use DBM_Filter ;
use SDBM_File; # or DB_File, GDBM_File, NDBM_File, or ODBM_File
$db = tie %hash, ...
$db->Filter_Push(Fetch => sub {...},
Store => sub {...});
$db->Filter_Push('my_filter1');
$db->Filter_Push('my_filter2', params...);
$db->Filter_Key_Push(...) ;
$db->Filter_Value_Push(...) ;
$db->Filter_Pop();
$db->Filtered();
package DBM_Filter::my_filter1;
sub Store { ... }
sub Fetch { ... }
1;
package DBM_Filter::my_filter2;
sub Filter
{
my @opts = @_;
...
return (
sub Store { ... },
sub Fetch { ... } );
}
1;
$some_hash{"abc"} = 42;
could potentially trigger two filters, one for the writing of the key ``abc'' and another for writing the value 42. Similarly, this snippet
my ($key, $value) = each %some_hash
will trigger two filters, one for the reading of the key and one for the reading of the value.
Like the existing DBM Filter functionality, this module arranges for the $_ variable to be populated with the key or value that a filter will check. This usually means that most DBM filters tend to be very short.
my $db = tie %hash, 'SDBM_File', ...
$db->Filter_Push( Store => sub { },
Fetch => sub { });
The code reference associated with "Store" will be called before any key/value is written to the database and the code reference associated with "Fetch" will be called after any key/value is read from the database.
For example, here is a sample filter that adds a trailing NULL character to all strings before they are written to the DBM file, and removes the trailing NULL when they are read from the DBM file
my $db = tie %hash, 'SDBM_File', ...
$db->Filter_Push( Store => sub { $_ .= "\x00" ; },
Fetch => sub { s/\x00$// ; });
Points to note:
The usage is for a canned filter is:
$db->Filter_Push("name", params)
where
DBM_Filter::null
DBM_Filter::utf8
The module that implements the canned filter can take one of two forms. Here is a template for the first
package DBM_Filter::null ;
use strict;
use warnings;
sub Store
{
# store code here
}
sub Fetch
{
# fetch code here
}
1;
Notes:
The second form allows the filter to hold state information using a closure, thus:
package DBM_Filter::encoding ;
use strict;
use warnings;
sub Filter
{
my @params = @_ ;
...
return {
Store => sub { $_ = $encoding->encode($_) },
Fetch => sub { $_ = $encoding->decode($_) }
} ;
}
1;
In this instance the ``Store'' and ``Fetch'' methods are encapsulated inside a ``Filter'' method.
The filter included are:
This module will ensure that all data written to the DBM will be encoded in UTF-8.
This module needs the Encode module.
Allows you to choose the character encoding will be store in the DBM file.
This filter will compress all data before it is written to the database and uncompressed it on reading.
This module needs Compress::Zlib.
This module is used when interoperating with a C/C++ application that uses a C int as either the key and/or value in the DBM file.
This module ensures that all data written to the DBM file is null terminated. This is useful when you have a perl script that needs to interoperate with a DBM file that a C program also uses. A fairly common issue is for the C application to include the terminating null in a string when it writes to the DBM file. This filter will ensure that all data written to the DBM file can be read by the C application.
If you don't provide an exact inverse transformation, you will find that code like this will not behave as you expect.
while (my ($k, $v) = each %hash)
{
...
}
Depending on the transformation, you will find that one or more of the following will happen
my $db = tie %hash, 'SDBM_File', ...
$db->Filter_Key_Push('int32') ;
$db->Filter_Value_Push('utf8');
$db->Filter_Value_Push('null');