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perldbmfilter ()
  • >> perldbmfilter (1) ( Solaris man: Команды и прикладные программы пользовательского уровня )
  • perldbmfilter (1) ( Разные man: Команды и прикладные программы пользовательского уровня )
  • 
    
    

    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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