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X11R7.4 - man page for perldbmfilter (x11r4 section 1)

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PERLDBMFILTER(1)		 Perl Programmers Reference Guide		 PERLDBMFILTER(1)

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 appli-
       cation 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 = "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} = "something";

       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 = "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
       DB_File, GDBM_File, NDBM_File, ODBM_File and SDBM_File.

AUTHOR
       Paul Marquess

perl v5.8.9				    2007-11-17				 PERLDBMFILTER(1)
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