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NAME

     perldbmfilter - Perl DBM Filters



SYNOPSIS
         $db = tie %hash, 'DBM', ...

         $old_filter = $db->filter_store_key  ( sub { ... } ) ;
         $old_filter = $db->filter_store_value( sub { ... } ) ;
         $old_filter = $db->filter_fetch_key  ( sub { ... } ) ;
         $old_filter = $db->filter_fetch_value( sub { ... } ) ;




DESCRIPTION
     The four `filter_*' methods shown above are available in all
     the DBM modules that ship with Perl, namely DB_File,
     GDBM_File, NDBM_File, ODBM_File and SDBM_File.

     Each of the methods work identically, and are used to
     install (or uninstall) a single DBM Filter. The only
     difference between them is the place that the filter is
     installed.

     To summarise:

     filter_store_key
          If a filter has been installed with this method, it
          will be invoked every time you write a key to a DBM
          database.

     filter_store_value
          If a filter has been installed with this method, it
          will be invoked every time you write a value to a DBM
          database.

     filter_fetch_key
          If a filter has been installed with this method, it
          will be invoked every time you read a key from a DBM
          database.

     filter_fetch_value
          If a filter has been installed with this method, it
          will be invoked every time you read a value from a DBM
          database.

     You can use any combination of the methods from none to all
     four.

     All filter methods return the existing filter, if present,
     or `undef' in not.

     To delete a filter pass `undef' to it.

     The Filter

     When each filter is called by Perl, a local copy of `$_'
     will contain the key or value to be filtered. Filtering is
     achieved by modifying the contents of `$_'. The return code
     from the filter is ignored.

     An Example -- the NULL termination problem.

     DBM Filters are useful for a class of problems where you
     always want to make the same transformation to all keys, all
     values or both.

     For example, consider the following scenario. You have a DBM
     database that you need to share with a third-party C
     application. The C application assumes that all keys and
     values are NULL terminated. Unfortunately when Perl writes
     to DBM databases it doesn't use NULL termination, so your
     Perl application will have to manage NULL termination
     itself. When you write to the database you will have to use
     something like this:

         $hash{"$key\0"} = "$value\0" ;

     Similarly the NULL needs to be taken into account when you
     are considering the length of existing keys/values.

     It would be much better if you could ignore the NULL
     terminations issue in the main application code and have a
     mechanism that automatically added the terminating NULL to
     all keys and values whenever you write to the database and
     have them removed when you read from the database. As I'm
     sure you have already guessed, this is a problem that DBM
     Filters can fix very easily.

         use strict ;
         use warnings ;
         use SDBM_File ;
         use Fcntl ;

         my %hash ;
         my $filename = "/tmp/filt" ;
         unlink $filename ;

         my $db = tie(%hash, 'SDBM_File', $filename, O_RDWR|O_CREAT, 0640)
           or die "Cannot open $filename: $!\n" ;



         # Install DBM Filters
         $db->filter_fetch_key  ( sub { s/\0$//    } ) ;
         $db->filter_store_key  ( sub { $_ .= "\0" } ) ;
         $db->filter_fetch_value(
             sub { no warnings 'uninitialized' ;s/\0$// } ) ;
         $db->filter_store_value( sub { $_ .= "\0" } ) ;

         $hash{"abc"} = "def" ;
         my $a = $hash{"ABC"} ;
         # ...
         undef $db ;
         untie %hash ;

     The code above uses SDBM_File, but it will work with any of
     the DBM modules.

     Hopefully the contents of each of the filters should be
     self-explanatory. Both "fetch" filters remove the
     terminating NULL, and both "store" filters add a terminating
     NULL.

     Another Example -- Key is a C int.

     Here is another real-life example. By default, whenever Perl
     writes to a DBM database it always writes the key and value
     as strings. So when you use this:

         $hash{12345} = "soemthing" ;

     the key 12345 will get stored in the DBM database as the 5
     byte string "12345". If you actually want the key to be
     stored in the DBM database as a C int, you will have to use
     `pack' when writing, and `unpack' when reading.

     Here is a DBM Filter that does it:

         use strict ;
         use warnings ;
         use DB_File ;
         my %hash ;
         my $filename = "/tmp/filt" ;
         unlink $filename ;

         my $db = tie %hash, 'DB_File', $filename, O_CREAT|O_RDWR, 0666, $DB_HASH
           or die "Cannot open $filename: $!\n" ;

         $db->filter_fetch_key  ( sub { $_ = unpack("i", $_) } ) ;
         $db->filter_store_key  ( sub { $_ = pack ("i", $_) } ) ;
         $hash{123} = "def" ;
         # ...
         undef $db ;
         untie %hash ;

     The code above uses DB_File, but again it will work with any
     of the DBM modules.

     This time only two filters have been used -- we only need to
     manipulate the contents of the key, so it wasn't necessary
     to install any value filters.



SEE ALSO
     the DB_File manpage, the GDBM_File manpage, the NDBM_File
     manpage, the ODBM_File manpage and the SDBM_File manpage.



AUTHOR
     Paul Marquess









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