then GDB would report a normal exit:
(gdb) has_exited_or_signalled The program has exited
The variable $_exception
is set to the exception object being
thrown at an exception-related catchpoint. See Set Catchpoints.
Arguments to a static probe. See Static Probe Points.
The variable $_sdata
contains extra collected static tracepoint
data. See Tracepoint Action Lists. Note that
could be empty, if not inspecting a trace buffer, or
if extra static tracepoint data has not been collected.
The variable $_siginfo
contains extra signal information
(see extra signal information). Note that $_siginfo
could be empty, if the application has not yet received any signals.
For example, it will be empty before you execute the run
The variable $_tlb
is automatically set when debugging
applications running on MS-Windows in native mode or connected to
gdbserver that supports the qGetTIBAddr
See General Query Packets.
This variable contains the address of the thread information block.
The number of the current inferior. See Debugging Multiple Inferiors and Programs.
The thread number of the current thread. See thread numbers.
The global number of the current thread. See global thread numbers.
Next: Registers, Previous: Convenience Vars, Up: Data [Contents][Index]
GDB also supplies some convenience functions. These have a syntax similar to convenience variables. A convenience function can be used in an expression just like an ordinary function; however, a convenience function is implemented internally to GDB.
These functions do not require GDB to be configured with
support, which means that they are always available.
$_isvoid (expr)
Return one if the expression expr is void
. Otherwise it
returns zero.
A void
expression is an expression where the type of the result
is void
. For example, you can examine a convenience variable
(see Convenience Variables) to check whether
it is void
(gdb) print $_exitcode $1 = void (gdb) print $_isvoid ($_exitcode) $2 = 1 (gdb) run Starting program: ./a.out [Inferior 1 (process 29572) exited normally] (gdb) print $_exitcode $3 = 0 (gdb) print $_isvoid ($_exitcode) $4 = 0
In the example above, we used $_isvoid
to check whether
is void
before and after the execution of the
program being debugged. Before the execution there is no exit code to
be examined, therefore $_exitcode
is void
. After the
execution the program being debugged returned zero, therefore
is zero, which means that it is not void
The void
expression can also be a call of a function from the
program being debugged. For example, given the following function:
void foo (void) { }
The result of calling it inside GDB is void
(gdb) print foo () $1 = void (gdb) print $_isvoid (foo ()) $2 = 1 (gdb) set $v = foo () (gdb) print $v $3 = void (gdb) print $_isvoid ($v) $4 = 1
These functions require GDB to be configured with
$_memeq(buf1, buf2, length)
Returns one if the length bytes at the addresses given by buf1 and buf2 are equal. Otherwise it returns zero.
$_regex(str, regex)
Returns one if the string str matches the regular expression
regex. Otherwise it returns zero.
The syntax of the regular expression is that specified by Python
regular expression support.
$_streq(str1, str2)
Returns one if the strings str1 and str2 are equal. Otherwise it returns zero.
Returns the length of string str.
$_caller_is(name[, number_of_frames])
Returns one if the calling function's name is equal to name. Otherwise it returns zero.
If the optional argument number_of_frames is provided, it is the number of frames up in the stack to look. The default is 1.
(gdb) backtrace #0 bottom_func () at testsuite/gdb.python/py-caller-is.c:21 #1 0x00000000004005a0 in middle_func () at testsuite/gdb.python/py-caller-is.c:27 #2 0x00000000004005ab in top_func () at testsuite/gdb.python/py-caller-is.c:33 #3 0x00000000004005b6 in main () at testsuite/gdb.python/py-caller-is.c:39 (gdb) print $_caller_is ("middle_func") $1 = 1 (gdb) print $_caller_is ("top_func", 2) $1 = 1
$_caller_matches(regexp[, number_of_frames])
Returns one if the calling function's name matches the regular expression regexp. Otherwise it returns zero.
If the optional argument number_of_frames is provided, it is the number of frames up in the stack to look. The default is 1.
$_any_caller_is(name[, number_of_frames])
Returns one if any calling function's name is equal to name. Otherwise it returns zero.
If the optional argument number_of_frames is provided, it is the number of frames up in the stack to look. The default is 1.
This function differs from $_caller_is
in that this function
checks all stack frames from the immediate caller to the frame specified
by number_of_frames, whereas $_caller_is
only checks the
frame specified by number_of_frames.
$_any_caller_matches(regexp[, number_of_frames])
Returns one if any calling function's name matches the regular expression regexp. Otherwise it returns zero.
If the optional argument number_of_frames is provided, it is the number of frames up in the stack to look. The default is 1.
This function differs from $_caller_matches
in that this function
checks all stack frames from the immediate caller to the frame specified
by number_of_frames, whereas $_caller_matches
only checks the
frame specified by number_of_frames.
Return the string representation of value.
This function is useful to obtain the textual label (enumerator) of an enumeration value. For example, assuming the variable node is of an enumerated type:
(gdb) printf "Visiting node of type %s\n", $_as_string(node) Visiting node of type NODE_INTEGER
GDB provides the ability to list and get help on convenience functions.
help function
Print a list of all convenience functions.
Next: Floating Point Hardware, Previous: Convenience Funs, Up: Data [Contents][Index]
You can refer to machine register contents, in expressions, as variables
with names starting with '$'. The names of registers are different
for each machine; use info registers
to see the names used on
your machine.
info registers
Print the names and values of all registers except floating-point and vector registers (in the selected stack frame).
info all-registers
Print the names and values of all registers, including floating-point and vector registers (in the selected stack frame).
info registers regname …
Print the relativized value of each specified register regname. As discussed in detail below, register values are normally relative to the selected stack frame. The regname may be any register name valid on the machine you are using, with or without the initial '$'.
GDB has four "standard" register names that are available (in
expressions) on most machines—whenever they do not conflict with an
architecture's canonical mnemonics for registers. The register names
and $sp
are used for the program counter register and
the stack pointer. $fp
is used for a register that contains a
pointer to the current stack frame, and $ps
is used for a
register that contains the processor status. For example,
you could print the program counter in hex with
p/x $pc
or print the instruction to be executed next with
x/i $pc
or add four to the stack pointer11 with
set $sp += 4
Whenever possible, these four standard register names are available on
your machine even though the machine has different canonical mnemonics,
so long as there is no conflict. The info registers
shows the canonical names. For example, on the SPARC, info
displays the processor status register as $psr
but you
can also refer to it as $ps
; and on x86-based machines $ps
is an alias for the EFLAGS register.
GDB always considers the contents of an ordinary register as an integer when the register is examined in this way. Some machines have special registers which can hold nothing but floating point; these registers are considered to have floating point values. There is no way to refer to the contents of an ordinary register as floating point value (although you can print it as a floating point value with 'print/f $regname').
Some registers have distinct "raw" and "virtual" data formats. This
means that the data format in which the register contents are saved by
the operating system is not the same one that your program normally
sees. For example, the registers of the 68881 floating point
coprocessor are always saved in "extended" (raw) format, but all C
programs expect to work with "double" (virtual) format. In such
cases, GDB normally works with the virtual format only (the format
that makes sense for your program), but the info registers
prints the data in both formats.
Some machines have special registers whose contents can be interpreted
in several different ways. For example, modern x86-based machines
have SSE and MMX registers that can hold several values packed
together in several different formats. GDB refers to such
registers in struct
(gdb) print $xmm1 $1 = { v4_float = {0, 3.43859137e-038, 1.54142831e-044, 1.821688e-044}, v2_double = {9.92129282474342e-303, 2.7585945287983262e-313}, v16_int8 = "\000\000\000\000\3706;\001\v\000\000\000\r\000\000", v8_int16 = {0, 0, 14072, 315, 11, 0, 13, 0}, v4_int32 = {0, 20657912, 11, 13}, v2_int64 = {88725056443645952, 55834574859}, uint128 = 0x0000000d0000000b013b36f800000000 }
To set values of such registers, you need to tell GDB which
view of the register you wish to change, as if you were assigning
value to a struct
(gdb) set $xmm1.uint128 = 0x000000000000000000000000FFFFFFFF
Normally, register values are relative to the selected stack frame (see Selecting a Frame). This means that you get the value that the register would contain if all stack frames farther in were exited and their saved registers restored. In order to see the true contents of hardware registers, you must select the innermost frame (with 'frame 0').
Usually ABIs reserve some registers as not needed to be saved by the callee (a.k.a.: "caller-saved", "call-clobbered" or "volatile" registers). It may therefore not be possible for GDB to know the value a register had before the call (in other words, in the outer frame), if the register value has since been changed by the callee. GDB tries to deduce where the inner frame saved ("callee-saved") registers, from the debug info, unwind info, or the machine code generated by your compiler. If some register is not saved, and GDB knows the register is "caller-saved" (via its own knowledge of the ABI, or because the debug/unwind info explicitly says the register's value is undefined), GDB displays '<not saved>' as the register's value. With targets that GDB has no knowledge of the register saving convention, if a register was not saved by the callee, then its value and location in the outer frame are assumed to be the same of the inner frame. This is usually harmless, because if the register is call-clobbered, the caller either does not care what is in the register after the call, or has code to restore the value that it does care about. Note, however, that if you change such a register in the outer frame, you may also be affecting the inner frame. Also, the more "outer" the frame is you're looking at, the more likely a call-clobbered register's value is to be wrong, in the sense that it doesn't actually represent the value the register had just before the call.
Next: Vector Unit, Previous: Registers, Up: Data [Contents][Index]
Depending on the configuration, GDB may be able to give you more information about the status of the floating point hardware.
info float
Display hardware-dependent information about the floating point unit. The exact contents and layout vary depending on the floating point chip. Currently, 'info float' is supported on the ARM and x86 machines.
Next: OS Information, Previous: Floating Point Hardware, Up: Data [Contents][Index]
Depending on the configuration, GDB may be able to give you more information about the status of the vector unit.
info vector
Display information about the vector unit. The exact contents and layout vary depending on the hardware.
Next: Memory Region Attributes, Previous: Vector Unit, Up: Data [Contents][Index]
GDB provides interfaces to useful OS facilities that can help you debug your program.
Some operating systems supply an auxiliary vector to programs at startup. This is akin to the arguments and environment that you specify for a program, but contains a system-dependent variety of binary values that tell system libraries important details about the hardware, operating system, and process. Each value's purpose is identified by an integer tag; the meanings are well-known but system-specific. Depending on the configuration and operating system facilities, GDB may be able to show you this information. For remote targets, this functionality may further depend on the remote stub's support of the 'qXfer:auxv:read' packet, see qXfer auxiliary vector read.
info auxv
Display the auxiliary vector of the inferior, which can be either a live process or a core dump file. GDB prints each tag value numerically, and also shows names and text descriptions for recognized tags. Some values in the vector are numbers, some bit masks, and some pointers to strings or other data. GDB displays each value in the most appropriate form for a recognized tag, and in hexadecimal for an unrecognized tag.
On some targets, GDB can access operating system-specific information and show it to you. The types of information available will differ depending on the type of operating system running on the target. The mechanism used to fetch the data is described in Operating System Information. For remote targets, this functionality depends on the remote stub's support of the 'qXfer:osdata:read' packet, see qXfer osdata read.
info os infotype
Display OS information of the requested type.
On GNU/Linux, the following values of infotype are valid:
Display the list of all CPUs/cores. For each CPU/core, GDB prints the available fields from /proc/cpuinfo. For each supported architecture different fields are available. Two common entries are processor which gives CPU number and bogomips; a system constant that is calculated during kernel initialization.
Display the list of open file descriptors on the target. For each file descriptor, GDB prints the identifier of the process owning the descriptor, the command of the owning process, the value of the descriptor, and the target of the descriptor.
Display the list of all loaded kernel modules on the target. For each module, GDB prints the module name, the size of the module in bytes, the number of times the module is used, the dependencies of the module, the status of the module, and the address of the loaded module in memory.
Display the list of all System V message queues on the target. For each message queue, GDB prints the message queue key, the message queue identifier, the access permissions, the current number of bytes on the queue, the current number of messages on the queue, the processes that last sent and received a message on the queue, the user and group of the owner and creator of the message queue, the times at which a message was last sent and received on the queue, and the time at which the message queue was last changed.
Display the list of processes on the target. For each process, GDB prints the process identifier, the name of the user, the command corresponding to the process, and the list of processor cores that the process is currently running on. (To understand what these properties mean, for this and the following info types, please consult the general GNU/Linux documentation.)
Display the list of process groups on the target. For each process, GDB prints the identifier of the process group that it belongs to, the command corresponding to the process group leader, the process identifier, and the command line of the process. The list is sorted first by the process group identifier, then by the process identifier, so that processes belonging to the same process group are grouped together and the process group leader is listed first.
Display the list of all System V semaphore sets on the target. For each semaphore set, GDB prints the semaphore set key, the semaphore set identifier, the access permissions, the number of semaphores in the set, the user and group of the owner and creator of the semaphore set, and the times at which the semaphore set was operated upon and changed.
Display the list of all System V shared-memory regions on the target. For each shared-memory region, GDB prints the region key, the shared-memory identifier, the access permissions, the size of the region, the process that created the region, the process that last attached to or detached from the region, the current number of live attaches to the region, and the times at which the region was last attached to, detach from, and changed.
Display the list of Internet-domain sockets on the target. For each socket, GDB prints the address and port of the local and remote endpoints, the current state of the connection, the creator of the socket, the IP address family of the socket, and the type of the connection.
Display the list of threads running on the target. For each thread, GDB prints the identifier of the process that the thread belongs to, the command of the process, the thread identifier, and the processor core that it is currently running on. The main thread of a process is not listed.
info os
If infotype is omitted, then list the possible values for infotype and the kind of OS information available for each infotype. If the target does not return a list of possible types, this command will report an error.
Next: Dump/Restore Files, Previous: OS Information, Up: Data [Contents][Index]
Memory region attributes allow you to describe special handling required by regions of your target's memory. GDB uses attributes to determine whether to allow certain types of memory accesses; whether to use specific width accesses; and whether to cache target memory. By default the description of memory regions is fetched from the target (if the current target supports this), but the user can override the fetched regions.
Defined memory regions can be individually enabled and disabled. When a memory region is disabled, GDB uses the default attributes when accessing memory in that region. Similarly, if no memory regions have been defined, GDB uses the default attributes when accessing all memory.
When a memory region is defined, it is given a number to identify it; to enable, disable, or remove a memory region, you specify that number.
mem lower upper attributes…
Define a memory region bounded by lower and upper with attributes attributes…, and add it to the list of regions monitored by GDB. Note that upper == 0 is a special case: it is treated as the target's maximum memory address. (0xffff on 16 bit targets, 0xffffffff on 32 bit targets, etc.)
mem auto
Discard any user changes to the memory regions and use target-supplied regions, if available, or no regions if the target does not support.
delete mem nums…
Remove memory regions nums… from the list of regions monitored by GDB.
disable mem nums…
Disable monitoring of memory regions nums…. A disabled memory region is not forgotten. It may be enabled again later.
enable mem nums…
Enable monitoring of memory regions nums….
info mem
Print a table of all defined memory regions, with the following columns for each region:
Enabled memory regions are marked with 'y'. Disabled memory regions are marked with 'n'.
The address defining the inclusive lower bound of the memory region.
The address defining the exclusive upper bound of the memory region.
The list of attributes set for this memory region.
The access mode attributes set whether GDB may make read or write accesses to a memory region.
While these attributes prevent GDB from performing invalid memory accesses, they do nothing to prevent the target system, I/O DMA, etc. from accessing memory.
Memory is read only.
Memory is write only.
Memory is read/write. This is the default.
The access size attribute tells GDB to use specific sized accesses in the memory region. Often memory mapped device registers require specific sized accesses. If no access size attribute is specified, GDB may use accesses of any size.
Use 8 bit memory accesses.
Use 16 bit memory accesses.
Use 32 bit memory accesses.
Use 64 bit memory accesses.
The data cache attributes set whether GDB will cache target memory. While this generally improves performance by reducing debug protocol overhead, it can lead to incorrect results because GDB does not know about volatile variables or memory mapped device registers.
Enable GDB to cache target memory.
Disable GDB from caching target memory. This is the default.
GDB can be instructed to refuse accesses to memory that is not explicitly described. This can be useful if accessing such regions has undesired effects for a specific target, or to provide better error checking. The following commands control this behaviour.
set mem inaccessible-by-default [on|off]
If on
is specified, make GDB treat memory not
explicitly described by the memory ranges as non-existent and refuse accesses
to such memory. The checks are only performed if there's at least one
memory range defined. If off
is specified, make GDB
treat the memory not explicitly described by the memory ranges as RAM.
The default value is on
show mem inaccessible-by-default
Show the current handling of accesses to unknown memory.
Next: Core File Generation, Previous: Memory Region Attributes, Up: Data [Contents][Index]
You can use the commands dump
, append
, and
to copy data between target memory and a file. The
and append
commands write data to a file, and the
command reads data from a file back into the inferior's
memory. Files may be in binary, Motorola S-record, Intel hex,
Tektronix Hex, or Verilog Hex format; however, GDB can only
append to binary files, and cannot read from Verilog Hex files.
dump [format] memory filename start_addr end_addr
dump [format] value filename expr
Dump the contents of memory from start_addr to end_addr, or the value of expr, to filename in the given format.
The format parameter may be any one of:
Raw binary form.
Intel hex format.
Motorola S-record format.
Tektronix Hex format.
Verilog Hex format.
GDB uses the same definitions of these formats as the GNU binary utilities, like 'objdump' and 'objcopy'. If format is omitted, GDB dumps the data in raw binary form.
append [binary] memory filename start_addr end_addr
append [binary] value filename expr
Append the contents of memory from start_addr to end_addr, or the value of expr, to the file filename, in raw binary form. (GDB can only append data to files in raw binary form.)
restore filename [binary] bias start end
Restore the contents of file filename into memory. The
command can automatically recognize any known BFD
file format, except for raw binary. To restore a raw binary file you
must specify the optional keyword binary
after the filename.
If bias is non-zero, its value will be added to the addresses contained in the file. Binary files always start at address zero, so they will be restored at address bias. Other bfd files have a built-in location; they will be restored at offset bias from that location.
If start and/or end are non-zero, then only data between file offset start and file offset end will be restored. These offsets are relative to the addresses in the file, before the bias argument is applied.
Next: Character Sets, Previous: Dump/Restore Files, Up: Data [Contents][Index]
A core file or core dump is a file that records the memory image of a running process and its process status (register values etc.). Its primary use is post-mortem debugging of a program that crashed while it ran outside a debugger. A program that crashes automatically produces a core file, unless this feature is disabled by the user. See Files, for information on invoking GDB in the post-mortem debugging mode.
Occasionally, you may wish to produce a core file of the program you are debugging in order to preserve a snapshot of its state. GDB has a special command for that.
generate-core-file [file]
gcore [file]
Produce a core dump of the inferior process. The optional argument file specifies the file name where to put the core dump. If not specified, the file name defaults to core.pid, where pid is the inferior process ID.
Note that this command is implemented only for some systems (as of this writing, GNU/Linux, FreeBSD, Solaris, and S390).
On GNU/Linux, this command can take into account the value of the file /proc/pid/coredump_filter when generating the core dump (see set use-coredump-filter).
set use-coredump-filter on
set use-coredump-filter off
Enable or disable the use of the file /proc/pid/coredump_filter when generating core dump files. This file is used by the Linux kernel to decide what types of memory mappings will be dumped or ignored when generating a core dump file. pid is the process ID of a currently running process.
To make use of this feature, you have to write in the
/proc/pid/coredump_filter file a value, in hexadecimal,
which is a bit mask representing the memory mapping types. If a bit
is set in the bit mask, then the memory mappings of the corresponding
types will be dumped; otherwise, they will be ignored. This
configuration is inherited by child processes. For more information
about the bits that can be set in the
/proc/pid/coredump_filter file, please refer to the
manpage of core(5)
By default, this option is on
. If this option is turned
, GDB does not read the coredump_filter file
and instead uses the same default value as the Linux kernel in order
to decide which pages will be dumped in the core dump file. This
value is currently 0x33
, which means that bits 0
(anonymous private mappings), 1
(anonymous shared mappings),
(ELF headers) and 5
(private huge pages) are active.
This will cause these memory mappings to be dumped automatically.
Next: Caching Target Data, Previous: Core File Generation, Up: Data [Contents][Index]
If the program you are debugging uses a different character set to represent characters and strings than the one GDB uses itself, GDB can automatically translate between the character sets for you. The character set GDB uses we call the host character set; the one the inferior program uses we call the target character set.
For example, if you are running GDB on a GNU/Linux system, which
uses the ISO Latin 1 character set, but you are using GDB's
remote protocol (see Remote Debugging) to debug a program
running on an IBM mainframe, which uses the EBCDIC character set,
then the host character set is Latin-1, and the target character set is
EBCDIC. If you give GDB the command set
target-charset EBCDIC-US
, then GDB translates between
EBCDIC and Latin 1 as you print character or string values, or use
character and string literals in expressions.
GDB has no way to automatically recognize which character set
the inferior program uses; you must tell it, using the set
command, described below.
Here are the commands for controlling GDB's character set support:
set target-charset charset
Set the current target character set to charset. To display the list of supported target character sets, type set target-charset TABTAB.
set host-charset charset
Set the current host character set to charset.
By default, GDB uses a host character set appropriate to the
system it is running on; you can override that default using the
set host-charset
command. On some systems, GDB cannot
automatically determine the appropriate host character set. In this
case, GDB uses 'UTF-8'.
GDB can only use certain character sets as its host character set. If you type set host-charset TABTAB, GDB will list the host character sets it supports.
set charset charset
Set the current host and target character sets to charset. As above, if you type set charset TABTAB, GDB will list the names of the character sets that can be used for both host and target.
show charset
Show the names of the current host and target character sets.
show host-charset
Show the name of the current host character set.
show target-charset
Show the name of the current target character set.
set target-wide-charset charset
Set the current target's wide character set to charset. This is
the character set used by the target's wchar_t
type. To
display the list of supported wide character sets, type
set target-wide-charset TABTAB.
show target-wide-charset
Show the name of the current target's wide character set.
Here is an example of GDB's character set support in action. Assume that the following source code has been placed in the file charset-test.c:
#include <stdio.h> char ascii_hello[] = {72, 101, 108, 108, 111, 44, 32, 119, 111, 114, 108, 100, 33, 10, 0}; char ibm1047_hello[] = {200, 133, 147, 147, 150, 107, 64, 166, 150, 153, 147, 132, 90, 37, 0}; main () { printf ("Hello, world!\n"); }
In this program, ascii_hello
and ibm1047_hello
are arrays
containing the string 'Hello, world!' followed by a newline,
encoded in the ASCII and IBM1047 character sets.
We compile the program, and invoke the debugger on it:
$ gcc -g charset-test.c -o charset-test $ gdb -nw charset-test GNU gdb 2001-12-19-cvs Copyright 2001 Free Software Foundation, Inc. … (gdb)
We can use the show charset
command to see what character sets
GDB is currently using to interpret and display characters and
(gdb) show charset The current host and target character set is `ISO-8859-1'. (gdb)
For the sake of printing this manual, let's use ASCII as our initial character set:
(gdb) set charset ASCII (gdb) show charset The current host and target character set is `ASCII'. (gdb)
Let's assume that ASCII is indeed the correct character set for our
host system — in other words, let's assume that if GDB prints
characters using the ASCII character set, our terminal will display
them properly. Since our current target character set is also
ASCII, the contents of ascii_hello
print legibly:
(gdb) print ascii_hello $1 = 0x401698 "Hello, world!\n" (gdb) print ascii_hello[0] $2 = 72 'H' (gdb)
GDB uses the target character set for character and string literals you use in expressions:
(gdb) print '+' $3 = 43 '+' (gdb)
The ASCII character set uses the number 43 to encode the '+' character.
GDB relies on the user to tell it which character set the
target program uses. If we print ibm1047_hello
while our target
character set is still ASCII, we get jibberish:
(gdb) print ibm1047_hello $4 = 0x4016a8 "\310\205\223\223\226k@\246\226\231\223\204Z%" (gdb) print ibm1047_hello[0] $5 = 200 '\310' (gdb)
If we invoke the set target-charset
followed by TABTAB,
GDB tells us the character sets it supports:
(gdb) set target-charset ASCII EBCDIC-US IBM1047 ISO-8859-1 (gdb) set target-charset
We can select IBM1047 as our target character set, and examine the
program's strings again. Now the ASCII string is wrong, but
GDB translates the contents of ibm1047_hello
from the
target character set, IBM1047, to the host character set,
ASCII, and they display correctly:
(gdb) set target-charset IBM1047 (gdb) show charset The current host character set is `ASCII'. The current target character set is `IBM1047'. (gdb) print ascii_hello $6 = 0x401698 "\110\145%%?\054\040\167?\162%\144\041\012" (gdb) print ascii_hello[0] $7 = 72 '\110' (gdb) print ibm1047_hello $8 = 0x4016a8 "Hello, world!\n" (gdb) print ibm1047_hello[0] $9 = 200 'H' (gdb)
As above, GDB uses the target character set for character and string literals you use in expressions:
(gdb) print '+' $10 = 78 '+' (gdb)
The IBM1047 character set uses the number 78 to encode the '+' character.
Next: Searching Memory, Previous: Character Sets, Up: Data [Contents][Index]
GDB caches data exchanged between the debugger and a target. Each cache is associated with the address space of the inferior. See Inferiors and Programs, about inferior and address space. Such caching generally improves performance in remote debugging (see Remote Debugging), because it reduces the overhead of the remote protocol by bundling memory reads and writes into large chunks. Unfortunately, simply caching everything would lead to incorrect results, since GDB does not necessarily know anything about volatile values, memory-mapped I/O addresses, etc. Furthermore, in non-stop mode (see Non-Stop Mode) memory can be changed while a gdb command is executing. Therefore, by default, GDB only caches data known to be on the stack12 or in the code segment. Other regions of memory can be explicitly marked as cacheable; see Memory Region Attributes.
set remotecache on
set remotecache off
This option no longer does anything; it exists for compatibility with old scripts.
show remotecache
Show the current state of the obsolete remotecache flag.
set stack-cache on
set stack-cache off
Enable or disable caching of stack accesses. When on
, use
caching. By default, this option is on
show stack-cache
Show the current state of data caching for memory accesses.
set code-cache on
set code-cache off
Enable or disable caching of code segment accesses. When on
use caching. By default, this option is on
. This improves
performance of disassembly in remote debugging.
show code-cache
Show the current state of target memory cache for code segment accesses.
info dcache [line]
Print the information about the performance of data cache of the current inferior's address space. The information displayed includes the dcache width and depth, and for each cache line, its number, address, and how many times it was referenced. This command is useful for debugging the data cache operation.
If a line number is specified, the contents of that line will be printed in hex.
set dcache size size
Set maximum number of entries in dcache (dcache depth above).
set dcache line-size line-size
Set number of bytes each dcache entry caches (dcache width above). Must be a power of 2.
show dcache size
Show maximum number of dcache entries. See info dcache.
show dcache line-size
Show default size of dcache lines.
Next: Value Sizes, Previous: Caching Target Data, Up: Data [Contents][Index]
Memory can be searched for a particular sequence of bytes with the
find [/sn] start_addr, +len, val1 [, val2, …]
find [/sn] start_addr, end_addr, val1 [, val2, …]
Search memory for the sequence of bytes specified by val1, val2, etc. The search begins at address start_addr and continues for either len bytes or through to end_addr inclusive.
s and n are optional parameters. They may be specified in either order, apart or together.
The size of each search query value.
halfwords (two bytes)
words (four bytes)
giant words (eight bytes)
All values are interpreted in the current language. This means, for example, that if the current source language is C/C++ then searching for the string "hello" includes the trailing '\0'.
If the value size is not specified, it is taken from the value's type in the current language. This is useful when one wants to specify the search pattern as a mixture of types. Note that this means, for example, that in the case of C-like languages a search for an untyped 0x42 will search for '(int) 0x42' which is typically four bytes.
The maximum number of matches to print. The default is to print all finds.
You can use strings as search values. Quote them with double-quotes
The string value is copied into the search pattern byte by byte,
regardless of the endianness of the target and the size specification.
The address of each match found is printed as well as a count of the number of matches found.
The address of the last value found is stored in convenience variable '$_'. A count of the number of matches is stored in '$numfound'.
For example, if stopped at the printf
in this function:
void hello () { static char hello[] = "hello-hello"; static struct { char c; short s; int i; } __attribute__ ((packed)) mixed = { 'c', 0x1234, 0x87654321 }; printf ("%s\n", hello); }
you get during debugging:
(gdb) find &hello[0], +sizeof(hello), "hello" 0x804956d <hello.1620+6> 1 pattern found (gdb) find &hello[0], +sizeof(hello), 'h', 'e', 'l', 'l', 'o' 0x8049567 <hello.1620> 0x804956d <hello.1620+6> 2 patterns found (gdb) find /b1 &hello[0], +sizeof(hello), 'h', 0x65, 'l' 0x8049567 <hello.1620> 1 pattern found (gdb) find &mixed, +sizeof(mixed), (char) 'c', (short) 0x1234, (int) 0x87654321 0x8049560 <mixed.1625> 1 pattern found (gdb) print $numfound $1 = 1 (gdb) print $_ $2 = (void *) 0x8049560
Previous: Searching Memory, Up: Data [Contents][Index]
Whenever GDB prints a value memory will be allocated within GDB to hold the contents of the value. It is possible in some languages with dynamic typing systems, that an invalid program may indicate a value that is incorrectly large, this in turn may cause GDB to try and allocate an overly large ammount of memory.
set max-value-size bytes
set max-value-size unlimited
Set the maximum size of memory that GDB will allocate for the contents of a value to bytes, trying to display a value that requires more memory than that will result in an error.
Setting this variable does not effect values that have already been allocated within GDB, only future allocations.
There's a minimum size that max-value-size
can be set to in
order that GDB can still operate correctly, this minimum is
currently 16 bytes.
The limit applies to the results of some subexpressions as well as to
complete expressions. For example, an expression denoting a simple
integer component, such as x.y.z
, may fail if the size of
x.y is dynamic and exceeds bytes. On the other hand,
GDB is sometimes clever; the expression A[i]
, where
A is an array variable with non-constant size, will generally
succeed regardless of the bounds on A, as long as the component
size is less than bytes.
The default value of max-value-size
is currently 64k.
show max-value-size
Show the maximum size of memory, in bytes, that GDB will allocate for the contents of a value.
Almost all compilers support optimization. With optimization disabled, the compiler generates assembly code that corresponds directly to your source code, in a simplistic way. As the compiler applies more powerful optimizations, the generated assembly code diverges from your original source code. With help from debugging information generated by the compiler, GDB can map from the running program back to constructs from your original source.
GDB is more accurate with optimization disabled. If you can recompile without optimization, it is easier to follow the progress of your program during debugging. But, there are many cases where you may need to debug an optimized version.
When you debug a program compiled with '-g -O', remember that the optimizer has rearranged your code; the debugger shows you what is really there. Do not be too surprised when the execution path does not exactly match your source file! An extreme example: if you define a variable, but never use it, GDB never sees that variable—because the compiler optimizes it out of existence.
Some things do not work as well with '-g -O' as with just '-g', particularly on machines with instruction scheduling. If in doubt, recompile with '-g' alone, and if this fixes the problem, please report it to us as a bug (including a test case!). See Variables, for more information about debugging optimized code.
• Inline Functions: | How GDB presents inlining | |
• Tail Call Frames: | GDB analysis of jumps to functions |
Next: Tail Call Frames, Up: Optimized Code [Contents][Index]
Inlining is an optimization that inserts a copy of the function
body directly at each call site, instead of jumping to a shared
routine. GDB displays inlined functions just like
non-inlined functions. They appear in backtraces. You can view their
arguments and local variables, step into them with step
, skip
them with next
, and escape from them with finish
You can check whether a function was inlined by using the
info frame
For GDB to support inlined functions, the compiler must record information about inlining in the debug information — GCC using the DWARF 2 format does this, and several other compilers do also. GDB only supports inlined functions when using DWARF 2. Versions of GCC before 4.1 do not emit two required attributes ('DW_AT_call_file' and 'DW_AT_call_line'); GDB does not display inlined function calls with earlier versions of GCC. It instead displays the arguments and local variables of inlined functions as local variables in the caller.
The body of an inlined function is directly included at its call site; unlike a non-inlined function, there are no instructions devoted to the call. GDB still pretends that the call site and the start of the inlined function are different instructions. Stepping to the call site shows the call site, and then stepping again shows the first line of the inlined function, even though no additional instructions are executed.
This makes source-level debugging much clearer; you can see both the
context of the call and then the effect of the call. Only stepping by
a single instruction using stepi
or nexti
does not do
this; single instruction steps always show the inlined body.
There are some ways that GDB does not pretend that inlined function calls are the same as normal calls:
command. This is a limitation of compiler-generated
debugging information; after finish
, you can step to the next line
and print a variable where your program stored the return value.
Previous: Inline Functions, Up: Optimized Code [Contents][Index]
Function B
can call function C
in its very last statement. In
unoptimized compilation the call of C
is immediately followed by return
instruction at the end of B
code. Optimizing compiler may replace the
call and return in function B
into one jump to function C
instead. Such use of a jump instruction is called tail call.
During execution of function C
, there will be no indication in the
function call stack frames that it was tail-called from B
. If function
regularly calls function B
which tail-calls function C
then GDB will see A
as the caller of C
. However, in
some cases GDB can determine that C
was tail-called from
, and it will then create fictitious call frame for that, with the
return address set up as if B
called C
This functionality is currently supported only by DWARF 2 debugging format and the compiler has to produce 'DW_TAG_GNU_call_site' tags. With GCC, you need to specify -O -g during compilation, to get this information.
info frame command (see Frame Info) will indicate the tail call frame
kind by text tail call frame
such as in this sample GDB output:
(gdb) x/i $pc - 2 0x40066b <b(int, double)+11>: jmp 0x400640 <c(int, double)> (gdb) info frame Stack level 1, frame at 0x7fffffffda30: rip = 0x40066d in b (amd64-entry-value.cc:59); saved rip 0x4004c5 tail call frame, caller of frame at 0x7fffffffda30 source language c++. Arglist at unknown address. Locals at unknown address, Previous frame's sp is 0x7fffffffda30
The detection of all the possible code path executions can find them ambiguous. There is no execution history stored (possible Reverse Execution is never used for this purpose) and the last known caller could have reached the known callee by multiple different jump sequences. In such case GDB still tries to show at least all the unambiguous top tail callers and all the unambiguous bottom tail calees, if any.
set debug entry-values
When set to on, enables printing of analysis messages for both frame argument values at function entry and tail calls. It will show all the possible valid tail calls code paths it has considered. It will also print the intersection of them with the final unambiguous (possibly partial or even empty) code path result.
show debug entry-values
Show the current state of analysis messages printing for both frame argument values at function entry and tail calls.
The analysis messages for tail calls can for example show why the virtual tail
call frame for function c
has not been recognized (due to the indirect
reference by variable x
static void __attribute__((noinline, noclone)) c (void); void (*x) (void) = c; static void __attribute__((noinline, noclone)) a (void) { x++; } static void __attribute__((noinline, noclone)) c (void) { a (); } int main (void) { x (); return 0; } Breakpoint 1, DW_OP_GNU_entry_value resolving cannot find DW_TAG_GNU_call_site 0x40039a in main a () at t.c:3 3 static void __attribute__((noinline, noclone)) a (void) { x++; } (gdb) bt #0 a () at t.c:3 #1 0x000000000040039a in main () at t.c:5
Another possibility is an ambiguous virtual tail call frames resolution:
int i; static void __attribute__((noinline, noclone)) f (void) { i++; } static void __attribute__((noinline, noclone)) e (void) { f (); } static void __attribute__((noinline, noclone)) d (void) { f (); } static void __attribute__((noinline, noclone)) c (void) { d (); } static void __attribute__((noinline, noclone)) b (void) { if (i) c (); else e (); } static void __attribute__((noinline, noclone)) a (void) { b (); } int main (void) { a (); return 0; } tailcall: initial: 0x4004d2(a) 0x4004ce(b) 0x4004b2(c) 0x4004a2(d) tailcall: compare: 0x4004d2(a) 0x4004cc(b) 0x400492(e) tailcall: reduced: 0x4004d2(a) | (gdb) bt #0 f () at t.c:2 #1 0x00000000004004d2 in a () at t.c:8 #2 0x0000000000400395 in main () at t.c:9
Frames #0 and #2 are real, #1 is a virtual tail call frame.
The code can have possible execution paths main→a→b→c→d→f
, GDB cannot find which one from the inferior state.
state shows some random possible calling sequence GDB
has found. It then finds another possible calling sequcen - that one is
prefixed by compare:
. The non-ambiguous intersection of these two is
printed as the reduced:
calling sequence. That one could have many
futher compare:
and reduced:
statements as long as there remain
any non-ambiguous sequence entries.
For the frame of function b
in both cases there are different possible
values (0x4004cc
or 0x4004ce
), therefore this frame is
also ambigous. The only non-ambiguous frame is the one for function a
therefore this one is displayed to the user while the ambiguous frames are
There can be also reasons why printing of frame argument values at function entry may fail:
int v; static void __attribute__((noinline, noclone)) c (int i) { v++; } static void __attribute__((noinline, noclone)) a (int i); static void __attribute__((noinline, noclone)) b (int i) { a (i); } static void __attribute__((noinline, noclone)) a (int i) { if (i) b (i - 1); else c (0); } int main (void) { a (5); return 0; } (gdb) bt #0 c (i=i@entry=0) at t.c:2 #1 0x0000000000400428 in a (DW_OP_GNU_entry_value resolving has found function "a" at 0x400420 can call itself via tail calls i=<optimized out>) at t.c:6 #2 0x000000000040036e in main () at t.c:7
GDB cannot find out from the inferior state if and how many times did
function a
call itself (via function b
) as these calls would be
tail calls. Such tail calls would modify thue i
variable, therefore
GDB cannot be sure the value it knows would be right - GDB
prints <optimized out>
Next: Tracepoints, Previous: Optimized Code, Up: Top [Contents][Index]
Some languages, such as C and C++, provide a way to define and invoke "preprocessor macros" which expand into strings of tokens. GDB can evaluate expressions containing macro invocations, show the result of macro expansion, and show a macro's definition, including where it was defined.
You may need to compile your program specially to provide GDB with information about preprocessor macros. Most compilers do not include macros in their debugging information, even when you compile with the -g flag. See Compilation.
A program may define a macro at one point, remove that definition later, and then provide a different definition after that. Thus, at different points in the program, a macro may have different definitions, or have no definition at all. If there is a current stack frame, GDB uses the macros in scope at that frame's source code line. Otherwise, GDB uses the macros in scope at the current listing location; see List.
Whenever GDB evaluates an expression, it always expands any macro invocations present in the expression. GDB also provides the following commands for working with macros explicitly.
macro expand expression
macro exp expression
Show the results of expanding all preprocessor macro invocations in expression. Since GDB simply expands macros, but does not parse the result, expression need not be a valid expression; it can be any string of tokens.
macro expand-once expression
macro exp1 expression
(This command is not yet implemented.) Show the results of expanding those preprocessor macro invocations that appear explicitly in expression. Macro invocations appearing in that expansion are left unchanged. This command allows you to see the effect of a particular macro more clearly, without being confused by further expansions. Since GDB simply expands macros, but does not parse the result, expression need not be a valid expression; it can be any string of tokens.
info macro [-a|-all] [--] macro
Show the current definition or all definitions of the named macro, and describe the source location or compiler command-line where that definition was established. The optional double dash is to signify the end of argument processing and the beginning of macro for non C-like macros where the macro may begin with a hyphen.
info macros location
Show all macro definitions that are in effect at the location specified by location, and describe the source location or compiler command-line where those definitions were established.
macro define macro replacement-list
macro define macro(arglist) replacement-list
Introduce a definition for a preprocessor macro named macro, invocations of which are replaced by the tokens given in replacement-list. The first form of this command defines an "object-like" macro, which takes no arguments; the second form defines a "function-like" macro, which takes the arguments given in arglist.
A definition introduced by this command is in scope in every
expression evaluated in GDB, until it is removed with the
macro undef
command, described below. The definition overrides
all definitions for macro present in the program being debugged,
as well as any previous user-supplied definition.
macro undef macro
Remove any user-supplied definition for the macro named macro.
This command only affects definitions provided with the macro
command, described above; it cannot remove definitions present
in the program being debugged.
macro list
List all the macros defined using the macro define
Here is a transcript showing the above commands in action. First, we show our source files:
$ cat sample.c #include <stdio.h> #include "sample.h" #define M 42 #define ADD(x) (M + x) main () { #define N 28 printf ("Hello, world!\n"); #undef N printf ("We're so creative.\n"); #define N 1729 printf ("Goodbye, world!\n"); } $ cat sample.h #define Q < $
Now, we compile the program using the GNU C compiler, GCC. We pass the -gdwarf-213 and -g3 flags to ensure the compiler includes information about preprocessor macros in the debugging information.
$ gcc -gdwarf-2 -g3 sample.c -o sample $
Now, we start GDB on our sample program:
$ gdb -nw sample GNU gdb 2002-05-06-cvs Copyright 2002 Free Software Foundation, Inc. GDB is free software, … (gdb)
We can expand macros and examine their definitions, even when the program is not running. GDB uses the current listing position to decide which macro definitions are in scope:
(gdb) list main 3 4 #define M 42 5 #define ADD(x) (M + x) 6 7 main () 8 { 9 #define N 28 10 printf ("Hello, world!\n"); 11 #undef N 12 printf ("We're so creative.\n"); (gdb) info macro ADD Defined at /home/jimb/gdb/macros/play/sample.c:5 #define ADD(x) (M + x) (gdb) info macro Q Defined at /home/jimb/gdb/macros/play/sample.h:1 included at /home/jimb/gdb/macros/play/sample.c:2 #define Q < (gdb) macro expand ADD(1) expands to: (42 + 1) (gdb) macro expand-once ADD(1) expands to: once (M + 1) (gdb)
In the example above, note that macro expand-once
expands only
the macro invocation explicit in the original text — the invocation of
— but does not expand the invocation of the macro M
which was introduced by ADD
Once the program is running, GDB uses the macro definitions in force at the source line of the current stack frame:
(gdb) break main Breakpoint 1 at 0x8048370: file sample.c, line 10. (gdb) run Starting program: /home/jimb/gdb/macros/play/sample Breakpoint 1, main () at sample.c:10 10 printf ("Hello, world!\n"); (gdb)
At line 10, the definition of the macro N
at line 9 is in force:
(gdb) info macro N Defined at /home/jimb/gdb/macros/play/sample.c:9 #define N 28 (gdb) macro expand N Q M expands to: 28 < 42 (gdb) print N Q M $1 = 1 (gdb)
As we step over directives that remove N
's definition, and then
give it a new definition, GDB finds the definition (or lack
thereof) in force at each point:
(gdb) next Hello, world! 12 printf ("We're so creative.\n"); (gdb) info macro N The symbol `N' has no definition as a C/C++ preprocessor macro at /home/jimb/gdb/macros/play/sample.c:12 (gdb) next We're so creative. 14 printf ("Goodbye, world!\n"); (gdb) info macro N Defined at /home/jimb/gdb/macros/play/sample.c:13 #define N 1729 (gdb) macro expand N Q M expands to: 1729 < 42 (gdb) print N Q M $2 = 0 (gdb)
In addition to source files, macros can be defined on the compilation command line using the -Dname=value syntax. For macros defined in such a way, GDB displays the location of their definition as line zero of the source file submitted to the compiler.
(gdb) info macro __STDC__ Defined at /home/jimb/gdb/macros/play/sample.c:0 -D__STDC__=1 (gdb)
In some applications, it is not feasible for the debugger to interrupt the program's execution long enough for the developer to learn anything helpful about its behavior. If the program's correctness depends on its real-time behavior, delays introduced by a debugger might cause the program to change its behavior drastically, or perhaps fail, even when the code itself is correct. It is useful to be able to observe the program's behavior without interrupting it.
Using GDB's trace
and collect
commands, you can
specify locations in the program, called tracepoints, and
arbitrary expressions to evaluate when those tracepoints are reached.
Later, using the tfind
command, you can examine the values
those expressions had when the program hit the tracepoints. The
expressions may also denote objects in memory—structures or arrays,
for example—whose values GDB should record; while visiting
a particular tracepoint, you may inspect those objects as if they were
in memory at that moment. However, because GDB records these
values without interacting with you, it can do so quickly and
unobtrusively, hopefully not disturbing the program's behavior.
The tracepoint facility is currently available only for remote targets. See Targets. In addition, your remote target must know how to collect trace data. This functionality is implemented in the remote stub; however, none of the stubs distributed with GDB support tracepoints as of this writing. The format of the remote packets used to implement tracepoints are described in Tracepoint Packets.
It is also possible to get trace data from a file, in a manner reminiscent
of corefiles; you specify the filename, and use tfind
to search
through the file. See Trace Files, for more details.
This chapter describes the tracepoint commands and features.
• Set Tracepoints: | ||
• Analyze Collected Data: | ||
• Tracepoint Variables: | ||
• Trace Files: |
Next: Analyze Collected Data, Up: Tracepoints [Contents][Index]
Before running such a trace experiment, an arbitrary number of tracepoints can be set. A tracepoint is actually a special type of breakpoint (see Set Breaks), so you can manipulate it using standard breakpoint commands. For instance, as with breakpoints, tracepoint numbers are successive integers starting from one, and many of the commands associated with tracepoints take the tracepoint number as their argument, to identify which tracepoint to work on.
For each tracepoint, you can specify, in advance, some arbitrary set of data that you want the target to collect in the trace buffer when it hits that tracepoint. The collected data can include registers, local variables, or global data. Later, you can use GDB commands to examine the values these data had at the time the tracepoint was hit.
Tracepoints do not support every breakpoint feature. Ignore counts on tracepoints have no effect, and tracepoints cannot run GDB commands when they are hit. Tracepoints may not be thread-specific either.
Some targets may support fast tracepoints, which are inserted in a different way (such as with a jump instead of a trap), that is faster but possibly restricted in where they may be installed.
Regular and fast tracepoints are dynamic tracing facilities, meaning that they can be used to insert tracepoints at (almost) any location in the target. Some targets may also support controlling static tracepoints from GDB. With static tracing, a set of instrumentation points, also known as markers, are embedded in the target program, and can be activated or deactivated by name or address. These are usually placed at locations which facilitate investigating what the target is actually doing. GDB's support for static tracing includes being able to list instrumentation points, and attach them with GDB defined high level tracepoints that expose the whole range of convenience of GDB's tracepoints support. Namely, support for collecting registers values and values of global or local (to the instrumentation point) variables; tracepoint conditions and trace state variables. The act of installing a GDB static tracepoint on an instrumentation point, or marker, is referred to as probing a static tracepoint marker.
supports tracepoints on some target systems.
See Tracepoints support in gdbserver
This section describes commands to set tracepoints and associated conditions and actions.
Next: Enable and Disable Tracepoints, Up: Set Tracepoints [Contents][Index]
trace location
The trace
command is very similar to the break
Its argument location can be any valid location.
See Specify Location. The trace
command defines a tracepoint,
which is a point in the target program where the debugger will briefly stop,
collect some data, and then allow the program to continue. Setting a tracepoint
or changing its actions takes effect immediately if the remote stub
supports the 'InstallInTrace' feature (see install tracepoint in tracing).
If remote stub doesn't support the 'InstallInTrace' feature, all
these changes don't take effect until the next tstart
command, and once a trace experiment is running, further changes will
not have any effect until the next trace experiment starts. In addition,
GDB supports pending tracepoints—tracepoints whose
address is not yet resolved. (This is similar to pending breakpoints.)
Pending tracepoints are not downloaded to the target and not installed
until they are resolved. The resolution of pending tracepoints requires
GDB support—when debugging with the remote target, and
GDB disconnects from the remote stub (see disconnected tracing), pending tracepoints can not be resolved (and downloaded to
the remote stub) while GDB is disconnected.
Here are some examples of using the trace
(gdb) trace foo.c:121 // a source file and line number (gdb) trace +2 // 2 lines forward (gdb) trace my_function // first source line of function (gdb) trace *my_function // EXACT start address of function (gdb) trace *0x2117c4 // an address
You can abbreviate trace
as tr
trace location if cond
Set a tracepoint with condition cond; evaluate the expression cond each time the tracepoint is reached, and collect data only if the value is nonzero—that is, if cond evaluates as true. See Tracepoint Conditions, for more information on tracepoint conditions.
ftrace location [ if cond ]
The ftrace
command sets a fast tracepoint. For targets that
support them, fast tracepoints will use a more efficient but possibly
less general technique to trigger data collection, such as a jump
instruction instead of a trap, or some sort of hardware support. It
may not be possible to create a fast tracepoint at the desired
location, in which case the command will exit with an explanatory
GDB handles arguments to ftrace
exactly as for
On 32-bit x86-architecture systems, fast tracepoints normally need to
be placed at an instruction that is 5 bytes or longer, but can be
placed at 4-byte instructions if the low 64K of memory of the target
program is available to install trampolines. Some Unix-type systems,
such as GNU/Linux, exclude low addresses from the program's
address space; but for instance with the Linux kernel it is possible
to let GDB use this area by doing a sysctl
to set the mmap_min_addr
kernel parameter, as in
sudo sysctl -w vm.mmap_min_addr=32768
which sets the low address to 32K, which leaves plenty of room for trampolines. The minimum address should be set to a page boundary.
strace location [ if cond ]
The strace
command sets a static tracepoint. For targets that
support it, setting a static tracepoint probes a static
instrumentation point, or marker, found at location. It may not
be possible to set a static tracepoint at the desired location, in
which case the command will exit with an explanatory message.
GDB handles arguments to strace
exactly as for
, with the addition that the user can also specify
-m marker
as location. This probes the marker
identified by the marker string identifier. This identifier
depends on the static tracepoint backend library your program is
using. You can find all the marker identifiers in the 'ID' field
of the info static-tracepoint-markers
command output.
See Listing Static Tracepoint
Markers. For example, in the following small program using the UST
tracing engine:
main () { trace_mark(ust, bar33, "str %s", "FOOBAZ"); }
the marker id is composed of joining the first two arguments to the
call with a slash, which translates to:
(gdb) info static-tracepoint-markers Cnt Enb ID Address What 1 n ust/bar33 0x0000000000400ddc in main at stexample.c:22 Data: "str %s" [etc...]
so you may probe the marker above with:
(gdb) strace -m ust/bar33
Static tracepoints accept an extra collect action — collect
. This collects arbitrary user data passed in the probe point
call to the tracing library. In the UST example above, you'll see
that the third argument to trace_mark
is a printf-like format
string. The user data is then the result of running that formating
string against the following arguments. Note that info
command output lists that format string in
the 'Data:' field.
You can inspect this data when analyzing the trace buffer, by printing the $_sdata variable like any other variable available to GDB. See Tracepoint Action Lists.
The convenience variable $tpnum
records the tracepoint number
of the most recently set tracepoint.
delete tracepoint [num]
Permanently delete one or more tracepoints. With no argument, the
default is to delete all tracepoints. Note that the regular
command can remove tracepoints also.
(gdb) delete trace 1 2 3 // remove three tracepoints (gdb) delete trace // remove all tracepoints
You can abbreviate this command as del tr
Next: Tracepoint Passcounts, Previous: Create and Delete Tracepoints, Up: Set Tracepoints [Contents][Index]
These commands are deprecated; they are equivalent to plain disable
and enable
disable tracepoint [num]
Disable tracepoint num, or all tracepoints if no argument
num is given. A disabled tracepoint will have no effect during
a trace experiment, but it is not forgotten. You can re-enable
a disabled tracepoint using the enable tracepoint
If the command is issued during a trace experiment and the debug target
has support for disabling tracepoints during a trace experiment, then the
change will be effective immediately. Otherwise, it will be applied to the
next trace experiment.
enable tracepoint [num]
Enable tracepoint num, or all tracepoints. If this command is issued during a trace experiment and the debug target supports enabling tracepoints during a trace experiment, then the enabled tracepoints will become effective immediately. Otherwise, they will become effective the next time a trace experiment is run.
Next: Tracepoint Conditions, Previous: Enable and Disable Tracepoints, Up: Set Tracepoints [Contents][Index]
passcount [n [num]]
Set the passcount of a tracepoint. The passcount is a way to
automatically stop a trace experiment. If a tracepoint's passcount is
n, then the trace experiment will be automatically stopped on
the n'th time that tracepoint is hit. If the tracepoint number
num is not specified, the passcount
command sets the
passcount of the most recently defined tracepoint. If no passcount is
given, the trace experiment will run until stopped explicitly by the
(gdb) passcount 5 2 // Stop on the 5th execution of
// tracepoint 2
(gdb) passcount 12 // Stop on the 12th execution of the
// most recently defined tracepoint.
(gdb) trace foo (gdb) pass 3 (gdb) trace bar (gdb) pass 2 (gdb) trace baz (gdb) pass 1 // Stop tracing when foo has been
// executed 3 times OR when bar has
// been executed 2 times
// OR when baz has been executed 1 time.
Next: Trace State Variables, Previous: Tracepoint Passcounts, Up: Set Tracepoints [Contents][Index]
The simplest sort of tracepoint collects data every time your program reaches a specified place. You can also specify a condition for a tracepoint. A condition is just a Boolean expression in your programming language (see Expressions). A tracepoint with a condition evaluates the expression each time your program reaches it, and data collection happens only if the condition is true.
Tracepoint conditions can be specified when a tracepoint is set, by
using 'if' in the arguments to the trace
See Setting Tracepoints. They can
also be set or changed at any time with the condition
just as with breakpoints.
Unlike breakpoint conditions, GDB does not actually evaluate the conditional expression itself. Instead, GDB encodes the expression into an agent expression (see Agent Expressions) suitable for execution on the target, independently of GDB. Global variables become raw memory locations, locals become stack accesses, and so forth.
For instance, suppose you have a function that is usually called frequently, but should not be called after an error has occurred. You could use the following tracepoint command to collect data about calls of that function that happen while the error code is propagating through the program; an unconditional tracepoint could end up collecting thousands of useless trace frames that you would have to search through.
(gdb) trace normal_operation if errcode > 0
Next: Tracepoint Actions, Previous: Tracepoint Conditions, Up: Set Tracepoints [Contents][Index]
A trace state variable is a special type of variable that is
created and managed by target-side code. The syntax is the same as
that for GDB's convenience variables (a string prefixed with "$"),
but they are stored on the target. They must be created explicitly,
using a tvariable
command. They are always 64-bit signed
Trace state variables are remembered by GDB, and downloaded to the target along with tracepoint information when the trace experiment starts. There are no intrinsic limits on the number of trace state variables, beyond memory limitations of the target.
Although trace state variables are managed by the target, you can use
them in print commands and expressions as if they were convenience
variables; GDB will get the current value from the target
while the trace experiment is running. Trace state variables share
the same namespace as other "$" variables, which means that you
cannot have trace state variables with names like $23
, nor can you have a trace state variable and a convenience
variable with the same name.
tvariable $name [ = expression ]
The tvariable
command creates a new trace state variable named
, and optionally gives it an initial value of
expression. The expression is evaluated when this command is
entered; the result will be converted to an integer if possible,
otherwise GDB will report an error. A subsequent
command specifying the same name does not create a
variable, but instead assigns the supplied initial value to the
existing variable of that name, overwriting any previous initial
value. The default initial value is 0.
info tvariables
List all the trace state variables along with their initial values. Their current values may also be displayed, if the trace experiment is currently running.
delete tvariable [ $name … ]
Delete the given trace state variables, or all of them if no arguments are specified.
Next: Listing Tracepoints, Previous: Trace State Variables, Up: Set Tracepoints [Contents][Index]
actions [num]
This command will prompt for a list of actions to be taken when the
tracepoint is hit. If the tracepoint number num is not
specified, this command sets the actions for the one that was most
recently defined (so that you can define a tracepoint and then say
without bothering about its number). You specify the
actions themselves on the following lines, one action at a time, and
terminate the actions list with a line containing just end
. So
far, the only defined actions are collect
, teval
, and
is actually equivalent to commands
(see Breakpoint Command Lists), except that only the defined
actions are allowed; any other GDB command is rejected.
To remove all actions from a tracepoint, type 'actions num' and follow it immediately with 'end'.
(gdb) collect data // collect some data (gdb) while-stepping 5 // single-step 5 times, collect data (gdb) end // signals the end of actions.
In the following example, the action list begins with collect
commands indicating the things to be collected when the tracepoint is
hit. Then, in order to single-step and collect additional data
following the tracepoint, a while-stepping
command is used,
followed by the list of things to be collected after each step in a
sequence of single steps. The while-stepping
command is
terminated by its own separate end
command. Lastly, the action
list is terminated by an end
(gdb) trace foo (gdb) actions Enter actions for tracepoint 1, one per line: > collect bar,baz > collect $regs > while-stepping 12 > collect $pc, arr[i] > end end
collect[/mods] expr1, expr2, …
Collect values of the given expressions when the tracepoint is hit. This command accepts a comma-separated list of any valid expressions. In addition to global, static, or local variables, the following special arguments are supported:
Collect all registers.
Collect all function arguments.
Collect all local variables.
Collect the return address. This is helpful if you want to see more of a backtrace.
Note: The return address location can not always be reliably determined up front, and the wrong address / registers may end up collected instead. On some architectures the reliability is higher for tracepoints at function entry, while on others it's the opposite. When this happens, backtracing will stop because the return address is found unavailable (unless another collect rule happened to match it).
Collects the number of arguments from the static probe at which the tracepoint is located. See Static Probe Points.
n is an integer between 0 and 11. Collects the nth argument from the static probe at which the tracepoint is located. See Static Probe Points.
Collect static tracepoint marker specific data. Only available for
static tracepoints. See Tracepoint Action
Lists. On the UST static tracepoints library backend, an
instrumentation point resembles a printf
function call. The
tracing library is able to collect user specified data formatted to a
character string using the format provided by the programmer that
instrumented the program. Other backends have similar mechanisms.
Here's an example of a UST marker call:
const char master_name[] = "$your_name"; trace_mark(channel1, marker1, "hello %s", master_name)
In this case, collecting $_sdata
collects the string
'hello $yourname'. When analyzing the trace buffer, you can
inspect '$_sdata' like any other variable available to
You can give several consecutive collect
commands, each one
with a single argument, or one collect
command with several
arguments separated by commas; the effect is the same.
The optional mods changes the usual handling of the arguments.
requests that pointers to chars be handled as strings, in
particular collecting the contents of the memory being pointed at, up
to the first zero. The upper bound is by default the value of the
print elements
variable; if s
is followed by a decimal
number, that is the upper bound instead. So for instance
'collect/s25 mystr' collects as many as 25 characters at
The command info scope
(see info scope) is
particularly useful for figuring out what data to collect.
teval expr1, expr2, …
Evaluate the given expressions when the tracepoint is hit. This
command accepts a comma-separated list of expressions. The results
are discarded, so this is mainly useful for assigning values to trace
state variables (see Trace State Variables) without adding those
values to the trace buffer, as would be the case if the collect
action were used.
while-stepping n
Perform n single-step instruction traces after the tracepoint,
collecting new data after each step. The while-stepping
command is followed by the list of what to collect while stepping
(followed by its own end
> while-stepping 12 > collect $regs, myglobal > end >
Note that $pc
is not automatically collected by
; you need to explicitly collect that register if
you need it. You may abbreviate while-stepping
as ws
set default-collect expr1, expr2, …
This variable is a list of expressions to collect at each tracepoint
hit. It is effectively an additional collect
action prepended
to every tracepoint action list. The expressions are parsed
individually for each tracepoint, so for instance a variable named
may be interpreted as a global for one tracepoint, and a
local for another, as appropriate to the tracepoint's location.
show default-collect
Show the list of expressions that are collected by default at each tracepoint hit.
Next: Listing Static Tracepoint Markers, Previous: Tracepoint Actions, Up: Set Tracepoints [Contents][Index]
info tracepoints [num…]
Display information about the tracepoint num. If you don't
specify a tracepoint number, displays information about all the
tracepoints defined so far. The format is similar to that used for
info breakpoints
; in fact, info tracepoints
is the same
command, simply restricting itself to tracepoints.
A tracepoint's listing may include additional information specific to tracing:
passcount n
(gdb) info trace Num Type Disp Enb Address What 1 tracepoint keep y 0x0804ab57 in foo() at main.cxx:7 while-stepping 20 collect globfoo, $regs end collect globfoo2 end pass count 1200 2 tracepoint keep y <MULTIPLE> collect $eip 2.1 y 0x0804859c in func4 at change-loc.h:35 installed on target 2.2 y 0xb7ffc480 in func4 at change-loc.h:35 installed on target 2.3 y <PENDING> set_tracepoint 3 tracepoint keep y 0x080485b1 in foo at change-loc.c:29 not installed on target (gdb)
This command can be abbreviated info tp
Next: Starting and Stopping Trace Experiments, Previous: Listing Tracepoints, Up: Set Tracepoints [Contents][Index]
info static-tracepoint-markers
Display information about all static tracepoint markers defined in the program.
For each marker, the following columns are printed:
An incrementing counter, output to help readability. This is not a stable identifier.
The marker ID, as reported by the target.
Probed markers are tagged with 'y'. 'n' identifies marks that are not enabled.
Where the marker is in your program, as a memory address.
Where the marker is in the source for your program, as a file and line number. If the debug information included in the program does not allow GDB to locate the source of the marker, this column will be left blank.
In addition, the following information may be printed for each marker:
User data passed to the tracing library by the marker call. In the UST backend, this is the format string passed as argument to the marker call.
The list of static tracepoints attached to the marker.
(gdb) info static-tracepoint-markers Cnt ID Enb Address What 1 ust/bar2 y 0x0000000000400e1a in main at stexample.c:25 Data: number1 %d number2 %d Probed by static tracepoints: #2 2 ust/bar33 n 0x0000000000400c87 in main at stexample.c:24 Data: str %s (gdb)
Next: Tracepoint Restrictions, Previous: Listing Static Tracepoint Markers, Up: Set Tracepoints [Contents][Index]
This command starts the trace experiment, and begins collecting data. It has the side effect of discarding all the data collected in the trace buffer during the previous trace experiment. If any arguments are supplied, they are taken as a note and stored with the trace experiment's state. The notes may be arbitrary text, and are especially useful with disconnected tracing in a multi-user context; the notes can explain what the trace is doing, supply user contact information, and so forth.
This command stops the trace experiment. If any arguments are supplied, they are recorded with the experiment as a note. This is useful if you are stopping a trace started by someone else, for instance if the trace is interfering with the system's behavior and needs to be stopped quickly.
Note: a trace experiment and data collection may stop automatically if any tracepoint's passcount is reached (see Tracepoint Passcounts), or if the trace buffer becomes full.
This command displays the status of the current trace data collection.
Here is an example of the commands we described so far:
(gdb) trace gdb_c_test (gdb) actions Enter actions for tracepoint #1, one per line. > collect $regs,$locals,$args > while-stepping 11 > collect $regs > end > end (gdb) tstart [time passes …] (gdb) tstop
You can choose to continue running the trace experiment even if
GDB disconnects from the target, voluntarily or
involuntarily. For commands such as detach
, the debugger will
ask what you want to do with the trace. But for unexpected
terminations (GDB crash, network outage), it would be
unfortunate to lose hard-won trace data, so the variable
lets you decide whether the trace should
continue running without GDB.
set disconnected-tracing on
set disconnected-tracing off
Choose whether a tracing run should continue to run if GDB
has disconnected from the target. Note that detach
will ask you directly what to do about a running trace no
matter what this variable's setting, so the variable is mainly useful
for handling unexpected situations, such as loss of the network.
show disconnected-tracing
Show the current choice for disconnected tracing.
When you reconnect to the target, the trace experiment may or may not still be running; it might have filled the trace buffer in the meantime, or stopped for one of the other reasons. If it is running, it will continue after reconnection.
Upon reconnection, the target will upload information about the tracepoints in effect. GDB will then compare that information to the set of tracepoints currently defined, and attempt to match them up, allowing for the possibility that the numbers may have changed due to creation and deletion in the meantime. If one of the target's tracepoints does not match any in GDB, the debugger will create a new tracepoint, so that you have a number with which to specify that tracepoint. This matching-up process is necessarily heuristic, and it may result in useless tracepoints being created; you may simply delete them if they are of no use.
If your target agent supports a circular trace buffer, then you can run a trace experiment indefinitely without filling the trace buffer; when space runs out, the agent deletes already-collected trace frames, oldest first, until there is enough room to continue collecting. This is especially useful if your tracepoints are being hit too often, and your trace gets terminated prematurely because the buffer is full. To ask for a circular trace buffer, simply set 'circular-trace-buffer' to on. You can set this at any time, including during tracing; if the agent can do it, it will change buffer handling on the fly, otherwise it will not take effect until the next run.
set circular-trace-buffer on
set circular-trace-buffer off
Choose whether a tracing run should use a linear or circular buffer for trace data. A linear buffer will not lose any trace data, but may fill up prematurely, while a circular buffer will discard old trace data, but it will have always room for the latest tracepoint hits.
show circular-trace-buffer
Show the current choice for the trace buffer. Note that this may not match the agent's current buffer handling, nor is it guaranteed to match the setting that might have been in effect during a past run, for instance if you are looking at frames from a trace file.
set trace-buffer-size n
set trace-buffer-size unlimited
Request that the target use a trace buffer of n bytes. Not all
targets will honor the request; they may have a compiled-in size for
the trace buffer, or some other limitation. Set to a value of
or -1
to let the target use whatever size it
likes. This is also the default.
show trace-buffer-size
Show the current requested size for the trace buffer. Note that this
will only match the actual size if the target supports size-setting,
and was able to handle the requested size. For instance, if the
target can only change buffer size between runs, this variable will
not reflect the change until the next run starts. Use tstatus
to get a report of the actual buffer size.
set trace-user text
show trace-user
set trace-notes text
Set the trace run's notes.
show trace-notes
Show the trace run's notes.
set trace-stop-notes text
Set the trace run's stop notes. The handling of the note is as for
arguments; the set command is convenient way to fix a
stop note that is mistaken or incomplete.
show trace-stop-notes
Show the trace run's stop notes.
Previous: Starting and Stopping Trace Experiments, Up: Set Tracepoints [Contents][Index]
There are a number of restrictions on the use of tracepoints. As described above, tracepoint data gathering occurs on the target without interaction from GDB. Thus the full capabilities of the debugger are not available during data gathering, and then at data examination time, you will be limited by only having what was collected. The following items describe some common problems, but it is not exhaustive, and you may run into additional difficulties not mentioned here.
or $args
, during while-stepping
behave erratically. The stepping action may enter a new scope (for
instance by stepping into a function), or the location of the variable
may change (for instance it is loaded into a register). The
tracepoint data recorded uses the location information for the
variables that is correct for the tracepoint location. When the
tracepoint is created, it is not possible, in general, to determine
where the steps of a while-stepping
sequence will advance the
program—particularly if a conditional branch is stepped.
can be used to collect the 50 element array pointed to
by ptr
*(unsigned char *)$esp@300
(adjust to use the name of the actual stack pointer register on your
target architecture, and the amount of stack you wish to capture).
Then the backtrace
command will show a partial backtrace when
using a trace frame. The number of stack frames that can be examined
depends on the sizes of the frames in the collected stack. Note that
if you ask for a block so large that it goes past the bottom of the
stack, the target agent may report an error trying to read from an
invalid address.
must be the same as the address of
the tracepoint and use that when you are looking at a trace frame
for that tracepoint. However, this cannot work if the tracepoint has
multiple locations (for instance if it was set in a function that was
inlined), or if it has a while-stepping
loop. In those cases
GDB will warn you that it can't infer $pc
, and default
it to zero.
Next: Tracepoint Variables, Previous: Set Tracepoints, Up: Tracepoints [Contents][Index]
After the tracepoint experiment ends, you use GDB commands
for examining the trace data. The basic idea is that each tracepoint
collects a trace snapshot every time it is hit and another
snapshot every time it single-steps. All these snapshots are
consecutively numbered from zero and go into a buffer, and you can
examine them later. The way you examine them is to focus on a
specific trace snapshot. When the remote stub is focused on a trace
snapshot, it will respond to all GDB requests for memory and
registers by reading from the buffer which belongs to that snapshot,
rather than from real memory or registers of the program being
debugged. This means that all GDB commands
, info registers
, backtrace
, etc.) will
behave as if we were currently debugging the program state as it was
when the tracepoint occurred. Any requests for data that are not in
the buffer will fail.
• tfind: | How to select a trace snapshot | |
• tdump: | How to display all data for a snapshot | |
• save tracepoints: | How to save tracepoints for a future run |
Next: tdump, Up: Analyze Collected Data [Contents][Index]
tfind n
The basic command for selecting a trace snapshot from the buffer is
tfind n
, which finds trace snapshot number n,
counting from zero. If no argument n is given, the next
snapshot is selected.
Here are the various forms of using the tfind
tfind start
Find the first snapshot in the buffer. This is a synonym for
tfind 0
(since 0 is the number of the first snapshot).
tfind none
Stop debugging trace snapshots, resume live debugging.
tfind end
Same as 'tfind none'.
No argument means find the next trace snapshot or find the first one if no trace snapshot is selected.
tfind -
Find the previous trace snapshot before the current one. This permits retracing earlier steps.
tfind tracepoint num
Find the next snapshot associated with tracepoint num. Search proceeds forward from the last examined trace snapshot. If no argument num is given, it means find the next snapshot collected for the same tracepoint as the current snapshot.
tfind pc addr
Find the next snapshot associated with the value addr of the program counter. Search proceeds forward from the last examined trace snapshot. If no argument addr is given, it means find the next snapshot with the same value of PC as the current snapshot.
tfind outside addr1, addr2
Find the next snapshot whose PC is outside the given range of addresses (exclusive).
tfind range addr1, addr2
Find the next snapshot whose PC is between addr1 and addr2 (inclusive).
tfind line [file:]n
Find the next snapshot associated with the source line n. If
the optional argument file is given, refer to line n in
that source file. Search proceeds forward from the last examined
trace snapshot. If no argument n is given, it means find the
next line other than the one currently being examined; thus saying
tfind line
repeatedly can appear to have the same effect as
stepping from line to line in a live debugging session.
The default arguments for the tfind
commands are specifically
designed to make it easy to scan through the trace buffer. For
instance, tfind
with no argument selects the next trace
snapshot, and tfind -
with no argument selects the previous
trace snapshot. So, by giving one tfind
command, and then
simply hitting RET repeatedly you can examine all the trace
snapshots in order. Or, by saying tfind -
and then hitting
RET repeatedly you can examine the snapshots in reverse order.
The tfind line
command with no argument selects the snapshot
for the next source line executed. The tfind pc
command with
no argument selects the next snapshot with the same program counter
(PC) as the current frame. The tfind tracepoint
command with
no argument selects the next trace snapshot collected by the same
tracepoint as the current one.
In addition to letting you scan through the trace buffer manually, these commands make it easy to construct GDB scripts that scan through the trace buffer and print out whatever collected data you are interested in. Thus, if we want to examine the PC, FP, and SP registers from each trace frame in the buffer, we can say this:
(gdb) tfind start (gdb) while ($trace_frame != -1) > printf "Frame %d, PC = %08X, SP = %08X, FP = %08X\n", \ $trace_frame, $pc, $sp, $fp > tfind > end Frame 0, PC = 0020DC64, SP = 0030BF3C, FP = 0030BF44 Frame 1, PC = 0020DC6C, SP = 0030BF38, FP = 0030BF44 Frame 2, PC = 0020DC70, SP = 0030BF34, FP = 0030BF44 Frame 3, PC = 0020DC74, SP = 0030BF30, FP = 0030BF44 Frame 4, PC = 0020DC78, SP = 0030BF2C, FP = 0030BF44 Frame 5, PC = 0020DC7C, SP = 0030BF28, FP = 0030BF44 Frame 6, PC = 0020DC80, SP = 0030BF24, FP = 0030BF44 Frame 7, PC = 0020DC84, SP = 0030BF20, FP = 0030BF44 Frame 8, PC = 0020DC88, SP = 0030BF1C, FP = 0030BF44 Frame 9, PC = 0020DC8E, SP = 0030BF18, FP = 0030BF44 Frame 10, PC = 00203F6C, SP = 0030BE3C, FP = 0030BF14
Or, if we want to examine the variable X
at each source line in
the buffer:
(gdb) tfind start (gdb) while ($trace_frame != -1) > printf "Frame %d, X == %d\n", $trace_frame, X > tfind line > end Frame 0, X = 1 Frame 7, X = 2 Frame 13, X = 255
Next: save tracepoints, Previous: tfind, Up: Analyze Collected Data [Contents][Index]
This command takes no arguments. It prints all the data collected at the current trace snapshot.
(gdb) trace 444 (gdb) actions Enter actions for tracepoint #2, one per line: > collect $regs, $locals, $args, gdb_long_test > end (gdb) tstart (gdb) tfind line 444 #0 gdb_test (p1=0x11, p2=0x22, p3=0x33, p4=0x44, p5=0x55, p6=0x66) at gdb_test.c:444 444 printp( "%s: arguments = 0x%X 0x%X 0x%X 0x%X 0x%X 0x%X\n", ) (gdb) tdump Data collected at tracepoint 2, trace frame 1: d0 0xc4aa0085 -995491707 d1 0x18 24 d2 0x80 128 d3 0x33 51 d4 0x71aea3d 119204413 d5 0x22 34 d6 0xe0 224 d7 0x380035 3670069 a0 0x19e24a 1696330 a1 0x3000668 50333288 a2 0x100 256 a3 0x322000 3284992 a4 0x3000698 50333336 a5 0x1ad3cc 1758156 fp 0x30bf3c 0x30bf3c sp 0x30bf34 0x30bf34 ps 0x0 0 pc 0x20b2c8 0x20b2c8 fpcontrol 0x0 0 fpstatus 0x0 0 fpiaddr 0x0 0 p = 0x20e5b4 "gdb-test" p1 = (void *) 0x11 p2 = (void *) 0x22 p3 = (void *) 0x33 p4 = (void *) 0x44 p5 = (void *) 0x55 p6 = (void *) 0x66 gdb_long_test = 17 '\021' (gdb)
works by scanning the tracepoint's current collection
actions and printing the value of each expression listed. So
can fail, if after a run, you change the tracepoint's
actions to mention variables that were not collected during the run.
Also, for tracepoints with while-stepping
loops, tdump
uses the collected value of $pc
to distinguish between trace
frames that were collected at the tracepoint hit, and frames that were
collected while stepping. This allows it to correctly choose whether
to display the basic list of collections, or the collections from the
body of the while-stepping loop. However, if $pc
was not collected,
then tdump
will always attempt to dump using the basic collection
list, and may fail if a while-stepping frame does not include all the
same data that is collected at the tracepoint hit.
Previous: tdump, Up: Analyze Collected Data [Contents][Index]
save tracepoints filename
This command saves all current tracepoint definitions together with
their actions and passcounts, into a file filename
suitable for use in a later debugging session. To read the saved
tracepoint definitions, use the source
command (see Command Files). The save-tracepoints
command is a deprecated
alias for save tracepoints
Next: Trace Files, Previous: Analyze Collected Data, Up: Tracepoints [Contents][Index]
(int) $trace_frame
The current trace snapshot (a.k.a. frame) number, or -1 if no snapshot is selected.
(int) $tracepoint
The tracepoint for the current trace snapshot.
(int) $trace_line
The line number for the current trace snapshot.
(char []) $trace_file
The source file for the current trace snapshot.
(char []) $trace_func
The name of the function containing $tracepoint
Note: $trace_file
is not suitable for use in printf
use output
Here's a simple example of using these convenience variables for stepping through all the trace snapshots and printing some of their data. Note that these are not the same as trace state variables, which are managed by the target.
(gdb) tfind start (gdb) while $trace_frame != -1 > output $trace_file > printf ", line %d (tracepoint #%d)\n", $trace_line, $tracepoint > tfind > end
Previous: Tracepoint Variables, Up: Tracepoints [Contents][Index]
In some situations, the target running a trace experiment may no
longer be available; perhaps it crashed, or the hardware was needed
for a different activity. To handle these cases, you can arrange to
dump the trace data into a file, and later use that file as a source
of trace data, via the target tfile
tsave [ -r ] filename
tsave [-ctf] dirname
Save the trace data to filename. By default, this command
assumes that filename refers to the host filesystem, so if
necessary GDB will copy raw trace data up from the target and
then save it. If the target supports it, you can also supply the
optional argument -r
("remote") to direct the target to save
the data directly into filename in its own filesystem, which may be
more efficient if the trace buffer is very large. (Note, however, that
target tfile
can only read from files accessible to the host.)
By default, this command will save trace frame in tfile format.
You can supply the optional argument -ctf
to save date in CTF
format. The Common Trace Format (CTF) is proposed as a trace format
that can be shared by multiple debugging and tracing tools. Please go to
' to get more information.
target tfile filename
target ctf dirname
Use the file named filename or directory named dirname as
a source of trace data. Commands that examine data work as they do with
a live target, but it is not possible to run any new trace experiments.
will report the state of the trace run at the moment
the data was saved, as well as the current trace frame you are examining.
Both filename and dirname must be on a filesystem accessible to
the host.
(gdb) target ctf ctf.ctf (gdb) tfind Found trace frame 0, tracepoint 2 39 ++a; /* set tracepoint 1 here */ (gdb) tdump Data collected at tracepoint 2, trace frame 0: i = 0 a = 0 b = 1 '\001' c = {"123", "456", "789", "123", "456", "789"} d = {{{a = 1, b = 2}, {a = 3, b = 4}}, {{a = 5, b = 6}, {a = 7, b = 8}}} (gdb) p b $1 = 1
Next: Languages, Previous: Tracepoints, Up: Top [Contents][Index]
If your program is too large to fit completely in your target system's memory, you can sometimes use overlays to work around this problem. GDB provides some support for debugging programs that use overlays.
• How Overlays Work: | A general explanation of overlays. | |
• Overlay Commands: | Managing overlays in GDB. | |
• Automatic Overlay Debugging: | GDB can find out which overlays are mapped by asking the inferior. | |
• Overlay Sample Program: | A sample program using overlays. |
Next: Overlay Commands, Up: Overlays [Contents][Index]
Suppose you have a computer whose instruction address space is only 64 kilobytes long, but which has much more memory which can be accessed by other means: special instructions, segment registers, or memory management hardware, for example. Suppose further that you want to adapt a program which is larger than 64 kilobytes to run on this system.
One solution is to identify modules of your program which are relatively independent, and need not call each other directly; call these modules overlays. Separate the overlays from the main program, and place their machine code in the larger memory. Place your main program in instruction memory, but leave at least enough space there to hold the largest overlay as well.
Now, to call a function located in an overlay, you must first copy that overlay's machine code from the large memory into the space set aside for it in the instruction memory, and then jump to its entry point there.
Data Instruction Larger Address Space Address Space Address Space +-----------+ +-----------+ +-----------+ | | | | | | +-----------+ +-----------+ +-----------+<-- overlay 1 | program | | main | .----| overlay 1 | load address | variables | | program | | +-----------+ | and heap | | | | | | +-----------+ | | | +-----------+<-- overlay 2 | | +-----------+ | | | load address +-----------+ | | | .-| overlay 2 | | | | | | | mapped --->+-----------+ | | +-----------+ address | | | | | | | overlay | <-' | | | | area | <---' +-----------+<-- overlay 3 | | <---. | | load address +-----------+ `--| overlay 3 | | | | | +-----------+ | | +-----------+ | | +-----------+ A code overlay
The diagram (see A code overlay) shows a system with separate data and instruction address spaces. To map an overlay, the program copies its code from the larger address space to the instruction address space. Since the overlays shown here all use the same mapped address, only one may be mapped at a time. For a system with a single address space for data and instructions, the diagram would be similar, except that the program variables and heap would share an address space with the main program and the overlay area.
An overlay loaded into instruction memory and ready for use is called a mapped overlay; its mapped address is its address in the instruction memory. An overlay not present (or only partially present) in instruction memory is called unmapped; its load address is its address in the larger memory. The mapped address is also called the virtual memory address, or VMA; the load address is also called the load memory address, or LMA.
Unfortunately, overlays are not a completely transparent way to adapt a program to limited instruction memory. They introduce a new set of global constraints you must keep in mind as you design your program:
The overlay system described above is rather simple, and could be improved in many ways:
Next: Automatic Overlay Debugging, Previous: How Overlays Work, Up: Overlays [Contents][Index]
To use GDB's overlay support, each overlay in your program must correspond to a separate section of the executable file. The section's virtual memory address and load memory address must be the overlay's mapped and load addresses. Identifying overlays with sections allows GDB to determine the appropriate address of a function or variable, depending on whether the overlay is mapped or not.
GDB's overlay commands all start with the word overlay
you can abbreviate this as ov
or ovly
. The commands are:
overlay off
Disable GDB's overlay support. When overlay support is disabled, GDB assumes that all functions and variables are always present at their mapped addresses. By default, GDB's overlay support is disabled.
overlay manual
Enable manual overlay debugging. In this mode, GDB
relies on you to tell it which overlays are mapped, and which are not,
using the overlay map-overlay
and overlay unmap-overlay
commands described below.
overlay map-overlay overlay
overlay map overlay
Tell GDB that overlay is now mapped; overlay must be the name of the object file section containing the overlay. When an overlay is mapped, GDB assumes it can find the overlay's functions and variables at their mapped addresses. GDB assumes that any other overlays whose mapped ranges overlap that of overlay are now unmapped.
overlay unmap-overlay overlay
overlay unmap overlay
Tell GDB that overlay is no longer mapped; overlay must be the name of the object file section containing the overlay. When an overlay is unmapped, GDB assumes it can find the overlay's functions and variables at their load addresses.
overlay auto
Enable automatic overlay debugging. In this mode, GDB consults a data structure the overlay manager maintains in the inferior to see which overlays are mapped. For details, see Automatic Overlay Debugging.
overlay load-target
overlay load
Re-read the overlay table from the inferior. Normally, GDB re-reads the table GDB automatically each time the inferior stops, so this command should only be necessary if you have changed the overlay mapping yourself using GDB. This command is only useful when using automatic overlay debugging.
overlay list-overlays
overlay list
Display a list of the overlays currently mapped, along with their mapped addresses, load addresses, and sizes.
Normally, when GDB prints a code address, it includes the name of the function the address falls in:
(gdb) print main $3 = {int ()} 0x11a0 <main>
When overlay debugging is enabled, GDB recognizes code in
unmapped overlays, and prints the names of unmapped functions with
asterisks around them. For example, if foo
is a function in an
unmapped overlay, GDB prints it this way:
(gdb) overlay list No sections are mapped. (gdb) print foo $5 = {int (int)} 0x100000 <*foo*>
When foo
's overlay is mapped, GDB prints the function's
name normally:
(gdb) overlay list Section .ov.foo.text, loaded at 0x100000 - 0x100034, mapped at 0x1016 - 0x104a (gdb) print foo $6 = {int (int)} 0x1016 <foo>
When overlay debugging is enabled, GDB can find the correct
address for functions and variables in an overlay, whether or not the
overlay is mapped. This allows most GDB commands, like
and disassemble
, to work normally, even on unmapped
code. However, GDB's breakpoint support has some limitations:
Next: Overlay Sample Program, Previous: Overlay Commands, Up: Overlays [Contents][Index]
GDB can automatically track which overlays are mapped and which
are not, given some simple co-operation from the overlay manager in the
inferior. If you enable automatic overlay debugging with the
overlay auto
command (see Overlay Commands), GDB
looks in the inferior's memory for certain variables describing the
current state of the overlays.
Here are the variables your overlay manager must define to support GDB's automatic overlay debugging:
:This variable must be an array of the following structures:
struct { /* The overlay's mapped address. */ unsigned long vma; /* The size of the overlay, in bytes. */ unsigned long size; /* The overlay's load address. */ unsigned long lma; /* Non-zero if the overlay is currently mapped; zero otherwise. */ unsigned long mapped; }
:This variable must be a four-byte signed integer, holding the total
number of elements in _ovly_table
To decide whether a particular overlay is mapped or not, GDB
looks for an entry in _ovly_table
whose vma
members equal the VMA and LMA of the overlay's section in the
executable file. When GDB finds a matching entry, it consults
the entry's mapped
member to determine whether the overlay is
currently mapped.
In addition, your overlay manager may define a function called
. If this function is defined, GDB
will silently set a breakpoint there. If the overlay manager then
calls this function whenever it has changed the overlay table, this
will enable GDB to accurately keep track of which overlays
are in program memory, and update any breakpoints that may be set
in overlays. This will allow breakpoints to work even if the
overlays are kept in ROM or other non-writable memory while they
are not being executed.
Previous: Automatic Overlay Debugging, Up: Overlays [Contents][Index]
When linking a program which uses overlays, you must place the overlays at their load addresses, while relocating them to run at their mapped addresses. To do this, you must write a linker script (see Overlay Description in Using ld: the GNU linker). Unfortunately, since linker scripts are specific to a particular host system, target architecture, and target memory layout, this manual cannot provide portable sample code demonstrating GDB's overlay support.
However, the GDB source distribution does contain an overlaid program, with linker scripts for a few systems, as part of its test suite. The program consists of the following files from gdb/testsuite/gdb.base:
The main program file.
A simple overlay manager, used by overlays.c.
Overlay modules, loaded and used by overlays.c.
Linker scripts for linking the test program on the d10v-elf
and m32r-elf
You can build the test program using the d10v-elf
cross-compiler like this:
$ d10v-elf-gcc -g -c overlays.c $ d10v-elf-gcc -g -c ovlymgr.c $ d10v-elf-gcc -g -c foo.c $ d10v-elf-gcc -g -c bar.c $ d10v-elf-gcc -g -c baz.c $ d10v-elf-gcc -g -c grbx.c $ d10v-elf-gcc -g overlays.o ovlymgr.o foo.o bar.o \ baz.o grbx.o -Wl,-Td10v.ld -o overlays
The build process is identical for any other architecture, except that
you must substitute the appropriate compiler and linker script for the
target system for d10v-elf-gcc
and d10v.ld
Although programming languages generally have common aspects, they are
rarely expressed in the same manner. For instance, in ANSI C,
dereferencing a pointer p
is accomplished by *p
, but in
Modula-2, it is accomplished by p^
. Values can also be
represented (and displayed) differently. Hex numbers in C appear as
'0x1ae', while in Modula-2 they appear as '1AEH'.
Language-specific information is built into GDB for some languages, allowing you to express operations like the above in your program's native language, and allowing GDB to output values in a manner consistent with the syntax of your program's native language. The language you use to build expressions is called the working language.
• Setting: | Switching between source languages | |
• Show: | Displaying the language | |
• Checks: | Type and range checks | |
• Supported Languages: | Supported languages | |
• Unsupported Languages: | Unsupported languages |
There are two ways to control the working language—either have GDB
set it automatically, or select it manually yourself. You can use the
set language
command for either purpose. On startup, GDB
defaults to setting the language automatically. The working language is
used to determine how expressions you type are interpreted, how values
are printed, etc.
In addition to the working language, every source file that
GDB knows about has its own working language. For some object
file formats, the compiler might indicate which language a particular
source file is in. However, most of the time GDB infers the
language from the name of the file. The language of a source file
controls whether C++ names are demangled—this way backtrace
show each frame appropriately for its own language. There is no way to
set the language of a source file from within GDB, but you can
set the language associated with a filename extension. See Displaying the Language.
This is most commonly a problem when you use a program, such
as cfront
or f2c
, that generates C but is written in
another language. In that case, make the
program use #line
directives in its C output; that way
GDB will know the correct language of the source code of the original
program, and will display that source code, not the generated C code.
• Filenames: | Filename extensions and languages. | |
• Manually: | Setting the working language manually | |
• Automatically: | Having GDB infer the source language |
If a source file name ends in one of the following extensions, then GDB infers that its language is the one indicated.
Ada source file.
C source file
C++ source file
D source file
Objective-C source file
Fortran source file
Modula-2 source file
Assembler source file. This actually behaves almost like C, but GDB does not skip over function prologues when stepping.
In addition, you may set the language associated with a filename extension. See Displaying the Language.
Next: Automatically, Previous: Filenames, Up: Setting [Contents][Index]
If you allow GDB to set the language automatically, expressions are interpreted the same way in your debugging session and your program.
If you wish, you may set the language manually. To do this, issue the
command 'set language lang', where lang is the name of
a language, such as
or modula-2
For a list of the supported languages, type 'set language'.
Setting the language manually prevents GDB from updating the working language automatically. This can lead to confusion if you try to debug a program when the working language is not the same as the source language, when an expression is acceptable to both languages—but means different things. For instance, if the current source file were written in C, and GDB was parsing Modula-2, a command such as:
print a = b + c
might not have the effect you intended. In C, this means to add
and c
and place the result in a
. The result
printed would be the value of a
. In Modula-2, this means to compare
to the result of b+c
, yielding a BOOLEAN
To have GDB set the working language automatically, use 'set language local' or 'set language auto'. GDB then infers the working language. That is, when your program stops in a frame (usually by encountering a breakpoint), GDB sets the working language to the language recorded for the function in that frame. If the language for a frame is unknown (that is, if the function or block corresponding to the frame was defined in a source file that does not have a recognized extension), the current working language is not changed, and GDB issues a warning.
This may not seem necessary for most programs, which are written entirely in one source language. However, program modules and libraries written in one source language can be used by a main program written in a different source language. Using 'set language auto' in this case frees you from having to set the working language manually.
The following commands help you find out which language is the working language, and also what language source files were written in.
show language
Display the current working language. This is the
language you can use with commands such as print
build and compute expressions that may involve variables in your program.
info frame
Display the source language for this frame. This language becomes the working language if you use an identifier from this frame. See Information about a Frame, to identify the other information listed here.
info source
Display the source language of this source file. See Examining the Symbol Table, to identify the other information listed here.
In unusual circumstances, you may have source files with extensions not in the standard list. You can then set the extension associated with a language explicitly:
set extension-language ext language
Tell GDB that source files with extension ext are to be assumed as written in the source language language.
info extensions
List all the filename extensions and the associated languages.
Next: Supported Languages, Previous: Show, Up: Languages [Contents][Index]
Some languages are designed to guard you against making seemingly common errors through a series of compile- and run-time checks. These include checking the type of arguments to functions and operators and making sure mathematical overflows are caught at run time. Checks such as these help to ensure a program's correctness once it has been compiled by eliminating type mismatches and providing active checks for range errors when your program is running.
By default GDB checks for these errors according to the
rules of the current source language. Although GDB does not check
the statements in your program, it can check expressions entered directly
into GDB for evaluation via the print
command, for example.
• Type Checking: | An overview of type checking | |
• Range Checking: | An overview of range checking |
Next: Range Checking, Up: Checks [Contents][Index]
Some languages, such as C and C++, are strongly typed, meaning that the arguments to operators and functions have to be of the correct type, otherwise an error occurs. These checks prevent type mismatch errors from ever causing any run-time problems. For example,
int klass::my_method(char *b) { return b ? 1 : 2; } (gdb) print obj.my_method (0) $1 = 2
(gdb) print obj.my_method (0x1234) Cannot resolve method klass::my_method to any overloaded instance
The second example fails because in C++ the integer constant '0x1234' is not type-compatible with the pointer parameter type.
For the expressions you use in GDB commands, you can tell GDB to not enforce strict type checking or to treat any mismatches as errors and abandon the expression; When type checking is disabled, GDB successfully evaluates expressions like the second example above.
Even if type checking is off, there may be other reasons
related to type that prevent GDB from evaluating an expression.
For instance, GDB does not know how to add an int
a struct foo
. These particular type errors have nothing to do
with the language in use and usually arise from expressions which make
little sense to evaluate anyway.
GDB provides some additional commands for controlling type checking:
set check type on
set check type off
Set strict type checking on or off. If any type mismatches occur in evaluating an expression while type checking is on, GDB prints a message and aborts evaluation of the expression.
show check type
Show the current setting of type checking and whether GDB is enforcing strict type checking rules.
Previous: Type Checking, Up: Checks [Contents][Index]
In some languages (such as Modula-2), it is an error to exceed the bounds of a type; this is enforced with run-time checks. Such range checking is meant to ensure program correctness by making sure computations do not overflow, or indices on an array element access do not exceed the bounds of the array.
For expressions you use in GDB commands, you can tell GDB to treat range errors in one of three ways: ignore them, always treat them as errors and abandon the expression, or issue warnings but evaluate the expression anyway.
A range error can result from numerical overflow, from exceeding an array index bound, or when you type a constant that is not a member of any type. Some languages, however, do not treat overflows as an error. In many implementations of C, mathematical overflow causes the result to "wrap around" to lower values—for example, if m is the largest integer value, and s is the smallest, then
m + 1 ⇒ s
This, too, is specific to individual languages, and in some cases specific to individual compilers or machines. See Supported Languages, for further details on specific languages.
GDB provides some additional commands for controlling the range checker:
set check range auto
Set range checking on or off based on the current working language. See Supported Languages, for the default settings for each language.
set check range on
set check range off
Set range checking on or off, overriding the default setting for the current working language. A warning is issued if the setting does not match the language default. If a range error occurs and range checking is on, then a message is printed and evaluation of the expression is aborted.
set check range warn
Output messages when the GDB range checker detects a range error, but attempt to evaluate the expression anyway. Evaluating the expression may still be impossible for other reasons, such as accessing memory that the process does not own (a typical example from many Unix systems).
show range
Show the current setting of the range checker, and whether or not it is being set automatically by GDB.
Next: Unsupported Languages, Previous: Checks, Up: Languages [Contents][Index]
GDB supports C, C++, D, Go, Objective-C, Fortran, Java,
OpenCL C, Pascal, Rust, assembly, Modula-2, and Ada.
Some GDB features may be used in expressions regardless of the
language you use: the GDB @
and ::
and the '{type}addr' construct (see Expressions) can be used with the constructs of any supported
The following sections detail to what degree each source language is supported by GDB. These sections are not meant to be language tutorials or references, but serve only as a reference guide to what the GDB expression parser accepts, and what input and output formats should look like for different languages. There are many good books written on each of these languages; please look to these for a language reference or tutorial.
• C: | C and C++ | |
• Go: | Go | |
• Objective-C: | Objective-C | |
• OpenCL C: | OpenCL C | |
• Fortran: | Fortran | |
• Pascal: | Pascal | |
• Rust: | Rust | |
• Modula-2: | Modula-2 | |
• Ada: | Ada |
Next: D, Up: Supported Languages [Contents][Index]
Since C and C++ are so closely related, many features of GDB apply to both languages. Whenever this is the case, we discuss those languages together.
The C++ debugging facilities are jointly implemented by the C++
compiler and GDB. Therefore, to debug your C++ code
effectively, you must compile your C++ programs with a supported
C++ compiler, such as GNU g++
, or the HP ANSI C++
compiler (aCC
• C Operators: | C and C++ operators | |
• C Constants: | C and C++ constants | |
• C Plus Plus Expressions: | C++ expressions | |
• C Defaults: | Default settings for C and C++ | |
• C Checks: | C and C++ type and range checks | |
• Debugging C: | GDB and C | |
• Debugging C Plus Plus: | GDB features for C++ | |
• Decimal Floating Point: | Numbers in Decimal Floating Point format |
Next: C Constants, Up: C [Contents][Index]
Operators must be defined on values of specific types. For instance,
is defined on numbers, but not on structures. Operators are
often defined on groups of types.
For the purposes of C and C++, the following definitions hold:
with any of its storage-class
specifiers; char
; enum
; and, for C++, bool
, double
, and
long double
(if supported by the target platform).
(type *)
The following operators are supported. They are listed here in order of increasing precedence:
The comma or sequencing operator. Expressions in a comma-separated list are evaluated from left to right, with the result of the entire expression being the last expression evaluated.
Assignment. The value of an assignment expression is the value assigned. Defined on scalar types.
Used in an expression of the form a op= b
and translated to a = a op b
and =
have the same precedence. The operator
op is any one of the operators |
, ^
, &
, >>
, +
, -
, *
, /
, %
The ternary operator. a ? b : c
can be thought
of as: if a then b else c. The argument a
should be of an integral type.
Logical OR. Defined on integral types.
Logical AND. Defined on integral types.
Bitwise OR. Defined on integral types.
Bitwise exclusive-OR. Defined on integral types.
Bitwise AND. Defined on integral types.
==, !=
Equality and inequality. Defined on scalar types. The value of these expressions is 0 for false and non-zero for true.
<, >, <=, >=
Less than, greater than, less than or equal, greater than or equal. Defined on scalar types. The value of these expressions is 0 for false and non-zero for true.
<<, >>
left shift, and right shift. Defined on integral types.
The GDB "artificial array" operator (see Expressions).
+, -
Addition and subtraction. Defined on integral types, floating-point types and pointer types.
*, /, %
Multiplication, division, and modulus. Multiplication and division are defined on integral and floating-point types. Modulus is defined on integral types.
++, --
Increment and decrement. When appearing before a variable, the operation is performed before the variable is used in an expression; when appearing after it, the variable's value is used before the operation takes place.
Pointer dereferencing. Defined on pointer types. Same precedence as
Address operator. Defined on variables. Same precedence as ++
For debugging C++, GDB implements a use of '&' beyond what is allowed in the C++ language itself: you can use '&(&ref)' to examine the address where a C++ reference variable (declared with '&ref') is stored.
Negative. Defined on integral and floating-point types. Same
precedence as ++
Logical negation. Defined on integral types. Same precedence as
Bitwise complement operator. Defined on integral types. Same precedence as
., ->
Structure member, and pointer-to-structure member. For convenience,
GDB regards the two as equivalent, choosing whether to dereference a
pointer based on the stored type information.
Defined on struct
and union
.*, ->*
Dereferences of pointers to members.
Array indexing. a[i]
is defined as
. Same precedence as ->
Function parameter list. Same precedence as ->
C++ scope resolution operator. Defined on struct
, union
and class
Doubled colons also represent the GDB scope operator
(see Expressions). Same precedence as ::
If an operator is redefined in the user code, GDB usually attempts to invoke the redefined version instead of using the operator's predefined meaning.
Next: C Plus Plus Expressions, Previous: C Operators, Up: C [Contents][Index]
GDB allows you to express the constants of C and C++ in the following ways:
(as opposed to the default double
) type; or with
a letter 'l' or 'L', which specifies a long double
), or a number—the ordinal value of the corresponding character
(usually its ASCII value). Within quotes, the single character may
be represented by a letter or by escape sequences, which are of
the form '\nnn', where nnn is the octal representation
of the character's ordinal value; or of the form '\x', where
'x' is a predefined special character—for example,
'\n' for newline.
Wide character constants can be written by prefixing a character constant with 'L', as in C. For example, 'L'x'' is the wide form of 'x'. The target wide character set is used when computing the value of this constant (see Character Sets).
). Any valid character constant (as described
above) may appear. Double quotes within the string must be preceded by
a backslash, so for instance '"a\"b'c"' is a string of five
Wide string constants can be written by prefixing a string constant with 'L', as in C. The target wide character set is used when computing the value of this constant (see Character Sets).
Next: C Defaults, Previous: C Constants, Up: C [Contents][Index]
GDB expression handling can interpret most C++ expressions.
Warning: GDB can only debug C++ code if you use the proper compiler and the proper debug format. Currently, GDB works best when debugging C++ code that is compiled with the most recent version of GCC possible. The DWARF debugging format is preferred; GCC defaults to this on most popular platforms. Other compilers and/or debug formats are likely to work badly or not at all when using GDB to debug C++ code. See Compilation.
count = aml->GetOriginal(x, y)
following the same rules as C++. using
declarations in the current scope are also respected by GDB.
It does perform integral conversions and promotions, floating-point promotions, arithmetic conversions, pointer conversions, conversions of class objects to base classes, and standard conversions such as those of functions or arrays to pointers; it requires an exact match on the number of function arguments.
Overload resolution is always performed, unless you have specified
set overload-resolution off
. See GDB Features for C++.
You must specify set overload-resolution off
in order to use an
explicit function signature to call an overloaded function, as in
p 'foo(char,int)'('x', 13)
The GDB command-completion facility can simplify this; see Command Completion.
In the parameter list shown when GDB displays a frame, the values of reference variables are not displayed (unlike other variables); this avoids clutter, since references are often used for large structures. The address of a reference variable is always shown, unless you have specified 'set print address off'.
expressions can use it just as expressions in your program do. Since
one scope may be defined in another, you can use ::
repeatedly if
necessary, for example in an expression like
'scope1::scope2::name'. GDB also allows
resolving name scope by reference to source files, in both C and C++
debugging (see Program Variables).
Next: C Checks, Previous: C Plus Plus Expressions, Up: C [Contents][Index]
If you allow GDB to set range checking automatically, it
defaults to off
whenever the working language changes to
C or C++. This happens regardless of whether you or GDB
selects the working language.
If you allow GDB to set the language automatically, it recognizes source files whose names end with .c, .C, or .cc, etc, and when GDB enters code compiled from one of these files, it sets the working language to C or C++. See Having GDB Infer the Source Language, for further details.
Next: Debugging C, Previous: C Defaults, Up: C [Contents][Index]
By default, when GDB parses C or C++ expressions, strict type checking is used. However, if you turn type checking off, GDB will allow certain non-standard conversions, such as promoting integer constants to pointers.
Range checking, if turned on, is done on mathematical operations. Array indices are not checked, since they are often used to index a pointer that is not itself an array.
Next: Debugging C Plus Plus, Previous: C Checks, Up: C [Contents][Index]
The set print union
and show print union
commands apply to
the union
type. When set to 'on', any union
that is
inside a struct
or class
is also printed. Otherwise, it
appears as '{...}'.
The @
operator aids in the debugging of dynamic arrays, formed
with pointers and a memory allocation function. See Expressions.
Next: Decimal Floating Point, Previous: Debugging C, Up: C [Contents][Index]
Some GDB commands are particularly useful with C++, and some are designed specifically for use with C++. Here is a summary:
breakpoint menus
When you want a breakpoint in a function whose name is overloaded, GDB has the capability to display a menu of possible breakpoint locations to help you specify which function definition you want. See Ambiguous Expressions.
rbreak regex
Setting breakpoints using regular expressions is helpful for setting breakpoints on overloaded functions that are not members of any special classes. See Setting Breakpoints.
catch throw
catch rethrow
catch catch
Debug C++ exception handling using these commands. See Setting Catchpoints.
ptype typename
Print inheritance relationships as well as other information for type typename. See Examining the Symbol Table.
info vtbl expression.
The info vtbl
command can be used to display the virtual
method tables of the object computed by expression. This shows
one entry per virtual table; there may be multiple virtual tables when
multiple inheritance is in use.
demangle name
Demangle name.
See Symbols, for a more complete description of the demangle
set print demangle
show print demangle
set print asm-demangle
show print asm-demangle
Control whether C++ symbols display in their source form, both when displaying code as C++ source and when displaying disassemblies. See Print Settings.
set print object
show print object
Choose whether to print derived (actual) or declared types of objects. See Print Settings.
set print vtbl
show print vtbl
Control the format for printing virtual function tables.
See Print Settings.
(The vtbl
commands do not work on programs compiled with the HP
ANSI C++ compiler (aCC
set overload-resolution on
Enable overload resolution for C++ expression evaluation. The default is on. For overloaded functions, GDB evaluates the arguments and searches for a function whose signature matches the argument types, using the standard C++ conversion rules (see C++ Expressions, for details). If it cannot find a match, it emits a message.
set overload-resolution off
Disable overload resolution for C++ expression evaluation. For overloaded functions that are not class member functions, GDB chooses the first function of the specified name that it finds in the symbol table, whether or not its arguments are of the correct type. For overloaded functions that are class member functions, GDB searches for a function whose signature exactly matches the argument types.
show overload-resolution
Show the current setting of overload resolution.
Overloaded symbol names
You can specify a particular definition of an overloaded symbol, using
the same notation that is used to declare such symbols in C++: type
rather than just symbol. You can
also use the GDB command-line word completion facilities to list the
available choices, or to finish the type list for you.
See Command Completion, for details on how to do this.
Previous: Debugging C Plus Plus, Up: C [Contents][Index]
GDB can examine, set and perform computations with numbers in
decimal floating point format, which in the C language correspond to the
, _Decimal64
and _Decimal128
types as
specified by the extension to support decimal floating-point arithmetic.
There are two encodings in use, depending on the architecture: BID (Binary Integer Decimal) for x86 and x86-64, and DPD (Densely Packed Decimal) for PowerPC and S/390. GDB will use the appropriate encoding for the configured target.
Because of a limitation in libdecnumber, the library used by GDB to manipulate decimal floating point numbers, it is not possible to convert (using a cast, for example) integers wider than 32-bit to decimal float.
In addition, in order to imitate GDB's behaviour with binary floating point computations, error checking in decimal float operations ignores underflow, overflow and divide by zero exceptions.
In the PowerPC architecture, GDB provides a set of pseudo-registers
to inspect _Decimal128
values stored in floating point registers.
See PowerPC for more details.
Next: Go, Previous: C, Up: Supported Languages [Contents][Index]
GDB can be used to debug programs written in D and compiled with GDC, LDC or DMD compilers. Currently GDB supports only one D specific feature — dynamic arrays.
Next: Objective-C, Previous: D, Up: Supported Languages [Contents][Index]
GDB can be used to debug programs written in Go and compiled with gccgo or 6g compilers.
Here is a summary of the Go-specific features and restrictions:
The current Go package
The name of the current package does not need to be specified when specifying global variables and functions.
For example, given the program:
package main var myglob = "Shall we?" func main () { // ... }
When stopped inside main
either of these work:
(gdb) p myglob (gdb) p main.myglob
Builtin Go types
The string
type is recognized by GDB and is printed
as a string.
Builtin Go functions
The GDB expression parser recognizes the unsafe.Sizeof
function and handles it internally.
Restrictions on Go expressions
All Go operators are supported except &^
The Go _
"blank identifier" is not supported.
Automatic dereferencing of pointers is not supported.
Next: OpenCL C, Previous: Go, Up: Supported Languages [Contents][Index]
This section provides information about some commands and command options that are useful for debugging Objective-C code. See also info classes, and info selectors, for a few more commands specific to Objective-C support.
• Method Names in Commands: | ||
• The Print Command with Objective-C: |
Next: The Print Command with Objective-C, Up: Objective-C [Contents][Index]
The following commands have been extended to accept Objective-C method names as line specifications:
info line
A fully qualified Objective-C method name is specified as
-[Class methodName]
where the minus sign is used to indicate an instance method and a
plus sign (not shown) is used to indicate a class method. The class
name Class and method name methodName are enclosed in
brackets, similar to the way messages are specified in Objective-C
source code. For example, to set a breakpoint at the create
instance method of class Fruit
in the program currently being
debugged, enter:
break -[Fruit create]
To list ten program lines around the initialize
class method,
list +[NSText initialize]
In the current version of GDB, the plus or minus sign is required. In future versions of GDB, the plus or minus sign will be optional, but you can use it to narrow the search. It is also possible to specify just a method name:
break create
You must specify the complete method name, including any colons. If
your program's source files contain more than one create
you'll be presented with a numbered list of classes that implement that
method. Indicate your choice by number, or type '0' to exit if
none apply.
As another example, to clear a breakpoint established at the
method of the NSWindow
class, enter:
clear -[NSWindow makeKeyAndOrderFront:]
Previous: Method Names in Commands, Up: Objective-C [Contents][Index]
The print command has also been extended to accept methods. For example:
print -[object hash]
will tell GDB to send the hash
message to object
and print the result. Also, an additional command has been added,
or po
for short, which is meant to print
the description of an object. However, this command may only work
with certain Objective-C libraries that have a particular hook
function, _NSPrintForDebugger
, defined.
Next: Fortran, Previous: Objective-C, Up: Supported Languages [Contents][Index]
This section provides information about GDBs OpenCL C support.
• OpenCL C Datatypes: | ||
• OpenCL C Expressions: | ||
• OpenCL C Operators: |
Next: OpenCL C Expressions, Up: OpenCL C [Contents][Index]
GDB supports the builtin scalar and vector datatypes specified
by OpenCL 1.1. In addition the half- and double-precision floating point
data types of the cl_khr_fp16
and cl_khr_fp64
extensions are also known to GDB.
Next: OpenCL C Operators, Previous: OpenCL C Datatypes, Up: OpenCL C [Contents][Index]
GDB supports accesses to vector components including the access as lvalue where possible. Since OpenCL C is based on C99 most C expressions supported by GDB can be used as well.
Previous: OpenCL C Expressions, Up: OpenCL C [Contents][Index]
GDB supports the operators specified by OpenCL 1.1 for scalar and vector data types.
Next: Pascal, Previous: OpenCL C, Up: Supported Languages [Contents][Index]
GDB can be used to debug programs written in Fortran, but it currently supports only the features of Fortran 77 language.
Some Fortran compilers (GNU Fortran 77 and Fortran 95 compilers among them) append an underscore to the names of variables and functions. When you debug programs compiled by those compilers, you will need to refer to variables and functions with a trailing underscore.
• Fortran Operators: | Fortran operators and expressions | |
• Fortran Defaults: | Default settings for Fortran | |
• Special Fortran Commands: | Special GDB commands for Fortran |
Next: Fortran Defaults, Up: Fortran [Contents][Index]
Operators must be defined on values of specific types. For instance,
is defined on numbers, but not on characters or other non-
arithmetic types. Operators are often defined on groups of types.
The exponentiation operator. It raises the first operand to the power of the second one.
The range operator. Normally used in the form of array(low:high) to represent a section of array.
The access component operator. Normally used to access elements in derived types. Also suitable for unions. As unions aren't part of regular Fortran, this can only happen when accessing a register that uses a gdbarch-defined union type.
Next: Special Fortran Commands, Previous: Fortran Operators, Up: Fortran [Contents][Index]
Fortran symbols are usually case-insensitive, so GDB by default uses case-insensitive matches for Fortran symbols. You can change that with the 'set case-insensitive' command, see Symbols, for the details.
Previous: Fortran Defaults, Up: Fortran [Contents][Index]
GDB has some commands to support Fortran-specific features, such as displaying common blocks.
info common [common-name]
This command prints the values contained in the Fortran COMMON
block whose name is common-name. With no argument, the names of
all COMMON
blocks visible at the current program location are
Next: Rust, Previous: Fortran, Up: Supported Languages [Contents][Index]
Debugging Pascal programs which use sets, subranges, file variables, or nested functions does not currently work. GDB does not support entering expressions, printing values, or similar features using Pascal syntax.
The Pascal-specific command set print pascal_static-members
controls whether static members of Pascal objects are displayed.
See pascal_static-members.
Next: Modula-2, Previous: Pascal, Up: Supported Languages [Contents][Index]
GDB supports the Rust Programming Language. Type- and value-printing, and expression parsing, are reasonably complete. However, there are a few peculiarities and holes to be aware of.
extern crate
That is, if GDB is stopped at a breakpoint in a function in
crate 'A', module 'B', then break B::f
will attempt
to set a breakpoint in a function named 'f' in a crate named
As a consequence of this approach, linespecs also cannot refer to items using 'self::' or 'super::'.
print ::x::y
will try to find the symbol
However, since it is useful to be able to refer to other crates when
debugging, GDB provides the extern
extension to
circumvent this. To use the extension, just put extern
a path expression to refer to the otherwise unavailable "global"
In the above example, if you wanted to refer to the symbol 'y' in
the crate 'x', you would use print extern x::y
or match
, or lambda expressions.
trait. Objects that may be created by the evaluator will
never be destroyed.
, where the parser would require
is not available.
statements are not available, so some names may not be
available in the crate.
Next: Ada, Previous: Rust, Up: Supported Languages [Contents][Index]
The extensions made to GDB to support Modula-2 only support output from the GNU Modula-2 compiler (which is currently being developed). Other Modula-2 compilers are not currently supported, and attempting to debug executables produced by them is most likely to give an error as GDB reads in the executable's symbol table.
• M2 Operators: | Built-in operators | |
• Built-In Func/Proc: | Built-in functions and procedures | |
• M2 Constants: | Modula-2 constants | |
• M2 Types: | Modula-2 types | |
• M2 Defaults: | Default settings for Modula-2 | |
• Deviations: | Deviations from standard Modula-2 | |
• M2 Checks: | Modula-2 type and range checks | |
• M2 Scope: | The scope operators :: and .
| |
• GDB/M2: | GDB and Modula-2 |
Next: Built-In Func/Proc, Up: Modula-2 [Contents][Index]
Operators must be defined on values of specific types. For instance,
is defined on numbers, but not on structures. Operators are
often defined on groups of types. For the purposes of Modula-2, the
following definitions hold:
, and
their subranges.
and its subranges.
and BITSET
The following operators are supported, and appear in order of increasing precedence:
Function argument or array index separator.
Assignment. The value of var :=
value is
<, >
Less than, greater than on integral, floating-point, or enumerated types.
<=, >=
Less than or equal to, greater than or equal to
on integral, floating-point and enumerated types, or set inclusion on
set types. Same precedence as <
=, <>, #
Equality and two ways of expressing inequality, valid on scalar types.
Same precedence as <
. In GDB scripts, only <>
available for inequality, since #
conflicts with the script
comment character.
Set membership. Defined on set types and the types of their members.
Same precedence as <
Boolean disjunction. Defined on boolean types.
Boolean conjunction. Defined on boolean types.
The GDB "artificial array" operator (see Expressions).
+, -
Addition and subtraction on integral and floating-point types, or union and difference on set types.
Multiplication on integral and floating-point types, or set intersection on set types.
Division on floating-point types, or symmetric set difference on set
types. Same precedence as *
Integer division and remainder. Defined on integral types. Same
precedence as *
Negative. Defined on INTEGER
and REAL
Pointer dereferencing. Defined on pointer types.
Boolean negation. Defined on boolean types. Same precedence as
field selector. Defined on RECORD
data. Same
precedence as ^
Array indexing. Defined on ARRAY
data. Same precedence as ^
Procedure argument list. Defined on PROCEDURE
objects. Same precedence
as ^
::, .
GDB and Modula-2 scope operators.
Warning: Set expressions and their operations are not yet supported, so GDB treats the use of the operator
, or the use of operators+
, ,<>
, and>=
on sets as an error.
Next: M2 Constants, Previous: M2 Operators, Up: Modula-2 [Contents][Index]
Modula-2 also makes available several built-in procedures and functions. In describing these, the following metavariables are used:
represents an ARRAY
represents a CHAR
constant or variable.
represents a variable or constant of integral type.
represents an identifier that belongs to a set. Generally used in the
same function with the metavariable s. The type of s should
be SET OF mtype
(where mtype is the type of m).
represents a variable or constant of integral or floating-point type.
represents a variable or constant of floating-point type.
represents a type.
represents a variable.
represents a variable or constant of one of many types. See the explanation of the function for details.
All Modula-2 built-in procedures also return a result, described below.
Returns the absolute value of n.
If c is a lower case letter, it returns its upper case equivalent, otherwise it returns its argument.
Returns the character whose ordinal value is i.
Decrements the value in the variable v by one. Returns the new value.
Decrements the value in the variable v by i. Returns the new value.
Removes the element m from the set s. Returns the new set.
Returns the floating point equivalent of the integer i.
Returns the index of the last member of a.
Increments the value in the variable v by one. Returns the new value.
Increments the value in the variable v by i. Returns the new value.
Adds the element m to the set s if it is not already there. Returns the new set.
Returns the maximum value of the type t.
Returns the minimum value of the type t.
Returns boolean TRUE if i is an odd number.
Returns the ordinal value of its argument. For example, the ordinal value of a character is its ASCII value (on machines supporting the ASCII character set). The argument x must be of an ordered type, which include integral, character and enumerated types.
Returns the size of its argument. The argument x can be a variable or a type.
Returns the integral part of r.
Returns the size of its argument. The argument x can be a variable or a type.
Returns the member of the type t whose ordinal value is i.
Warning: Sets and their operations are not yet supported, so GDB treats the use of procedures
as an error.
Next: M2 Types, Previous: Built-In Func/Proc, Up: Modula-2 [Contents][Index]
GDB allows you to express the constants of Modula-2 in the following ways:
) or double ("
). They may
also be expressed by their ordinal value (their ASCII value, usually)
followed by a 'C'.
) or double ("
Escape sequences in the style of C are also allowed. See C and C++ Constants, for a brief explanation of escape
Next: M2 Defaults, Previous: M2 Constants, Up: Modula-2 [Contents][Index]
Currently GDB can print the following data types in Modula-2 syntax: array types, record types, set types, pointer types, procedure types, enumerated types, subrange types and base types. You can also print the contents of variables declared using these type. This section gives a number of simple source code examples together with sample GDB sessions.
The first example contains the following section of code:
VAR s: SET OF CHAR ; r: [20..40] ;
and you can request GDB to interrogate the type and value of
and s
(gdb) print s {'A'..'C', 'Z'} (gdb) ptype s SET OF CHAR (gdb) print r 21 (gdb) ptype r [20..40]
Likewise if your source code declares s
VAR s: SET ['A'..'Z'] ;
then you may query the type of s
(gdb) ptype s type = SET ['A'..'Z']
Note that at present you cannot interactively manipulate set expressions using the debugger.
The following example shows how you might declare an array in Modula-2 and how you can interact with GDB to print its type and contents:
VAR s: ARRAY [-10..10] OF CHAR ;
(gdb) ptype s ARRAY [-10..10] OF CHAR
Note that the array handling is not yet complete and although the type
is printed correctly, expression handling still assumes that all
arrays have a lower bound of zero and not -10
as in the example
Here are some more type related Modula-2 examples:
TYPE colour = (blue, red, yellow, green) ; t = [blue..yellow] ; VAR s: t ; BEGIN s := blue ;
The GDB interaction shows how you can query the data type and value of a variable.
(gdb) print s $1 = blue (gdb) ptype t type = [blue..yellow]
In this example a Modula-2 array is declared and its contents
displayed. Observe that the contents are written in the same way as
their C
VAR s: ARRAY [1..5] OF CARDINAL ; BEGIN s[1] := 1 ;
(gdb) print s $1 = {1, 0, 0, 0, 0} (gdb) ptype s type = ARRAY [1..5] OF CARDINAL
The Modula-2 language interface to GDB also understands pointer types as shown in this example:
VAR s: POINTER TO ARRAY [1..5] OF CARDINAL ; BEGIN NEW(s) ; s^[1] := 1 ;
and you can request that GDB describes the type of s
(gdb) ptype s type = POINTER TO ARRAY [1..5] OF CARDINAL
GDB handles compound types as we can see in this example. Here we combine array types, record types, pointer types and subrange types:
TYPE foo = RECORD f1: CARDINAL ; f2: CHAR ; f3: myarray ; END ; myarray = ARRAY myrange OF CARDINAL ; myrange = [-2..2] ; VAR s: POINTER TO ARRAY myrange OF foo ;
and you can ask GDB to describe the type of s
as shown
(gdb) ptype s type = POINTER TO ARRAY [-2..2] OF foo = RECORD f1 : CARDINAL; f2 : CHAR; f3 : ARRAY [-2..2] OF CARDINAL; END
Next: Deviations, Previous: M2 Types, Up: Modula-2 [Contents][Index]
If type and range checking are set automatically by GDB, they
both default to on
whenever the working language changes to
Modula-2. This happens regardless of whether you or GDB
selected the working language.
If you allow GDB to set the language automatically, then entering code compiled from a file whose name ends with .mod sets the working language to Modula-2. See Having GDB Infer the Source Language, for further details.
Next: M2 Checks, Previous: M2 Defaults, Up: Modula-2 [Contents][Index]
A few changes have been made to make Modula-2 programs easier to debug. This is done primarily via loosening its type strictness:
) returns the value of its right-hand
Next: M2 Scope, Previous: Deviations, Up: Modula-2 [Contents][Index]
Warning: in this release, GDB does not yet perform type or range checking.
GDB considers two Modula-2 variables type equivalent if:
t1 = t2
As long as type checking is enabled, any attempt to combine variables whose types are not equivalent is an error.
Range checking is done on all mathematical operations, assignment, array index bounds, and all built-in functions and procedures.
and .
There are a few subtle differences between the Modula-2 scope operator
) and the GDB scope operator (::
). The two have
similar syntax:
module . id scope :: id
where scope is the name of a module or a procedure, module the name of a module, and id is any declared identifier within your program, except another module.
Using the ::
operator makes GDB search the scope
specified by scope for the identifier id. If it is not
found in the specified scope, then GDB searches all scopes
enclosing the one specified by scope.
Using the .
operator makes GDB search the current scope for
the identifier specified by id that was imported from the
definition module specified by module. With this operator, it is
an error if the identifier id was not imported from definition
module module, or if id is not an identifier in
Some GDB commands have little use when debugging Modula-2 programs.
Five subcommands of set print
and show print
specifically to C and C++: 'vtbl', 'demangle',
'asm-demangle', 'object', and 'union'. The first four
apply to C++, and the last to the C union
type, which has no direct
analogue in Modula-2.
The @
operator (see Expressions), while available
with any language, is not useful with Modula-2. Its
intent is to aid the debugging of dynamic arrays, which cannot be
created in Modula-2 as they can in C or C++. However, because an
address can be specified by an integral constant, the construct
'{type}adrexp' is still useful.
In GDB scripts, the Modula-2 inequality operator #
interpreted as the beginning of a comment. Use <>
Previous: Modula-2, Up: Supported Languages [Contents][Index]
The extensions made to GDB for Ada only support output from the GNU Ada (GNAT) compiler. Other Ada compilers are not currently supported, and attempting to debug executables produced by them is most likely to be difficult.
• Ada Mode Intro: | General remarks on the Ada syntax and semantics supported by Ada mode in GDB. | |
• Omissions from Ada: | Restrictions on the Ada expression syntax. | |
• Additions to Ada: | Extensions of the Ada expression syntax. | |
• Overloading support for Ada: | Support for expressions involving overloaded subprograms. | |
• Stopping Before Main Program: | Debugging the program during elaboration. | |
• Ada Exceptions: | Ada Exceptions | |
• Ada Tasks: | Listing and setting breakpoints in tasks. | |
• Ada Tasks and Core Files: | Tasking Support when Debugging Core Files | |
• Ravenscar Profile: | Tasking Support when using the Ravenscar Profile | |
• Ada Glitches: | Known peculiarities of Ada mode. |
Next: Omissions from Ada, Up: Ada [Contents][Index]
The Ada mode of GDB supports a fairly large subset of Ada expression syntax, with some extensions. The philosophy behind the design of this subset is
Thus, for brevity, the debugger acts as if all names declared in user-written packages are directly visible, even if they are not visible according to Ada rules, thus making it unnecessary to fully qualify most names with their packages, regardless of context. Where this causes ambiguity, GDB asks the user's intent.
The debugger will start in Ada mode if it detects an Ada main program. As for other languages, it will enter Ada mode when stopped in a program that was translated from an Ada source file.
While in Ada mode, you may use '--' for comments. This is useful mostly for documenting command files. The standard GDB comment ('#') still works at the beginning of a line in Ada mode, but not in the middle (to allow based literals).
Next: Additions to Ada, Previous: Ada Mode Intro, Up: Ada [Contents][Index]
Here are the notable omissions from the subset:
) operator.
are not available and
concatenation is not implemented. Thus, escape characters in strings are
not currently available.
, or
, not
, and relational tests other than equality)
are not implemented.
(gdb) set An_Array := (1, 2, 3, 4, 5, 6) (gdb) set An_Array := (1, others => 0) (gdb) set An_Array := (0|4 => 1, 1..3 => 2, 5 => 6) (gdb) set A_2D_Array := ((1, 2, 3), (4, 5, 6), (7, 8, 9)) (gdb) set A_Record := (1, "Peter", True); (gdb) set A_Record := (Name => "Peter", Id => 1, Alive => True)
Changing a
discriminant's value by assigning an aggregate has an
undefined effect if that discriminant is used within the record.
However, you can first modify discriminants by directly assigning to
them (which normally would not be allowed in Ada), and then performing an
aggregate assignment. For example, given a variable A_Rec
declared to have a type such as:
type Rec (Len : Small_Integer := 0) is record Id : Integer; Vals : IntArray (1 .. Len); end record;
you can assign a value with a different size of Vals
with two
(gdb) set A_Rec.Len := 4 (gdb) set A_Rec := (Id => 42, Vals => (1, 2, 3, 4))
As this example also illustrates, GDB is very loose about the usual
rules concerning aggregates. You may leave out some of the
components of an array or record aggregate (such as the Len
component in the assignment to A_Rec
above); they will retain their
original values upon assignment. You may freely use dynamic values as
indices in component associations. You may even use overlapping or
redundant component associations, although which component values are
assigned in such cases is not defined.
operator is not implemented.
and False
, when not part of a qualified name,
are interpreted as if implicitly prefixed by Standard
, regardless of
Should your program
redefine these names in a package or procedure (at best a dubious practice),
you will have to use fully qualified names to access their new definitions.
Next: Overloading support for Ada, Previous: Omissions from Ada, Up: Ada [Contents][Index]
As it does for other languages, GDB makes certain generic extensions to Ada (see Expressions):
displays the values of E and the
N-1 adjacent variables following it in memory as an array. In
Ada, this operator is generally not necessary, since its prime use is
in displaying parts of an array, and slicing will usually do this in
Ada. However, there are occasional uses when debugging programs in
which certain debugging information has been optimized away.
means "the variable named var that
appears in function or file B." When B is a file name,
you must typically surround it in single quotes.
{type} addr
means "the variable of type
type that appears at address addr."
In addition, GDB provides a few other shortcuts and outright additions specific to Ada:
(gdb) set x := y + 3 (gdb) print A(tmp := y + 1)
(gdb) break f (gdb) condition 1 (report(i); k += 1; A(k) > 100)
"One line.["0a"]Next line.["0a"]"
contains an ASCII newline character (Ada.Characters.Latin_1.LF
after each period.
(gdb) print 'max(x, y)
(3 => 10, 17, 1)
That is, in contrast to valid Ada, only the first component has a =>
(gdb) print <JMPBUF_SAVE>[0]
Next: Stopping Before Main Program, Previous: Additions to Ada, Up: Ada [Contents][Index]
The debugger supports limited overloading. Given a subprogram call in which
the function symbol has multiple definitions, it will use the number of
actual parameters and some information about their types to attempt to narrow
the set of definitions. It also makes very limited use of context, preferring
procedures to functions in the context of the call
command, and
functions to procedures elsewhere.
If, after narrowing, the set of matching definitions still contains more than one definition, GDB will display a menu to query which one it should use, for instance:
(gdb) print f(1) Multiple matches for f [0] cancel [1] foo.f (integer) return boolean at foo.adb:23 [2] foo.f (foo.new_integer) return boolean at foo.adb:28 >
In this case, just select one menu entry either to cancel expression evaluation (type 0 and press RET) or to continue evaluation with a specific instance (type the corresponding number and press RET).
Here are a couple of commands to customize GDB's behavior in this case:
set ada print-signatures
Control whether parameter types and return types are displayed in overloads
selection menus. It is on
by default.
See Overloading support for Ada.
show ada print-signatures
Show the current setting for displaying parameter types and return types in overloads selection menu. See Overloading support for Ada.
Next: Ada Exceptions, Previous: Overloading support for Ada, Up: Ada [Contents][Index]
It is sometimes necessary to debug the program during elaboration, and
before reaching the main procedure.
As defined in the Ada Reference
Manual, the elaboration code is invoked from a procedure called
. To run your program up to the beginning of
elaboration, simply use the following two commands:
tbreak adainit
and run
Next: Ada Tasks, Previous: Stopping Before Main Program, Up: Ada [Contents][Index]
A command is provided to list all Ada exceptions:
info exceptions
info exceptions regexp
The info exceptions
command allows you to list all Ada exceptions
defined within the program being debugged, as well as their addresses.
With a regular expression, regexp, as argument, only those exceptions
whose names match regexp are listed.
Below is a small example, showing how the command can be used, first without argument, and next with a regular expression passed as an argument.
(gdb) info exceptions All defined Ada exceptions: constraint_error: 0x613da0 program_error: 0x613d20 storage_error: 0x613ce0 tasking_error: 0x613ca0 const.aint_global_e: 0x613b00 (gdb) info exceptions const.aint All Ada exceptions matching regular expression "const.aint": constraint_error: 0x613da0 const.aint_global_e: 0x613b00
It is also possible to ask GDB to stop your program's execution when an exception is raised. For more details, see Set Catchpoints.
Next: Ada Tasks and Core Files, Previous: Ada Exceptions, Up: Ada [Contents][Index]
Support for Ada tasks is analogous to that for threads (see Threads). GDB provides the following task-related commands:
info tasks
This command shows a list of current Ada tasks, as in the following example:
(gdb) info tasks ID TID P-ID Pri State Name 1 8088000 0 15 Child Activation Wait main_task 2 80a4000 1 15 Accept Statement b 3 809a800 1 15 Child Activation Wait a * 4 80ae800 3 15 Runnable c
In this listing, the asterisk before the last task indicates it to be the task currently being inspected.
Represents GDB's internal task number.
The Ada task ID.
The parent's task ID (GDB's internal task number).
The base priority of the task.
Current state of the task.
The task has been created but has not been activated. It cannot be executing.
The task is not blocked for any reason known to Ada. (It may be waiting for a mutex, though.) It is conceptually "executing" in normal mode.
The task is terminated, in the sense of ARM 9.3 (5). Any dependents that were waiting on terminate alternatives have been awakened and have terminated themselves.
Child Activation Wait
The task is waiting for created tasks to complete activation.
Accept Statement
The task is waiting on an accept or selective wait statement.
Waiting on entry call
The task is waiting on an entry call.
Async Select Wait
The task is waiting to start the abortable part of an asynchronous select statement.
Delay Sleep
The task is waiting on a select statement with only a delay alternative open.
Child Termination Wait
The task is sleeping having completed a master within itself, and is waiting for the tasks dependent on that master to become terminated or waiting on a terminate Phase.
Wait Child in Term Alt
The task is sleeping waiting for tasks on terminate alternatives to finish terminating.
Accepting RV with taskno
The task is accepting a rendez-vous with the task taskno.
Name of the task in the program.
info task taskno
This command shows detailled informations on the specified task, as in the following example:
(gdb) info tasks ID TID P-ID Pri State Name 1 8077880 0 15 Child Activation Wait main_task * 2 807c468 1 15 Runnable task_1 (gdb) info task 2 Ada Task: 0x807c468 Name: task_1 Thread: 0x807f378 Parent: 1 (main_task) Base Priority: 15 State: Runnable
This command prints the ID of the current task.
(gdb) info tasks ID TID P-ID Pri State Name 1 8077870 0 15 Child Activation Wait main_task * 2 807c458 1 15 Runnable t (gdb) task [Current task is 2]
task taskno
This command is like the thread thread-id
command (see Threads). It switches the context of debugging
from the current task to the given task.
(gdb) info tasks ID TID P-ID Pri State Name 1 8077870 0 15 Child Activation Wait main_task * 2 807c458 1 15 Runnable t (gdb) task 1 [Switching to task 1] #0 0x8067726 in pthread_cond_wait () (gdb) bt #0 0x8067726 in pthread_cond_wait () #1 0x8056714 in system.os_interface.pthread_cond_wait () #2 0x805cb63 in system.task_primitives.operations.sleep () #3 0x806153e in system.tasking.stages.activate_tasks () #4 0x804aacc in un () at un.adb:5
break location task taskno
break location task taskno if …
These commands are like the break … thread …
command (see Thread Stops). The
location argument specifies source lines, as described
in Specify Location.
Use the qualifier 'task taskno' with a breakpoint command to specify that you only want GDB to stop the program when a particular Ada task reaches this breakpoint. The taskno is one of the numeric task identifiers assigned by GDB, shown in the first column of the 'info tasks' display.
If you do not specify 'task taskno' when you set a breakpoint, the breakpoint applies to all tasks of your program.
You can use the task
qualifier on conditional breakpoints as
well; in this case, place 'task taskno' before the
breakpoint condition (before the if
For example,
(gdb) info tasks ID TID P-ID Pri State Name 1 140022020 0 15 Child Activation Wait main_task 2 140045060 1 15 Accept/Select Wait t2 3 140044840 1 15 Runnable t1 * 4 140056040 1 15 Runnable t3 (gdb) b 15 task 2 Breakpoint 5 at 0x120044cb0: file test_task_debug.adb, line 15. (gdb) cont Continuing. task # 1 running task # 2 running Breakpoint 5, test_task_debug () at test_task_debug.adb:15 15 flush; (gdb) info tasks ID TID P-ID Pri State Name 1 140022020 0 15 Child Activation Wait main_task * 2 140045060 1 15 Runnable t2 3 140044840 1 15 Runnable t1 4 140056040 1 15 Delay Sleep t3
Next: Ravenscar Profile, Previous: Ada Tasks, Up: Ada [Contents][Index]
When inspecting a core file, as opposed to debugging a live program, tasking support may be limited or even unavailable, depending on the platform being used. For instance, on x86-linux, the list of tasks is available, but task switching is not supported.
On certain platforms, the debugger needs to perform some memory writes in order to provide Ada tasking support. When inspecting a core file, this means that the core file must be opened with read-write privileges, using the command '"set write on"' (see Patching). Under these circumstances, you should make a backup copy of the core file before inspecting it with GDB.
Next: Ada Glitches, Previous: Ada Tasks and Core Files, Up: Ada [Contents][Index]
The Ravenscar Profile is a subset of the Ada tasking features, specifically designed for systems with safety-critical real-time requirements.
set ravenscar task-switching on
Allows task switching when debugging a program that uses the Ravenscar Profile. This is the default.
set ravenscar task-switching off
Turn off task switching when debugging a program that uses the Ravenscar Profile. This is mostly intended to disable the code that adds support for the Ravenscar Profile, in case a bug in either GDB or in the Ravenscar runtime is preventing GDB from working properly. To be effective, this command should be run before the program is started.
show ravenscar task-switching
Show whether it is possible to switch from task to task in a program using the Ravenscar Profile.
Previous: Ravenscar Profile, Up: Ada [Contents][Index]
Besides the omissions listed previously (see Omissions from Ada), we know of several problems with and limitations of Ada mode in GDB, some of which will be fixed with planned future releases of the debugger and the GNU Ada compiler.
for any of
the standard symbols defined by the Ada language. GDB knows about
this: it will strip the prefix from names when you use it, and will never
look for a name you have so qualified among local symbols, nor match against
symbols in other packages or subprograms. If you have
defined entities anywhere in your program other than parameters and
local variables whose simple names match names in Standard
GNAT's lack of qualification here can cause confusion. When this happens,
you can usually resolve the confusion
by qualifying the problematic names with package
Older versions of the compiler sometimes generate erroneous debugging information, resulting in the debugger incorrectly printing the value of affected entities. In some cases, the debugger is able to work around an issue automatically. In other cases, the debugger is able to work around the issue, but the work-around has to be specifically enabled.
set ada trust-PAD-over-XVS on
Configure GDB to strictly follow the GNAT encoding when computing the
value of Ada entities, particularly when PAD
and PAD___XVS
types are involved (see ada/exp_dbug.ads
in the GCC sources for
a complete description of the encoding used by the GNAT compiler).
This is the default.
set ada trust-PAD-over-XVS off
This is related to the encoding using by the GNAT compiler. If GDB
sometimes prints the wrong value for certain entities, changing ada
to off
activates a work-around which may fix
the issue. It is always safe to set ada trust-PAD-over-XVS
, but this incurs a slight performance penalty, so it is
recommended to leave this setting to on
unless necessary.
Internally, the debugger also relies on the compiler following a number of conventions known as the 'GNAT Encoding', all documented in gcc/ada/exp_dbug.ads in the GCC sources. This encoding describes how the debugging information should be generated for certain types. In particular, this convention makes use of descriptive types, which are artificial types generated purely to help the debugger.
These encodings were defined at a time when the debugging information format used was not powerful enough to describe some of the more complex types available in Ada. Since DWARF allows us to express nearly all Ada features, the long-term goal is to slowly replace these descriptive types by their pure DWARF equivalent. To facilitate that transition, a new maintenance option is available to force the debugger to ignore those descriptive types. It allows the user to quickly evaluate how well GDB works without them.
maintenance ada set ignore-descriptive-types [on|off]
Control whether the debugger should ignore descriptive types.
The default is not to ignore descriptives types (off
maintenance ada show ignore-descriptive-types
Show if descriptive types are ignored by GDB.
Previous: Supported Languages, Up: Languages [Contents][Index]
In addition to the other fully-supported programming languages,
GDB also provides a pseudo-language, called minimal
It does not represent a real programming language, but provides a set
of capabilities close to what the C or assembly languages provide.
This should allow most simple operations to be performed while debugging
an application that uses a language currently not supported by GDB.
If the language is set to auto
, GDB will automatically
select this language if the current frame corresponds to an unsupported
The commands described in this chapter allow you to inquire about the symbols (names of variables, functions and types) defined in your program. This information is inherent in the text of your program and does not change as your program executes. GDB finds it in your program's symbol table, in the file indicated when you started GDB (see Choosing Files), or by one of the file-management commands (see Commands to Specify Files).
Occasionally, you may need to refer to symbols that contain unusual characters, which GDB ordinarily treats as word delimiters. The most frequent case is in referring to static variables in other source files (see Program Variables). File names are recorded in object files as debugging symbols, but GDB would ordinarily parse a typical file name, like foo.c, as the three words 'foo' '.' 'c'. To allow GDB to recognize 'foo.c' as a single symbol, enclose it in single quotes; for example,
p 'foo.c'::x
looks up the value of x
in the scope of the file foo.c.
set case-sensitive on
set case-sensitive off
set case-sensitive auto
Normally, when GDB looks up symbols, it matches their names
with case sensitivity determined by the current source language.
Occasionally, you may wish to control that. The command set
lets you do that by specifying on
case-sensitive matches or off
for case-insensitive ones. If
you specify auto
, case sensitivity is reset to the default
suitable for the source language. The default is case-sensitive
matches for all languages except for Fortran, for which the default is
case-insensitive matches.
show case-sensitive
This command shows the current setting of case sensitivity for symbols lookups.
set print type methods
set print type methods on
set print type methods off
Normally, when GDB prints a class, it displays any methods
declared in that class. You can control this behavior either by
passing the appropriate flag to ptype
, or using set
print type methods
. Specifying on
will cause GDB to
display the methods; this is the default. Specifying off
cause GDB to omit the methods.
show print type methods
This command shows the current setting of method display when printing classes.
set print type typedefs
set print type typedefs on
set print type typedefs off
Normally, when GDB prints a class, it displays any typedefs
defined in that class. You can control this behavior either by
passing the appropriate flag to ptype
, or using set
print type typedefs
. Specifying on
will cause GDB to
display the typedef definitions; this is the default. Specifying
will cause GDB to omit the typedef definitions.
Note that this controls whether the typedef definition itself is
printed, not whether typedef names are substituted when printing other
show print type typedefs
This command shows the current setting of typedef display when printing classes.
info address symbol
Describe where the data for symbol is stored. For a register variable, this says which register it is kept in. For a non-register local variable, this prints the stack-frame offset at which the variable is always stored.
Note the contrast with 'print &symbol', which does not work at all for a register variable, and for a stack local variable prints the exact address of the current instantiation of the variable.
info symbol addr
Print the name of a symbol which is stored at the address addr. If no symbol is stored exactly at addr, GDB prints the nearest symbol and an offset from it:
(gdb) info symbol 0x54320 _initialize_vx + 396 in section .text
This is the opposite of the info address
command. You can use
it to find out the name of a variable or a function given its address.
For dynamically linked executables, the name of executable or shared library containing the symbol is also printed:
(gdb) info symbol 0x400225 _start + 5 in section .text of /tmp/a.out (gdb) info symbol 0x2aaaac2811cf __read_nocancel + 6 in section .text of /usr/lib64/libc.so.6
demangle [-l language] [--] name
Demangle name. If language is provided it is the name of the language to demangle name in. Otherwise name is demangled in the current language.
The '--' option specifies the end of options, and is useful when name begins with a dash.
The parameter demangle-style
specifies how to interpret the kind
of mangling used. See Print Settings.
whatis[/flags] [arg]
Print the data type of arg, which can be either an expression
or a name of a data type. With no argument, print the data type of
, the last value in the value history.
If arg is an expression (see Expressions), it is not actually evaluated, and any side-effecting operations (such as assignments or function calls) inside it do not take place.
If arg is a variable or an expression, whatis
prints its
literal type as it is used in the source code. If the type was
defined using a typedef
, whatis
will not print
the data type underlying the typedef
. If the type of the
variable or the expression is a compound data type, such as
or class
, whatis
never prints their
fields or methods. It just prints the struct
name (a.k.a. its tag). If you want to see the members of
such a compound data type, use ptype
If arg is a type name that was defined using typedef
unrolls only one level of that typedef
Unrolling means that whatis
will show the underlying type used
in the typedef
declaration of arg. However, if that
underlying type is also a typedef
, whatis
will not
unroll it.
For C code, the type names may also have the form 'class class-name', 'struct struct-tag', 'union union-tag' or 'enum enum-tag'.
flags can be used to modify how the type is displayed. Available flags are:
Display in "raw" form. Normally, GDB substitutes template
parameters and typedefs defined in a class when printing the class'
members. The /r
flag disables this.
Do not print methods defined in the class.
Print methods defined in the class. This is the default, but the flag
exists in case you change the default with set print type methods
Do not print typedefs defined in the class. Note that this controls whether the typedef definition itself is printed, not whether typedef names are substituted when printing other types.
Print typedefs defined in the class. This is the default, but the flag
exists in case you change the default with set print type typedefs
ptype[/flags] [arg]
accepts the same arguments as whatis
, but prints a
detailed description of the type, instead of just the name of the type.
See Expressions.
Contrary to whatis
, ptype
always unrolls any
s in its argument declaration, whether the argument is
a variable, expression, or a data type. This means that ptype
of a variable or an expression will not print literally its type as
present in the source code—use whatis
for that. typedef
s at
the pointer or reference targets are also unrolled. Only typedef
s of
fields, methods and inner class typedef
s of struct
es and union
s are not unrolled even with ptype
For example, for this variable declaration:
typedef double real_t; struct complex { real_t real; double imag; }; typedef struct complex complex_t; complex_t var; real_t *real_pointer_var;
the two commands give this output:
(gdb) whatis var type = complex_t (gdb) ptype var type = struct complex { real_t real; double imag; } (gdb) whatis complex_t type = struct complex (gdb) whatis struct complex type = struct complex (gdb) ptype struct complex type = struct complex { real_t real; double imag; } (gdb) whatis real_pointer_var type = real_t * (gdb) ptype real_pointer_var type = double *
As with whatis
, using ptype
without an argument refers to
the type of $
, the last value in the value history.
Sometimes, programs use opaque data types or incomplete specifications of complex data structure. If the debug information included in the program does not allow GDB to display a full declaration of the data type, it will say '<incomplete type>'. For example, given these declarations:
struct foo; struct foo *fooptr;
but no definition for struct foo
itself, GDB will say:
(gdb) ptype foo $1 = <incomplete type>
"Incomplete type" is C terminology for data types that are not completely specified.
info types regexp
info types
Print a brief description of all types whose names match the regular
expression regexp (or all types in your program, if you supply
no argument). Each complete typename is matched as though it were a
complete line; thus, 'i type value' gives information on all
types in your program whose names include the string value
, but
'i type ^value$' gives information only on types whose complete
name is value
This command differs from ptype
in two ways: first, like
, it does not print a detailed description; second, it
lists all source files where a type is defined.
info type-printers
Versions of GDB that ship with Python scripting enabled may
have "type printers" available. When using ptype
, these printers are consulted when the name of a type
is needed. See Type Printing API, for more information on writing
type printers.
info type-printers
displays all the available type printers.
enable type-printer name…
disable type-printer name…
These commands can be used to enable or disable type printers.
info scope location
List all the variables local to a particular scope. This command accepts a location argument—a function name, a source line, or an address preceded by a '*', and prints all the variables local to the scope defined by that location. (See Specify Location, for details about supported forms of location.) For example:
(gdb) info scope command_line_handler Scope for command_line_handler: Symbol rl is an argument at stack/frame offset 8, length 4. Symbol linebuffer is in static storage at address 0x150a18, length 4. Symbol linelength is in static storage at address 0x150a1c, length 4. Symbol p is a local variable in register $esi, length 4. Symbol p1 is a local variable in register $ebx, length 4. Symbol nline is a local variable in register $edx, length 4. Symbol repeat is a local variable at frame offset -8, length 4.
This command is especially useful for determining what data to collect during a trace experiment, see collect.
info source
Show information about the current source file—that is, the source file for the function containing the current point of execution:
info sources
Print the names of all source files in your program for which there is debugging information, organized into two lists: files whose symbols have already been read, and files whose symbols will be read when needed.
info functions
Print the names and data types of all defined functions.
info functions regexp
Print the names and data types of all defined functions
whose names contain a match for regular expression regexp.
Thus, 'info fun step' finds all functions whose names
include step
; 'info fun ^step' finds those whose names
start with step
. If a function name contains characters
that conflict with the regular expression language (e.g.
'operator*()'), they may be quoted with a backslash.
info variables
Print the names and data types of all variables that are defined outside of functions (i.e. excluding local variables).
info variables regexp
Print the names and data types of all variables (except for local variables) whose names contain a match for regular expression regexp.
info classes
info classes regexp
Display all Objective-C classes in your program, or (with the regexp argument) all those matching a particular regular expression.
info selectors
info selectors regexp
Display all Objective-C selectors in your program, or (with the regexp argument) all those matching a particular regular expression.
set opaque-type-resolution on
Tell GDB to resolve opaque types. An opaque type is a type
declared as a pointer to a struct
, class
, or
—for example, struct MyType *
—that is used in one
source file although the full declaration of struct MyType
is in
another source file. The default is on.
A change in the setting of this subcommand will not take effect until the next time symbols for a file are loaded.
set opaque-type-resolution off
Tell GDB not to resolve opaque types. In this case, the type is printed as follows:
{<no data fields>}
show opaque-type-resolution
Show whether opaque types are resolved or not.
set print symbol-loading
set print symbol-loading full
set print symbol-loading brief
set print symbol-loading off
The set print symbol-loading
command allows you to control the
printing of messages when GDB loads symbol information.
By default a message is printed for the executable and one for each
shared library, and normally this is what you want. However, when
debugging apps with large numbers of shared libraries these messages
can be annoying.
When set to brief
a message is printed for each executable,
and when GDB loads a collection of shared libraries at once
it will only print one message regardless of the number of shared
libraries. When set to off
no messages are printed.
show print symbol-loading
Show whether messages will be printed when a GDB command entered from the keyboard causes symbol information to be loaded.
maint print symbols filename
maint print psymbols filename
maint print msymbols filename
Write a dump of debugging symbol data into the file filename.
These commands are used to debug the GDB symbol-reading code. Only
symbols with debugging data are included. If you use 'maint print
symbols', GDB includes all the symbols for which it has already
collected full details: that is, filename reflects symbols for
only those files whose symbols GDB has read. You can use the
command info sources
to find out which files these are. If you
use 'maint print psymbols' instead, the dump shows information about
symbols that GDB only knows partially—that is, symbols defined in
files that GDB has skimmed, but not yet read completely. Finally,
'maint print msymbols' dumps just the minimal symbol information
required for each object file from which GDB has read some symbols.
See Commands to Specify Files, for a discussion of how
GDB reads symbols (in the description of symbol-file
maint info symtabs [ regexp ]
maint info psymtabs [ regexp ]
List the struct symtab
or struct partial_symtab
structures whose names match regexp. If regexp is not
given, list them all. The output includes expressions which you can
copy into a GDB debugging this one to examine a particular
structure in more detail. For example:
(gdb) maint info psymtabs dwarf2read { objfile /home/gnu/build/gdb/gdb ((struct objfile *) 0x82e69d0) { psymtab /home/gnu/src/gdb/dwarf2read.c ((struct partial_symtab *) 0x8474b10) readin no fullname (null) text addresses 0x814d3c8 -- 0x8158074 globals (* (struct partial_symbol **) 0x8507a08 @ 9) statics (* (struct partial_symbol **) 0x40e95b78 @ 2882) dependencies (none) } } (gdb) maint info symtabs (gdb)
We see that there is one partial symbol table whose filename contains the string 'dwarf2read', belonging to the 'gdb' executable; and we see that GDB has not read in any symtabs yet at all. If we set a breakpoint on a function, that will cause GDB to read the symtab for the compilation unit containing that function:
(gdb) break dwarf2_psymtab_to_symtab Breakpoint 1 at 0x814e5da: file /home/gnu/src/gdb/dwarf2read.c, line 1574. (gdb) maint info symtabs { objfile /home/gnu/build/gdb/gdb ((struct objfile *) 0x82e69d0) { symtab /home/gnu/src/gdb/dwarf2read.c ((struct symtab *) 0x86c1f38) dirname (null) fullname (null) blockvector ((struct blockvector *) 0x86c1bd0) (primary) linetable ((struct linetable *) 0x8370fa0) debugformat DWARF 2 } } (gdb)
maint info line-table [ regexp ]
List the struct linetable
from all struct symtab
instances whose name matches regexp. If regexp is not
given, list the struct linetable
from all struct symtab
maint set symbol-cache-size size
Set the size of the symbol cache to size. The default size is intended to be good enough for debugging most applications. This option exists to allow for experimenting with different sizes.
maint show symbol-cache-size
Show the size of the symbol cache.
maint print symbol-cache
Print the contents of the symbol cache. This is useful when debugging symbol cache issues.
maint print symbol-cache-statistics
Print symbol cache usage statistics. This helps determine how well the cache is being utilized.
maint flush-symbol-cache
Flush the contents of the symbol cache, all entries are removed. This command is useful when debugging the symbol cache. It is also useful when collecting performance data.
Once you think you have found an error in your program, you might want to find out for certain whether correcting the apparent error would lead to correct results in the rest of the run. You can find the answer by experiment, using the GDB features for altering execution of the program.
For example, you can store new values into variables or memory locations, give your program a signal, restart it at a different address, or even return prematurely from a function.
• Assignment: | Assignment to variables | |
• Jumping: | Continuing at a different address | |
• Signaling: | Giving your program a signal | |
• Returning: | Returning from a function | |
• Calling: | Calling your program's functions | |
• Patching: | Patching your program | |
• Compiling and Injecting Code: | Compiling and injecting code in GDB |
To alter the value of a variable, evaluate an assignment expression. See Expressions. For example,
print x=4
stores the value 4 into the variable x
, and then prints the
value of the assignment expression (which is 4).
See Using GDB with Different Languages, for more
information on operators in supported languages.
If you are not interested in seeing the value of the assignment, use the
command instead of the print
command. set
really the same as print
except that the expression's value is
not printed and is not put in the value history (see Value History). The expression is evaluated only for its effects.
If the beginning of the argument string of the set
appears identical to a set
subcommand, use the set
command instead of just set
. This command is identical
to set
except for its lack of subcommands. For example, if your
program has a variable width
, you get an error if you try to set
a new value with just 'set width=13', because GDB has the
command set width
(gdb) whatis width type = double (gdb) p width $4 = 13 (gdb) set width=47 Invalid syntax in expression.
The invalid expression, of course, is '=47'. In
order to actually set the program's variable width
, use
(gdb) set var width=47
Because the set
command has many subcommands that can conflict
with the names of program variables, it is a good idea to use the
set variable
command instead of just set
. For example, if
your program has a variable g
, you run into problems if you try
to set a new value with just 'set g=4', because GDB has
the command set gnutarget
, abbreviated set g
(gdb) whatis g type = double (gdb) p g $1 = 1 (gdb) set g=4 (gdb) p g $2 = 1 (gdb) r The program being debugged has been started already. Start it from the beginning? (y or n) y Starting program: /home/smith/cc_progs/a.out "/home/smith/cc_progs/a.out": can't open to read symbols: Invalid bfd target. (gdb) show g The current BFD target is "=4".
The program variable g
did not change, and you silently set the
to an invalid value. In order to set the variable
, use
(gdb) set var g=4
GDB allows more implicit conversions in assignments than C; you can freely store an integer value into a pointer variable or vice versa, and you can convert any structure to any other structure that is the same length or shorter.
To store values into arbitrary places in memory, use the '{…}'
construct to generate a value of specified type at a specified address
(see Expressions). For example, {int}0x83040
to memory location 0x83040
as an integer (which implies a certain size
and representation in memory), and
set {int}0x83040 = 4
stores the value 4 into that memory location.
Next: Signaling, Previous: Assignment, Up: Altering [Contents][Index]
Ordinarily, when you continue your program, you do so at the place where
it stopped, with the continue
command. You can instead continue at
an address of your own choosing, with the following commands:
jump location
j location
Resume execution at location. Execution stops again immediately
if there is a breakpoint there. See Specify Location, for a description
of the different forms of location. It is common
practice to use the tbreak
command in conjunction with
. See Setting Breakpoints.
The jump
command does not change the current stack frame, or
the stack pointer, or the contents of any memory location or any
register other than the program counter. If location is in
a different function from the one currently executing, the results may
be bizarre if the two functions expect different patterns of arguments or
of local variables. For this reason, the jump
command requests
confirmation if the specified line is not in the function currently
executing. However, even bizarre results are predictable if you are
well acquainted with the machine-language code of your program.
On many systems, you can get much the same effect as the jump
command by storing a new value into the register $pc
. The
difference is that this does not start your program running; it only
changes the address of where it will run when you continue. For
set $pc = 0x485
makes the next continue
command or stepping command execute at
address 0x485
, rather than at the address where your program stopped.
See Continuing and Stepping.
The most common occasion to use the jump
command is to back
up—perhaps with more breakpoints set—over a portion of a program
that has already executed, in order to examine its execution in more
signal signal
Resume execution where your program is stopped, but immediately give it the
signal signal. The signal can be the name or the number of a
signal. For example, on many systems signal 2
and signal
are both ways of sending an interrupt signal.
Alternatively, if signal is zero, continue execution without
giving a signal. This is useful when your program stopped on account of
a signal and would ordinarily see the signal when resumed with the
command; 'signal 0' causes it to resume without a
Note: When resuming a multi-threaded program, signal is delivered to the currently selected thread, not the thread that last reported a stop. This includes the situation where a thread was stopped due to a signal. So if you want to continue execution suppressing the signal that stopped a thread, you should select that same thread before issuing the 'signal 0' command. If you issue the 'signal 0' command with another thread as the selected one, GDB detects that and asks for confirmation.
Invoking the signal
command is not the same as invoking the
utility from the shell. Sending a signal with kill
causes GDB to decide what to do with the signal depending on
the signal handling tables (see Signals). The signal
passes the signal directly to your program.
does not repeat when you press RET a second time
after executing the command.
queue-signal signal
Queue signal to be delivered immediately to the current thread
when execution of the thread resumes. The signal can be the name or
the number of a signal. For example, on many systems signal 2
signal SIGINT
are both ways of sending an interrupt signal.
The handling of the signal must be set to pass the signal to the program,
otherwise GDB will report an error.
You can control the handling of signals from GDB with the
command (see Signals).
Alternatively, if signal is zero, any currently queued signal
for the current thread is discarded and when execution resumes no signal
will be delivered. This is useful when your program stopped on account
of a signal and would ordinarily see the signal when resumed with the
This command differs from the signal
command in that the signal
is just queued, execution is not resumed. And queue-signal
be used to pass a signal whose handling state has been set to nopass
(see Signals).
See stepping into signal handlers, for information on how stepping commands behave when the thread has a signal queued.
return expression
You can cancel execution of a function call with the return
command. If you give an
expression argument, its value is used as the function's return
When you use return
, GDB discards the selected stack frame
(and all frames within it). You can think of this as making the
discarded frame return prematurely. If you wish to specify a value to
be returned, give that value as the argument to return
This pops the selected stack frame (see Selecting a Frame), and any other frames inside of it, leaving its caller as the innermost remaining frame. That frame becomes selected. The specified value is stored in the registers used for returning values of functions.
The return
command does not resume execution; it leaves the
program stopped in the state that would exist if the function had just
returned. In contrast, the finish
command (see Continuing and Stepping) resumes execution until the
selected stack frame returns naturally.
GDB needs to know how the expression argument should be set for
the inferior. The concrete registers assignment depends on the OS ABI and the
type being returned by the selected stack frame. For example it is common for
OS ABI to return floating point values in FPU registers while integer values in
CPU registers. Still some ABIs return even floating point values in CPU
registers. Larger integer widths (such as long long int
) also have
specific placement rules. GDB already knows the OS ABI from its
current target so it needs to find out also the type being returned to make the
assignment into the right register(s).
Normally, the selected stack frame has debug info. GDB will always
use the debug info instead of the implicit type of expression when the
debug info is available. For example, if you type return -1, and the
function in the current stack frame is declared to return a long long
, GDB transparently converts the implicit int
value of -1
into a long long int
Breakpoint 1, func () at gdb.base/return-nodebug.c:29 29 return 31; (gdb) return -1 Make func return now? (y or n) y #0 0x004004f6 in main () at gdb.base/return-nodebug.c:43 43 printf ("result=%lld\n", func ()); (gdb)
However, if the selected stack frame does not have a debug info, e.g., if the
function was compiled without debug info, GDB has to find out the type
to return from user. Specifying a different type by mistake may set the value
in different inferior registers than the caller code expects. For example,
typing return -1 with its implicit type int
would set only a part
of a long long int
result for a debug info less function (on 32-bit
architectures). Therefore the user is required to specify the return type by
an appropriate cast explicitly:
Breakpoint 2, 0x0040050b in func () (gdb) return -1 Return value type not available for selected stack frame. Please use an explicit cast of the value to return. (gdb) return (long long int) -1 Make selected stack frame return now? (y or n) y #0 0x00400526 in main () (gdb)
print expr
Evaluate the expression expr and display the resulting value. The expression may include calls to functions in the program being debugged.
call expr
Evaluate the expression expr without displaying void
returned values.
You can use this variant of the print
command if you want to
execute a function from your program that does not return anything
(a.k.a. a void function), but without cluttering the output
with void
returned values that GDB will otherwise
print. If the result is not void, it is printed and saved in the
value history.
It is possible for the function you call via the print
command to generate a signal (e.g., if there's a bug in
the function, or if you passed it incorrect arguments). What happens
in that case is controlled by the set unwindonsignal
Similarly, with a C++ program it is possible for the function you
call via the print
or call
command to generate an
exception that is not handled due to the constraints of the dummy
frame. In this case, any exception that is raised in the frame, but has
an out-of-frame exception handler will not be found. GDB builds a
dummy-frame for the inferior function call, and the unwinder cannot
seek for exception handlers outside of this dummy-frame. What happens
in that case is controlled by the
set unwind-on-terminating-exception
set unwindonsignal
Set unwinding of the stack if a signal is received while in a function that GDB called in the program being debugged. If set to on, GDB unwinds the stack it created for the call and restores the context to what it was before the call. If set to off (the default), GDB stops in the frame where the signal was received.
show unwindonsignal
Show the current setting of stack unwinding in the functions called by GDB.
set unwind-on-terminating-exception
Set unwinding of the stack if a C++ exception is raised, but left unhandled while in a function that GDB called in the program being debugged. If set to on (the default), GDB unwinds the stack it created for the call and restores the context to what it was before the call. If set to off, GDB the exception is delivered to the default C++ exception handler and the inferior terminated.
show unwind-on-terminating-exception
Show the current setting of stack unwinding in the functions called by GDB.
Sometimes, a function you wish to call is actually a weak alias for another function. In such case, GDB might not pick up the type information, including the types of the function arguments, which causes GDB to call the inferior function incorrectly. As a result, the called function will function erroneously and may even crash. A solution to that is to use the name of the aliased function instead.
Next: Compiling and Injecting Code, Previous: Calling, Up: Altering [Contents][Index]
By default, GDB opens the file containing your program's executable code (or the corefile) read-only. This prevents accidental alterations to machine code; but it also prevents you from intentionally patching your program's binary.
If you'd like to be able to patch the binary, you can specify that
explicitly with the set write
command. For example, you might
want to turn on internal debugging flags, or even to make emergency
set write on
set write off
If you specify 'set write on', GDB opens executable and core files for both reading and writing; if you specify set write off (the default), GDB opens them read-only.
If you have already loaded a file, you must load it again (using the
or core-file
command) after changing set
, for your new setting to take effect.
show write
Display whether executable files and core files are opened for writing as well as reading.
GDB supports on-demand compilation and code injection into programs running under GDB. GCC 5.0 or higher built with libcc1.so must be installed for this functionality to be enabled. This functionality is implemented with the following commands.
compile code source-code
compile code -raw -- source-code
Compile source-code with the compiler language found as the current language in GDB (see Languages). If compilation and injection is not supported with the current language specified in GDB, or the compiler does not support this feature, an error message will be printed. If source-code compiles and links successfully, GDB will load the object-code emitted, and execute it within the context of the currently selected inferior. It is important to note that the compiled code is executed immediately. After execution, the compiled code is removed from GDB and any new types or variables you have defined will be deleted.
The command allows you to specify source-code in two ways. The simplest method is to provide a single line of code to the command. E.g.:
compile code printf ("hello world\n");
If you specify options on the command line as well as source code, they may conflict. The '--' delimiter can be used to separate options from actual source code. E.g.:
compile code -r -- printf ("hello world\n");
Alternatively you can enter source code as multiple lines of text. To enter this mode, invoke the 'compile code' command without any text following the command. This will start the multiple-line editor and allow you to type as many lines of source code as required. When you have completed typing, enter 'end' on its own line to exit the editor.
compile code >printf ("hello\n"); >printf ("world\n"); >end
Specifying '-raw', prohibits GDB from wrapping the
provided source-code in a callable scope. In this case, you must
specify the entry point of the code by defining a function named
. The '-raw' code cannot access variables of the
inferior. Using '-raw' option may be needed for example when
source-code requires '#include' lines which may conflict with
inferior symbols otherwise.
compile file filename
compile file -raw filename
Like compile code
, but take the source code from filename.
compile file /home/user/example.c
compile print expr
compile print /f expr
Compile and execute expr with the compiler language found as the current language in GDB (see Languages). By default the value of expr is printed in a format appropriate to its data type; you can choose a different format by specifying '/f', where f is a letter specifying the format; see Output Formats.
compile print
compile print /f
Alternatively you can enter the expression (source code producing it) as multiple lines of text. To enter this mode, invoke the 'compile print' command without any text following the command. This will start the multiple-line editor.
The process of compiling and injecting the code can be inspected using:
set debug compile
Turns on or off display of GDB process of compiling and injecting the code. The default is off.
show debug compile
Displays the current state of displaying GDB process of compiling and injecting the code.
commandGDB needs to specify the right compilation options for the code to be injected, in part to make its ABI compatible with the inferior and in part to make the injected code compatible with GDB's injecting process.
The options used, in increasing precedence:
)These options depend on target processor type and target operating
system, usually they specify at least 32-bit (-m32
) or 64-bit
) compilation option.
GCC (since version 4.7) stores the options used for compilation
into DW_AT_producer
part of DWARF debugging information according
to the GCC option -grecord-gcc-switches
. One has to
explicitly specify -g
during inferior compilation otherwise
GCC produces no DWARF. This feature is only relevant for
platforms where -g
produces DWARF by default, otherwise one may
try to enforce DWARF by using -gdwarf-4
set compile-args
You can override compilation options using the following command:
set compile-args
Set compilation options used for compiling and injecting code with the
commands. These options override any conflicting ones
from the target architecture and/or options stored during inferior
show compile-args
Displays the current state of compilation options override. This does not show all the options actually used during compilation, use set debug compile for that.
commandThere are a few caveats to keep in mind when using the compile
command. As the caveats are different per language, the table below
highlights specific issues on a per language basis.
When the language in GDB is set to 'C', the compiler will
attempt to compile the source code with a 'C' compiler. The source
code provided to the compile
command will have much the same
access to variables and types as it normally would if it were part of
the program currently being debugged in GDB.
Below is a sample program that forms the basis of the examples that follow. This program has been compiled and loaded into GDB, much like any other normal debugging session.
void function1 (void) { int i = 42; printf ("function 1\n"); } void function2 (void) { int j = 12; function1 (); } int main(void) { int k = 6; int *p; function2 (); return 0; }
For the purposes of the examples in this section, the program above has
been compiled, loaded into GDB, stopped at the function
, and GDB is awaiting input from the user.
To access variables and types for any program in GDB, the
program must be compiled and packaged with debug information. The
command is not an exception to this rule. Without debug
information, you can still use the compile
command, but you will
be very limited in what variables and types you can access.
So with that in mind, the example above has been compiled with debug
information enabled. The compile
command will have access to
all variables and types (except those that may have been optimized
out). Currently, as GDB has stopped the program in the
function, the compile
command would have access to
the variable k
. You could invoke the compile
and type some source code to set the value of k
. You can also
read it, or do anything with that variable you would normally do in
. Be aware that changes to inferior variables in the
command are persistent. In the following example:
compile code k = 3;
the variable k
is now 3. It will retain that value until
something else in the example program changes it, or another
command changes it.
Normal scope and access rules apply to source code compiled and
injected by the compile
command. In the example, the variables
and k
are not accessible yet, because the program is
currently stopped in the main
function, where these variables
are not in scope. Therefore, the following command
compile code j = 3;
will result in a compilation error message.
Once the program is continued, execution will bring these variables in
scope, and they will become accessible; then the code you specify via
the compile
command will be able to access them.
You can create variables and types with the compile
command as
part of your source code. Variables and types that are created as part
of the compile
command are not visible to the rest of the program for
the duration of its run. This example is valid:
compile code int ff = 5; printf ("ff is %d\n", ff);
However, if you were to type the following into GDB after that command has completed:
compile code printf ("ff is %d\n'', ff);
a compiler error would be raised as the variable ff
no longer
exists. Object code generated and injected by the compile
command is removed when its execution ends. Caution is advised
when assigning to program variables values of variables created by the
code submitted to the compile
command. This example is valid:
compile code int ff = 5; k = ff;
The value of the variable ff
is assigned to k
. The variable
does not require the existence of ff
to maintain the value
it has been assigned. However, pointers require particular care in
assignment. If the source code compiled with the compile
changed the address of a pointer in the example program, perhaps to a
variable created in the compile
command, that pointer would point
to an invalid location when the command exits. The following example
would likely cause issues with your debugged program:
compile code int ff = 5; p = &ff;
In this example, p
would point to ff
when the
command is executing the source code provided to it.
However, as variables in the (example) program persist with their
assigned values, the variable p
would point to an invalid
location when the command exists. A general rule should be followed
in that you should either assign NULL
to any assigned pointers,
or restore a valid location to the pointer before the command exits.
Similar caution must be exercised with any structs, unions, and typedefs
defined in compile
command. Types defined in the compile
command will no longer be available in the next compile
Therefore, if you cast a variable to a type defined in the
command, care must be taken to ensure that any future
need to resolve the type can be achieved.
(gdb) compile code static struct a { int a; } v = { 42 }; argv = &v; (gdb) compile code printf ("%d\n", ((struct a *) argv)->a); gdb command line:1:36: error: dereferencing pointer to incomplete type âstruct aâ Compilation failed. (gdb) compile code struct a { int a; }; printf ("%d\n", ((struct a *) argv)->a); 42
Variables that have been optimized away by the compiler are not
accessible to the code submitted to the compile
Access to those variables will generate a compiler error which GDB
will print to the console.
commandGDB needs to find GCC for the inferior being debugged which
may not be obvious for remote targets of different architecture than where
GDB is running. Environment variable PATH
shell that executed GDB, not the one set by GDB
command set environment
). See Environment. PATH
GDB host is searched for GCC binary matching the
target architecture and operating system.
Specifically PATH
is searched for binaries matching regular expression
according to the inferior target being
debugged. arch is processor name — multiarch is supported, so for
example both i386
and x86_64
targets look for pattern
and both s390
and s390x
targets look
for pattern s390x?
. os is currently supported only for
pattern linux(-gnu)?
GDB needs to know the file name of the program to be debugged, both in order to read its symbol table and in order to start your program. To debug a core dump of a previous run, you must also tell GDB the name of the core dump file.
• Files: | Commands to specify files | |
• File Caching: | Information about GDB's file caching | |
• Separate Debug Files: | Debugging information in separate files | |
• MiniDebugInfo: | Debugging information in a special section | |
• Index Files: | Index files speed up GDB | |
• Symbol Errors: | Errors reading symbol files | |
• Data Files: | GDB data files |
Next: File Caching, Up: GDB Files [Contents][Index]
You may want to specify executable and core dump file names. The usual way to do this is at start-up time, using the arguments to GDB's start-up commands (see Getting In and Out of GDB).
Occasionally it is necessary to change to a different file during a
GDB session. Or you may run GDB and forget to
specify a file you want to use. Or you are debugging a remote target
via gdbserver
(see Using the gdbserver
Program). In these situations the GDB commands to specify
new files are useful.
file filename
Use filename as the program to be debugged. It is read for its
symbols and for the contents of pure memory. It is also the program
executed when you use the run
command. If you do not specify a
directory and the file is not found in the GDB working directory,
GDB uses the environment variable PATH
as a list of
directories to search, just as the shell does when looking for a program
to run. You can change the value of this variable, for both GDB
and your program, using the path
You can load unlinked object .o files into GDB using
the file
command. You will not be able to "run" an object
file, but you can disassemble functions and inspect variables. Also,
if the underlying BFD functionality supports it, you could use
gdb -write to patch object files using this technique. Note
that GDB can neither interpret nor modify relocations in this
case, so branches and some initialized variables will appear to go to
the wrong place. But this feature is still handy from time to time.
with no argument makes GDB discard any information it
has on both executable file and the symbol table.
exec-file [ filename ]
Specify that the program to be run (but not the symbol table) is found
in filename. GDB searches the environment variable PATH
if necessary to locate your program. Omitting filename means to
discard information on the executable file.
symbol-file [ filename ]
Read symbol table information from file filename. PATH
searched when necessary. Use the file
command to get both symbol
table and program to run from the same file.
with no argument clears out GDB information on your
program's symbol table.
The symbol-file
command causes GDB to forget the contents of
some breakpoints and auto-display expressions. This is because they may
contain pointers to the internal data recording symbols and data types,
which are part of the old symbol table data being discarded inside
does not repeat if you press RET again after
executing it once.
When GDB is configured for a particular environment, it
understands debugging information in whatever format is the standard
generated for that environment; you may use either a GNU compiler, or
other compilers that adhere to the local conventions.
Best results are usually obtained from GNU compilers; for example,
using GCC
you can generate debugging information for
optimized code.
For most kinds of object files, with the exception of old SVR3 systems
using COFF, the symbol-file
command does not normally read the
symbol table in full right away. Instead, it scans the symbol table
quickly to find which source files and which symbols are present. The
details are read later, one source file at a time, as they are needed.
The purpose of this two-stage reading strategy is to make GDB
start up faster. For the most part, it is invisible except for
occasional pauses while the symbol table details for a particular source
file are being read. (The set verbose
command can turn these
pauses into messages if desired. See Optional
Warnings and Messages.)
We have not implemented the two-stage strategy for COFF yet. When the
symbol table is stored in COFF format, symbol-file
reads the
symbol table data in full right away. Note that "stabs-in-COFF"
still does the two-stage strategy, since the debug info is actually
in stabs format.
symbol-file [ -readnow ] filename
file [ -readnow ] filename
You can override the GDB two-stage strategy for reading symbol tables by using the '-readnow' option with any of the commands that load symbol table information, if you want to be sure GDB has the entire symbol table available.
core-file [filename]
Specify the whereabouts of a core dump file to be used as the "contents of memory". Traditionally, core files contain only some parts of the address space of the process that generated them; GDB can access the executable file itself for other parts.
with no argument specifies that no core file is
to be used.
Note that the core file is ignored when your program is actually running
under GDB. So, if you have been running your program and you
wish to debug a core file instead, you must kill the subprocess in which
the program is running. To do this, use the kill
(see Killing the Child Process).
add-symbol-file filename address
add-symbol-file filename address [ -readnow ]
add-symbol-file filename address -s section address …
The add-symbol-file
command reads additional symbol table
information from the file filename. You would use this command
when filename has been dynamically loaded (by some other means)
into the program that is running. The address should give the memory
address at which the file has been loaded; GDB cannot figure
this out for itself. You can additionally specify an arbitrary number
of '-s section address' pairs, to give an explicit
section name and base address for that section. You can specify any
address as an expression.
The symbol table of the file filename is added to the symbol table
originally read with the symbol-file
command. You can use the
command any number of times; the new symbol data
thus read is kept in addition to the old.
Changes can be reverted using the command remove-symbol-file
Although filename is typically a shared library file, an executable file, or some other object file which has been fully relocated for loading into a process, you can also load symbolic information from relocatable .o files, as long as:
Some embedded operating systems, like Sun Chorus and VxWorks, can load
relocatable files into an already running program; such systems
typically make the requirements above easy to meet. However, it's
important to recognize that many native systems use complex link
procedures (.linkonce
section factoring and C++ constructor table
assembly, for example) that make the requirements difficult to meet. In
general, one cannot assume that using add-symbol-file
to read a
relocatable object file's symbolic information will have the same effect
as linking the relocatable object file into the program in the normal
does not repeat if you press RET after using it.
remove-symbol-file filename
remove-symbol-file -a address
Remove a symbol file added via the add-symbol-file
command. The
file to remove can be identified by its filename or by an address
that lies within the boundaries of this symbol file in memory. Example:
(gdb) add-symbol-file /home/user/gdb/mylib.so 0x7ffff7ff9480 add symbol table from file "/home/user/gdb/mylib.so" at .text_addr = 0x7ffff7ff9480 (y or n) y Reading symbols from /home/user/gdb/mylib.so...done. (gdb) remove-symbol-file -a 0x7ffff7ff9480 Remove symbol table from file "/home/user/gdb/mylib.so"? (y or n) y (gdb)
does not repeat if you press RET after using it.
add-symbol-file-from-memory address
Load symbols from the given address in a dynamically loaded
object file whose image is mapped directly into the inferior's memory.
For example, the Linux kernel maps a syscall DSO
into each
process's address space; this DSO provides kernel-specific code for
some system calls. The argument can be any expression whose
evaluation yields the address of the file's shared object file header.
For this command to work, you must have used symbol-file
commands in advance.
section section addr
The section
command changes the base address of the named
section of the exec file to addr. This can be used if the
exec file does not contain section addresses, (such as in the
format), or when the addresses specified in the file
itself are wrong. Each section must be changed separately. The
info files
command, described below, lists all the sections and
their addresses.
info files
info target
info files
and info target
are synonymous; both print the
current target (see Specifying a Debugging Target),
including the names of the executable and core dump files currently in
use by GDB, and the files from which symbols were loaded. The
command help target
lists all possible targets rather than
current ones.
maint info sections
Another command that can give you extra information about program sections
is maint info sections
. In addition to the section information
displayed by info files
, this command displays the flags and file
offset of each section in the executable and core dump files. In addition,
maint info sections
provides the following command options (which
may be arbitrarily combined):
Display sections for all loaded object files, including shared libraries.
Display info only for named sections.
Display info only for sections for which section-flags are true. The section flags that GDB currently knows about are:
Section will have space allocated in the process when loaded. Set for all sections except those containing debug information.
Section will be loaded from the file into the child process memory.
Set for pre-initialized code and data, clear for .bss
Section needs to be relocated before loading.
Section cannot be modified by the child process.
Section contains executable code only.
Section contains data only (no executable code).
Section will reside in ROM.
Section contains data for constructor/destructor lists.
Section is not empty.
An instruction to the linker to not output the section.
A notification to the linker that the section contains COFF shared library information.
Section contains common symbols.
set trust-readonly-sections on
Tell GDB that readonly sections in your object file really are read-only (i.e. that their contents will not change). In that case, GDB can fetch values from these sections out of the object file, rather than from the target program. For some targets (notably embedded ones), this can be a significant enhancement to debugging performance.
The default is off.
set trust-readonly-sections off
Tell GDB not to trust readonly sections. This means that the contents of the section might change while the program is running, and must therefore be fetched from the target when needed.
show trust-readonly-sections
Show the current setting of trusting readonly sections.
All file-specifying commands allow both absolute and relative file names as arguments. GDB always converts the file name to an absolute file name and remembers it that way.
GDB supports GNU/Linux, MS-Windows, SunOS, Darwin/Mach-O, SVr4, IBM RS/6000 AIX, QNX Neutrino, FDPIC (FR-V), and DSBT (TIC6X) shared libraries.
On MS-Windows GDB must be linked with the Expat library to support shared libraries. See Expat.
GDB automatically loads symbol definitions from shared libraries
when you use the run
command, or when you examine a core file.
(Before you issue the run
command, GDB does not understand
references to a function in a shared library, however—unless you are
debugging a core file).
There are times, however, when you may wish to not automatically load symbol definitions from shared libraries, such as when they are particularly large or there are many of them.
To control the automatic loading of shared library symbols, use the commands:
set auto-solib-add mode
If mode is on
, symbols from all shared object libraries
will be loaded automatically when the inferior begins execution, you
attach to an independently started inferior, or when the dynamic linker
informs GDB that a new library has been loaded. If mode
is off
, symbols must be loaded manually, using the
command. The default value is on
If your program uses lots of shared libraries with debug info that takes large amounts of memory, you can decrease the GDB memory footprint by preventing it from automatically loading the symbols from shared libraries. To that end, type set auto-solib-add off before running the inferior, then load each library whose debug symbols you do need with sharedlibrary regexp, where regexp is a regular expression that matches the libraries whose symbols you want to be loaded.
show auto-solib-add
Display the current autoloading mode.
To explicitly load shared library symbols, use the sharedlibrary
info share regex
info sharedlibrary regex
Print the names of the shared libraries which are currently loaded that match regex. If regex is omitted then print all shared libraries that are loaded.
info dll regex
This is an alias of info sharedlibrary
sharedlibrary regex
share regex
Load shared object library symbols for files matching a
Unix regular expression.
As with files loaded automatically, it only loads shared libraries
required by your program for a core file or after typing run
. If
regex is omitted all shared libraries required by your program are
Unload all shared object library symbols. This discards all symbols that have been loaded from all shared libraries. Symbols from shared libraries that were loaded by explicit user requests are not discarded.
Sometimes you may wish that GDB stops and gives you control
when any of shared library events happen. The best way to do this is
to use catch load
and catch unload
(see Set Catchpoints).
GDB also supports the the set stop-on-solib-events
command for this. This command exists for historical reasons. It is
less useful than setting a catchpoint, because it does not allow for
conditions or commands as a catchpoint does.
set stop-on-solib-events
This command controls whether GDB should give you control when the dynamic linker notifies it about some shared library event. The most common event of interest is loading or unloading of a new shared library.
show stop-on-solib-events
Show whether GDB stops and gives you control when shared library events happen.
Shared libraries are also supported in many cross or remote debugging configurations. GDB needs to have access to the target's libraries; this can be accomplished either by providing copies of the libraries on the host system, or by asking GDB to automatically retrieve the libraries from the target. If copies of the target libraries are provided, they need to be the same as the target libraries, although the copies on the target can be stripped as long as the copies on the host are not.
For remote debugging, you need to tell GDB where the target libraries are, so that it can load the correct copies—otherwise, it may try to load the host's libraries. GDB has two variables to specify the search directories for target libraries.
set sysroot path
Use path as the system root for the program being debugged. Any
absolute shared library paths will be prefixed with path; many
runtime loaders store the absolute paths to the shared library in the
target program's memory. When starting processes remotely, and when
attaching to already-running processes (local or remote), their
executable filenames will be prefixed with path if reported to
GDB as absolute by the operating system. If you use
set sysroot
to find executables and shared libraries, they need
to be laid out in the same way that they are on the target, with
e.g. a /bin, /lib and /usr/lib hierarchy under
If path starts with the sequence target: and the target
system is remote then GDB will retrieve the target binaries
from the remote system. This is only supported when using a remote
target that supports the remote get
command (see Sending files to a remote system). The part of path
following the initial target: (if present) is used as system
root prefix on the remote file system. If path starts with the
sequence remote: this is converted to the sequence
target: by set sysroot
14. If you want
to specify a local system root using a directory that happens to be
named target: or remote:, you need to use some
equivalent variant of the name like ./target:.
For targets with an MS-DOS based filesystem, such as MS-Windows and SymbianOS, GDB tries prefixing a few variants of the target absolute file name with path. But first, on Unix hosts, GDB converts all backslash directory separators into forward slashes, because the backslash is not a directory separator on Unix:
c:\foo\bar.dll ⇒ c:/foo/bar.dll
Then, GDB attempts prefixing the target file name with path, and looks for the resulting file name in the host file system:
c:/foo/bar.dll ⇒ /path/to/sysroot/c:/foo/bar.dll
If that does not find the binary, GDB tries removing the ':' character from the drive spec, both for convenience, and, for the case of the host file system not supporting file names with colons:
c:/foo/bar.dll ⇒ /path/to/sysroot/c/foo/bar.dll
This makes it possible to have a system root that mirrors a target with more than one drive. E.g., you may want to setup your local copies of the target system shared libraries like so (note 'c' vs 'z'):
/path/to/sysroot/c/sys/bin/foo.dll /path/to/sysroot/c/sys/bin/bar.dll /path/to/sysroot/z/sys/bin/bar.dll
and point the system root at /path/to/sysroot, so that GDB can find the correct copies of both c:\sys\bin\foo.dll, and z:\sys\bin\bar.dll.
If that still does not find the binary, GDB tries removing the whole drive spec from the target file name:
c:/foo/bar.dll ⇒ /path/to/sysroot/foo/bar.dll
This last lookup makes it possible to not care about the drive name, if you don't want or need to.
The set solib-absolute-prefix
command is an alias for set
You can set the default system root by using the configure-time '--with-sysroot' option. If the system root is inside GDB's configured binary prefix (set with '--prefix' or '--exec-prefix'), then the default system root will be updated automatically if the installed GDB is moved to a new location.
show sysroot
Display the current executable and shared library prefix.
set solib-search-path path
If this variable is set, path is a colon-separated list of directories to search for shared libraries. 'solib-search-path' is used after 'sysroot' fails to locate the library, or if the path to the library is relative instead of absolute. If you want to use 'solib-search-path' instead of 'sysroot', be sure to set 'sysroot' to a nonexistent directory to prevent GDB from finding your host's libraries. 'sysroot' is preferred; setting it to a nonexistent directory may interfere with automatic loading of shared library symbols.
show solib-search-path
Display the current shared library search path.
set target-file-system-kind kind
Set assumed file system kind for target reported file names.
Shared library file names as reported by the target system may not
make sense as is on the system GDB is running on. For
example, when remote debugging a target that has MS-DOS based file
system semantics, from a Unix host, the target may be reporting to
GDB a list of loaded shared libraries with file names such as
c:\Windows\kernel32.dll. On Unix hosts, there's no concept of
drive letters, so the 'c:\' prefix is not normally understood as
indicating an absolute file name, and neither is the backslash
normally considered a directory separator character. In that case,
the native file system would interpret this whole absolute file name
as a relative file name with no directory components. This would make
it impossible to point GDB at a copy of the remote target's
shared libraries on the host using set sysroot
, and impractical
with set solib-search-path
. Setting
to dos-based
tells GDB
to interpret such file names similarly to how the target would, and to
map them to file names valid on GDB's native file system
semantics. The value of kind can be "auto"
, in addition
to one of the supported file system kinds. In that case, GDB
tries to determine the appropriate file system variant based on the
current target's operating system (see Configuring the
Current ABI). The supported file system settings are:
Instruct GDB to assume the target file system is of Unix kind. Only file names starting the forward slash ('/') character are considered absolute, and the directory separator character is also the forward slash.
Instruct GDB to assume the target file system is DOS based. File names starting with either a forward slash, or a drive letter followed by a colon (e.g., 'c:'), are considered absolute, and both the slash ('/') and the backslash ('\\') characters are considered directory separators.
Instruct GDB to use the file system kind associated with the target operating system (see Configuring the Current ABI). This is the default.
When processing file names provided by the user, GDB
frequently needs to compare them to the file names recorded in the
program's debug info. Normally, GDB compares just the
base names of the files as strings, which is reasonably fast
even for very large programs. (The base name of a file is the last
portion of its name, after stripping all the leading directories.)
This shortcut in comparison is based upon the assumption that files
cannot have more than one base name. This is usually true, but
references to files that use symlinks or similar filesystem
facilities violate that assumption. If your program records files
using such facilities, or if you provide file names to GDB
using symlinks etc., you can set basenames-may-differ
to instruct GDB to completely canonicalize each
pair of file names it needs to compare. This will make file-name
comparisons accurate, but at a price of a significant slowdown.
set basenames-may-differ
Set whether a source file may have multiple base names.
show basenames-may-differ
Show whether a source file may have multiple base names.
Next: Separate Debug Files, Previous: Files, Up: GDB Files [Contents][Index]
To speed up file loading, and reduce memory usage, GDB will
reuse the bfd
objects used to track open files. See BFD in The Binary File Descriptor Library. The following commands
allow visibility and control of the caching behavior.
maint info bfds
This prints information about each bfd
object that is known to
maint set bfd-sharing
maint show bfd-sharing
Control whether bfd
objects can be shared. When sharing is
enabled GDB reuses already open bfd
objects rather
than reopening the same file. Turning sharing off does not cause
already shared bfd
objects to be unshared, but all future files
that are opened will create a new bfd
object. Similarly,
re-enabling sharing does not cause multiple existing bfd
objects to be collapsed into a single shared bfd
set debug bfd-cache level
Turns on debugging of the bfd cache, setting the level to level.
show debug bfd-cache
Show the current debugging level of the bfd cache.
Next: MiniDebugInfo, Previous: File Caching, Up: GDB Files [Contents][Index]
GDB allows you to put a program's debugging information in a file separate from the executable itself, in a way that allows GDB to find and load the debugging information automatically. Since debugging information can be very large—sometimes larger than the executable code itself—some systems distribute debugging information for their executables in separate files, which users can install only when they need to debug a problem.
GDB supports two ways of specifying the separate debug info file:
Depending on the way the debug info file is specified, GDB uses two different methods of looking for the debug file:
So, for example, suppose you ask GDB to debug
/usr/bin/ls, which has a debug link that specifies the
file ls.debug, and a build ID whose value in hex is
. If the list of the global debug directories includes
/usr/lib/debug, then GDB will look for the following
debug information files, in the indicated order:
Global debugging info directories default to what is set by GDB configure option --with-separate-debug-dir. During GDB run you can also set the global debugging info directories, and view the list GDB is currently using.
set debug-file-directory directories
Set the directories which GDB searches for separate debugging information files to directory. Multiple path components can be set concatenating them by a path separator.
show debug-file-directory
Show the directories GDB searches for separate debugging information files.
A debug link is a special section of the executable file named
. The section must contain:
Any executable file format can carry a debug link, as long as it can
contain a section named .gnu_debuglink
with the contents
described above.
The build ID is a special section in the executable file (and in other
ELF binary files that GDB may consider). This section is
often named .note.gnu.build-id
, but that name is not mandatory.
It contains unique identification for the built files—the ID remains
the same across multiple builds of the same build tree. The default
algorithm SHA1 produces 160 bits (40 hexadecimal characters) of the
content for the build ID string. The same section with an identical
value is present in the original built binary with symbols, in its
stripped variant, and in the separate debugging information file.
The debugging information file itself should be an ordinary
executable, containing a full set of linker symbols, sections, and
debugging information. The sections of the debugging information file
should have the same names, addresses, and sizes as the original file,
but they need not contain any data—much like a .bss
in an ordinary executable.
The GNU binary utilities (Binutils) package includes the 'objcopy' utility that can produce the separated executable / debugging information file pairs using the following commands:
objcopy --only-keep-debug foo foo.debug strip -g foo
These commands remove the debugging information from the executable file foo and place it in the file foo.debug. You can use the first, second or both methods to link the two files:
objcopy --add-gnu-debuglink=foo.debug foo
Ulrich Drepper's elfutils package, starting with version 0.53, contains
a version of the strip
command such that the command strip foo -f
foo.debug has the same functionality as the two objcopy
commands and
the ln -s
command above, together.
ld --build-id
the GCC counterpart gcc -Wl,--build-id
. Build ID support plus
compatibility fixes for debug files separation are present in GNU binary
utilities (Binutils) package since version 2.18.
The CRC used in .gnu_debuglink
is the CRC-32 defined in
IEEE 802.3 using the polynomial:
x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1
The function is computed byte at a time, taking the least
significant bit of each byte first. The initial pattern
is used, to ensure leading zeros affect the CRC and
the final result is inverted to ensure trailing zeros also affect the
Note: This is the same CRC polynomial as used in handling the
Remote Serial Protocol qCRC
packet (see qCRC packet).
However in the case of the Remote Serial Protocol, the CRC is computed
most significant bit first, and the result is not inverted, so
trailing zeros have no effect on the CRC value.
To complete the description, we show below the code of the function
which produces the CRC used in .gnu_debuglink
. Inverting the
initially supplied crc
argument means that an initial call to
this function passing in zero will start computing the CRC using
unsigned long gnu_debuglink_crc32 (unsigned long crc, unsigned char *buf, size_t len) { static const unsigned long crc32_table[256] = { 0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419, 0x706af48f, 0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4, 0xe0d5e91e, 0x97d2d988, 0x09b64c2b, 0x7eb17cbd, 0xe7b82d07, 0x90bf1d91, 0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de, 0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7, 0x136c9856, 0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9, 0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e, 0xd56041e4, 0xa2677172, 0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b, 0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940, 0x32d86ce3, 0x45df5c75, 0xdcd60dcf, 0xabd13d59, 0x26d930ac, 0x51de003a, 0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423, 0xcfba9599, 0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924, 0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d, 0x76dc4190, 0x01db7106, 0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f, 0x9fbfe4a5, 0xe8b8d433, 0x7807c9a2, 0x0f00f934, 0x9609a88e, 0xe10e9818, 0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01, 0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e, 0x6c0695ed, 0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6, 0x12b7e950, 0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3, 0xfbd44c65, 0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2, 0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb, 0x4369e96a, 0x346ed9fc, 0xad678846, 0xda60b8d0, 0x44042d73, 0x33031de5, 0xaa0a4c5f, 0xdd0d7cc9, 0x5005713c, 0x270241aa, 0xbe0b1010, 0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f, 0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17, 0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad, 0xedb88320, 0x9abfb3b6, 0x03b6e20c, 0x74b1d29a, 0xead54739, 0x9dd277af, 0x04db2615, 0x73dc1683, 0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8, 0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1, 0xf00f9344, 0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb, 0x196c3671, 0x6e6b06e7, 0xfed41b76, 0x89d32be0, 0x10da7a5a, 0x67dd4acc, 0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5, 0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252, 0xd1bb67f1, 0xa6bc5767, 0x3fb506dd, 0x48b2364b, 0xd80d2bda, 0xaf0a1b4c, 0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55, 0x316e8eef, 0x4669be79, 0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236, 0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f, 0xc5ba3bbe, 0xb2bd0b28, 0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31, 0x2cd99e8b, 0x5bdeae1d, 0x9b64c2b0, 0xec63f226, 0x756aa39c, 0x026d930a, 0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713, 0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38, 0x92d28e9b, 0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21, 0x86d3d2d4, 0xf1d4e242, 0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1, 0x18b74777, 0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c, 0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45, 0xa00ae278, 0xd70dd2ee, 0x4e048354, 0x3903b3c2, 0xa7672661, 0xd06016f7, 0x4969474d, 0x3e6e77db, 0xaed16a4a, 0xd9d65adc, 0x40df0b66, 0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9, 0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605, 0xcdd70693, 0x54de5729, 0x23d967bf, 0xb3667a2e, 0xc4614ab8, 0x5d681b02, 0x2a6f2b94, 0xb40bbe37, 0xc30c8ea1, 0x5a05df1b, 0x2d02ef8d }; unsigned char *end; crc = ~crc & 0xffffffff; for (end = buf + len; buf < end; ++buf) crc = crc32_table[(crc ^ *buf) & 0xff] ^ (crc >> 8); return ~crc & 0xffffffff; }
This computation does not apply to the "build ID" method.
Next: Index Files, Previous: Separate Debug Files, Up: GDB Files [Contents][Index]
Some systems ship pre-built executables and libraries that have a special '.gnu_debugdata' section. This feature is called MiniDebugInfo. This section holds an LZMA-compressed object and is used