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           $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 { ... } ) ;


       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 dif­
       ference between them is the place that the filter is

       To summarise:

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

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

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

            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
       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 ter­
       minations 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 { $_ .= "\0" } ) ;
               sub { no warnings 'uninitialized' ;s/\0$// } ) ;
           $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 termi­
       nating NULL, and both "store" filters add a terminating

       Another Example -- Key is a C int.

           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 neces­
       sary to install any value filters.


       DB_File, GDBM_File, NDBM_File, ODBM_File and SDBM_File.


       Paul Marquess

perl v5.8.1                 2003-09-02           PERLDBMFILTER(1)

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