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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 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} = "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 = "/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

Index for Section 1

Alphabetical listing for P

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