1 DBM_Filter(3) Perl Programmers Reference Guide DBM_Filter(3) NNAAMMEE DBM_Filter -- Filter DBM keys/values SSYYNNOOPPSSIISS use DBM_Filter ; use SDBM_File; # or DB_File, or GDBM_File, or NDBM_File, or ODBM_File $db = tie %hash, ... $db->Filter_Push(Fetch => sub {...}, Store => sub {...}); $db->Filter_Push('my_filter1'); $db->Filter_Push('my_filter2', params...); $db->Filter_Key_Push(...) ; $db->Filter_Value_Push(...) ; $db->Filter_Pop(); $db->Filtered(); package DBM_Filter::my_filter1; sub Store { ... } sub Fetch { ... } 1; package DBM_Filter::my_filter2; sub Filter { my @opts = @_; ... return ( sub Store { ... }, sub Fetch { ... } ); } 1; DDEESSCCRRIIPPTTIIOONN This module provides an interface that allows filters to be applied to tied Hashes associated with DBM files. It builds on the DBM Filter hooks that are present in all the *DB*_File modules included with the standard Perl source distribution from version 5.6.1 onwards. In addi- tion to the *DB*_File modules distributed with Perl, the BerkeleyDB module, available on CPAN, supports the DBM Filter hooks. See perldbm- filter for more details on the DBM Filter hooks. WWhhaatt iiss aa DDBBMM FFiilltteerr?? A DBM Filter allows the keys and/or values in a tied hash to be modi- fied by some user-defined code just before it is written to the DBM file and just after it is read back from the DBM file. For example, this snippet of code $some_hash{"abc"} = 42; could potentially trigger two filters, one for the writing of the key "abc" and another for writing the value 42. Similarly, this snippet my ($key, $value) = each %some_hash will trigger two filters, one for the reading of the key and one for the reading of the value. Like the existing DBM Filter functionality, this module arranges for the $_ variable to be populated with the key or value that a filter will check. This usually means that most DBM filters tend to be very short. SSoo wwhhaatt''ss nneeww?? The main enhancements over the standard DBM Filter hooks are: +o A cleaner interface. +o The ability to easily apply multiple filters to a single DBM file. +o The ability to create "canned" filters. These allow commonly used filters to be packaged into a stand-alone module. MMEETTHHOODDSS This module will arrange for the following methods to be available via the object returned from the "tie" call. $$ddbb-->>_F_i_l_t_e_r___P_u_s_h_(_) $$ddbb-->>_F_i_l_t_e_r___K_e_y___P_u_s_h_(_) $$ddbb-->>_F_i_l_t_e_r___V_a_l_u_e___P_u_s_h_(_) Add a filter to filter stack for the database, $db. The three formats vary only in whether they apply to the DBM key, the DBM value or both. Filter_Push The filter is applied to _b_o_t_h keys and values. Filter_Key_Push The filter is applied to the key _o_n_l_y. Filter_Value_Push The filter is applied to the value _o_n_l_y. $$ddbb-->>_F_i_l_t_e_r___P_o_p_(_) Removes the last filter that was applied to the DBM file associated with $db, if present. $$ddbb-->>_F_i_l_t_e_r_e_d_(_) Returns TRUE if there are any filters applied to the DBM associated with $db. Otherwise returns FALSE. WWrriittiinngg aa FFiilltteerr Filters can be created in two main ways IImmmmeeddiiaattee FFiilltteerrss An immediate filter allows you to specify the filter code to be used at the point where the filter is applied to a dbm. In this mode the Fil- ter_*_Push methods expects to receive exactly two parameters. my $db = tie %hash, 'SDBM_File', ... $db->Filter_Push( Store => sub { }, Fetch => sub { }); The code reference associated with "Store" will be called before any key/value is written to the database and the code reference associated with "Fetch" will be called after any key/value is read from the data- base. For example, here is a sample filter that adds a trailing NULL charac- ter to all strings before they are written to the DBM file, and removes the trailing NULL when they are read from the DBM file my $db = tie %hash, 'SDBM_File', ... $db->Filter_Push( Store => sub { $_ .= "\x00" ; }, Fetch => sub { s/\x00$// ; }); Points to note: 1. Both the Store and Fetch filters manipulate $_. CCaannnneedd FFiilltteerrss Immediate filters are useful for one-off situations. For more generic problems it can be useful to package the filter up in its own module. The usage is for a canned filter is: $db->Filter_Push("name", params) where "name" is the name of the module to load. If the string specified does not contain the package separator characters "::", it is assumed to refer to the full module name "DBM_Filter::name". This means that the full names for canned filters, "null" and "utf8", included with this module are: DBM_Filter::null DBM_Filter::utf8 params any optional parameters that need to be sent to the filter. See the encode filter for an example of a module that uses parameters. The module that implements the canned filter can take one of two forms. Here is a template for the first package DBM_Filter::null ; use strict; use warnings; sub Store { # store code here } sub Fetch { # fetch code here } 1; Notes: 1. The package name uses the "DBM_Filter::" prefix. 2. The module _m_u_s_t have both a Store and a Fetch method. If only one is present, or neither are present, a fatal error will be thrown. The second form allows the filter to hold state information using a closure, thus: package DBM_Filter::encoding ; use strict; use warnings; sub Filter { my @params = @_ ; ... return { Store => sub { $_ = $encoding->encode($_) }, Fetch => sub { $_ = $encoding->decode($_) } } ; } 1; In this instance the "Store" and "Fetch" methods are encapsulated inside a "Filter" method. FFiilltteerrss IInncclluuddeedd A number of canned filers are provided with this module. They cover a number of the main areas that filters are needed when interfacing with DBM files. They also act as templates for your own filters. The filter included are: * utf8 This module will ensure that all data written to the DBM will be encoded in UTF-8. This module needs the Encode module. * encode Allows you to choose the character encoding will be store in the DBM file. * compress This filter will compress all data before it is written to the database and uncompressed it on reading. This module needs Compress::Zlib. * int32 This module is used when interoperating with a C/C++ application that uses a C int as either the key and/or value in the DBM file. * null This module ensures that all data written to the DBM file is null terminated. This is useful when you have a perl script that needs to interoperate with a DBM file that a C program also uses. A fairly common issue is for the C application to include the termi- nating null in a string when it writes to the DBM file. This fil- ter will ensure that all data written to the DBM file can be read by the C application. NNOOTTEESS MMaaiinnttaaiinn RRoouunndd TTrriipp IInntteeggrriittyy When writing a DBM filter it is _v_e_r_y important to ensure that it is possible to retrieve all data that you have written when the DBM filter is in place. In practice, this means that whatever transformation is applied to the data in the Store method, the _e_x_a_c_t inverse operation should be applied in the Fetch method. If you don't provide an exact inverse transformation, you will find that code like this will not behave as you expect. while (my ($k, $v) = each %hash) { ... } Depending on the transformation, you will find that one or more of the following will happen 1 The loop will never terminate. 2 Too few records will be retrieved. 3 Too many will be retrieved. 4 The loop will do the right thing for a while, but it will unex- pectedly fail. DDoonn''tt mmiixx ffiilltteerreedd && nnoonn--ffiilltteerreedd ddaattaa iinn tthhee ssaammee ddaattaabbaassee ffiillee.. This is just a restatement of the previous section. Unless you are com- pletely certain you know what you are doing, avoid mixing filtered & non-filtered data. EEXXAAMMPPLLEE Say you need to interoperate with a legacy C application that stores keys as C ints and the values and null terminated UTF-8 strings. Here is how you would set that up my $db = tie %hash, 'SDBM_File', ... $db->Filter_Key_Push('int32') ; $db->Filter_Value_Push('utf8'); $db->Filter_Value_Push('null'); SSEEEE AALLSSOO , GDBM_File, NDBM_File, ODBM_File, SDBM_File, perldbmfilter AAUUTTHHOORR Paul Marquess perl v5.8.8 2006-06-14 DBM_Filter(3)