perldbmfilter (1)

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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 modules that ship with Perl, namely DB_File, GDBM_File, NDBM_File, ODBM_File and SDBM_File.

Each of the methods works identically, and is used to install (or uninstall) a single

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" if 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.

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

database that you need to share with a third-party C application. The C application assumes that all keys and values are terminated. Unfortunately when Perl writes to databases it doesn't use termination, so your Perl application will have to manage termination itself. When you write to the database you will have to use something like this:

    $hash{"$key\0"} = "$value\0";

Similarly the

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

terminations issue in the main application code and have a mechanism that automatically added the terminating 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 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

modules.

Hopefully the contents of each of the filters should be self-explanatory. Both ``fetch'' filters remove the terminating

and both ``store'' filters add a terminating

Another Example: Key is a C int.

Here is another real-life example. By default, whenever Perl writes to a 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

database as the 5 byte string ``12345''. If you actually want the key to be stored in the database as a C int, you will have to use "pack" when writing, and "unpack" when reading.

Here is a

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

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