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   1  =head1 NAME
   2  
   3  perldbmfilter - Perl DBM Filters
   4  
   5  =head1 SYNOPSIS
   6  
   7      $db = tie %hash, 'DBM', ...
   8  
   9      $old_filter = $db->filter_store_key  ( sub { ... } );
  10      $old_filter = $db->filter_store_value( sub { ... } );
  11      $old_filter = $db->filter_fetch_key  ( sub { ... } );
  12      $old_filter = $db->filter_fetch_value( sub { ... } );
  13  
  14  =head1 DESCRIPTION
  15  
  16  The four C<filter_*> methods shown above are available in all the DBM
  17  modules that ship with Perl, namely DB_File, GDBM_File, NDBM_File,
  18  ODBM_File and SDBM_File.
  19  
  20  Each of the methods work identically, and are used to install (or
  21  uninstall) a single DBM Filter. The only difference between them is the
  22  place that the filter is installed.
  23  
  24  To summarise:
  25  
  26  =over 5
  27  
  28  =item B<filter_store_key>
  29  
  30  If a filter has been installed with this method, it will be invoked
  31  every time you write a key to a DBM database.
  32  
  33  =item B<filter_store_value>
  34  
  35  If a filter has been installed with this method, it will be invoked
  36  every time you write a value to a DBM database.
  37  
  38  
  39  =item B<filter_fetch_key>
  40  
  41  If a filter has been installed with this method, it will be invoked
  42  every time you read a key from a DBM database.
  43  
  44  =item B<filter_fetch_value>
  45  
  46  If a filter has been installed with this method, it will be invoked
  47  every time you read a value from a DBM database.
  48  
  49  =back
  50  
  51  You can use any combination of the methods from none to all four.
  52  
  53  All filter methods return the existing filter, if present, or C<undef>
  54  in not.
  55  
  56  To delete a filter pass C<undef> to it.
  57  
  58  =head2 The Filter
  59  
  60  When each filter is called by Perl, a local copy of C<$_> will contain
  61  the key or value to be filtered. Filtering is achieved by modifying
  62  the contents of C<$_>. The return code from the filter is ignored.
  63  
  64  =head2 An Example -- the NULL termination problem.
  65  
  66  DBM Filters are useful for a class of problems where you I<always>
  67  want to make the same transformation to all keys, all values or both.
  68  
  69  For example, consider the following scenario. You have a DBM database
  70  that you need to share with a third-party C application. The C application
  71  assumes that I<all> keys and values are NULL terminated. Unfortunately
  72  when Perl writes to DBM databases it doesn't use NULL termination, so
  73  your Perl application will have to manage NULL termination itself. When
  74  you write to the database you will have to use something like this:
  75  
  76      $hash{"$key\0"} = "$value\0";
  77  
  78  Similarly the NULL needs to be taken into account when you are considering
  79  the length of existing keys/values.
  80  
  81  It would be much better if you could ignore the NULL terminations issue
  82  in the main application code and have a mechanism that automatically
  83  added the terminating NULL to all keys and values whenever you write to
  84  the database and have them removed when you read from the database. As I'm
  85  sure you have already guessed, this is a problem that DBM Filters can
  86  fix very easily.
  87  
  88      use strict;
  89      use warnings;
  90      use SDBM_File;
  91      use Fcntl;
  92  
  93      my %hash;
  94      my $filename = "filt";
  95      unlink $filename;
  96  
  97      my $db = tie(%hash, 'SDBM_File', $filename, O_RDWR|O_CREAT, 0640)
  98        or die "Cannot open $filename: $!\n";
  99  
 100      # Install DBM Filters
 101      $db->filter_fetch_key  ( sub { s/\0$//    } );
 102      $db->filter_store_key  ( sub { $_ .= "\0" } );
 103      $db->filter_fetch_value( 
 104          sub { no warnings 'uninitialized'; s/\0$// } );
 105      $db->filter_store_value( sub { $_ .= "\0" } );
 106  
 107      $hash{"abc"} = "def";
 108      my $a = $hash{"ABC"};
 109      # ...
 110      undef $db;
 111      untie %hash;
 112  
 113  The code above uses SDBM_File, but it will work with any of the DBM
 114  modules.
 115  
 116  Hopefully the contents of each of the filters should be
 117  self-explanatory. Both "fetch" filters remove the terminating NULL,
 118  and both "store" filters add a terminating NULL.
 119  
 120  
 121  =head2 Another Example -- Key is a C int.
 122  
 123  Here is another real-life example. By default, whenever Perl writes to
 124  a DBM database it always writes the key and value as strings. So when
 125  you use this:
 126  
 127      $hash{12345} = "something";
 128  
 129  the key 12345 will get stored in the DBM database as the 5 byte string
 130  "12345". If you actually want the key to be stored in the DBM database
 131  as a C int, you will have to use C<pack> when writing, and C<unpack>
 132  when reading.
 133  
 134  Here is a DBM Filter that does it:
 135  
 136      use strict;
 137      use warnings;
 138      use DB_File;
 139      my %hash;
 140      my $filename = "filt";
 141      unlink $filename;
 142  
 143  
 144      my $db = tie %hash, 'DB_File', $filename, O_CREAT|O_RDWR, 0666, $DB_HASH 
 145        or die "Cannot open $filename: $!\n";
 146  
 147      $db->filter_fetch_key  ( sub { $_ = unpack("i", $_) } );
 148      $db->filter_store_key  ( sub { $_ = pack ("i", $_) } );
 149      $hash{123} = "def";
 150      # ...
 151      undef $db;
 152      untie %hash;
 153  
 154  The code above uses DB_File, but again it will work with any of the
 155  DBM modules.
 156  
 157  This time only two filters have been used -- we only need to manipulate
 158  the contents of the key, so it wasn't necessary to install any value
 159  filters.
 160  
 161  =head1 SEE ALSO
 162  
 163  L<DB_File>, L<GDBM_File>, L<NDBM_File>, L<ODBM_File> and L<SDBM_File>.
 164  
 165  =head1 AUTHOR
 166  
 167  Paul Marquess
 168