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kimmel's scratchpad

by kimmel (Beadle)
on Jun 19, 2011 at 18:12 UTC ( #910460=scratchpad: print w/ replies, xml ) Need Help??

working title: Debugging: Inspecting a variable as the value changes

This is a simple tutorial on how to find out when a variable's value changes without needing prior knowledge of the codebase.

It is common practice when writing/debugging/refactoring/messing around with a program that you want to know when a certain variable is set or its value changes. For example I was looking at this piece code
if ( $show_banner == 1 ) { print STDERR "$app_name $version ($app_www)\n"; print STDERR "$copyright $author, $author_email\n\n"; }
and I realized it was never getting executed. I could have just sprinkled in some print $show_banner; statements but this code was written by another programmer so I am not familiar with its structure and flow so I would waste time learning the code to know the best places to put a print statement.

There has to be a faster way to track this variable and I know this is a pretty common problem. So what is the Perl answer you ask? CPAN. There I found the Tie::Trace module.

Tie::Trace allows you to watch a variable and every time the value changes you get a debug message. Here is how I added it to my existing program.

use Tie::Trace qw( watch ); my $show_banner = 1; watch $show_banner;
Then I ran my application and got the following output.
main:: $show_banner => 0 at ./ line 156.
From there I ran the test suite to see if $show_banner changed due to any edge cases. Each test in the test suite produced the same above output.

Now I knew where the value was being changed. From there it was a simple change to an if statement and a rerun of the application test suite to verify no regressions were introduced.

The watch function provided by Tie::Trace does all the heavy lifting by issuing print Dumper statements on the variable as it changes. This is an invaluable tool for following value changes of variables as a program executes.

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