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.IX Title "PERLDBMFILTER 1"
.TH PERLDBMFILTER 1 2026-01-18 "perl v5.42.2" "Perl Programmers Reference Guide"
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.SH NAME
perldbmfilter \- Perl DBM Filters
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 1
\&    $db = tie %hash, \*(AqDBM\*(Aq, ...
\&
\&    $old_filter = $db\->filter_store_key  ( sub { ... } );
\&    $old_filter = $db\->filter_store_value( sub { ... } );
\&    $old_filter = $db\->filter_fetch_key  ( sub { ... } );
\&    $old_filter = $db\->filter_fetch_value( sub { ... } );
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
The four \f(CW\*(C`filter_*\*(C'\fR methods shown above are available in all the DBM
modules that ship with Perl, namely DB_File, GDBM_File, NDBM_File,
ODBM_File and SDBM_File.
.PP
Each of the methods works identically, and is used to install (or
uninstall) a single DBM Filter. The only difference between them is the
place that the filter is installed.
.PP
To summarise:
.IP \fBfilter_store_key\fR 5
.IX Item "filter_store_key"
If a filter has been installed with this method, it will be invoked
every time you write a key to a DBM database.
.IP \fBfilter_store_value\fR 5
.IX Item "filter_store_value"
If a filter has been installed with this method, it will be invoked
every time you write a value to a DBM database.
.IP \fBfilter_fetch_key\fR 5
.IX Item "filter_fetch_key"
If a filter has been installed with this method, it will be invoked
every time you read a key from a DBM database.
.IP \fBfilter_fetch_value\fR 5
.IX Item "filter_fetch_value"
If a filter has been installed with this method, it will be invoked
every time you read a value from a DBM database.
.PP
You can use any combination of the methods from none to all four.
.PP
All filter methods return the existing filter, if present, or \f(CW\*(C`undef\*(C'\fR
if not.
.PP
To delete a filter pass \f(CW\*(C`undef\*(C'\fR to it.
.SS "The Filter"
.IX Subsection "The Filter"
When each filter is called by Perl, a local copy of \f(CW$_\fR will contain
the key or value to be filtered. Filtering is achieved by modifying
the contents of \f(CW$_\fR. The return code from the filter is ignored.
.SS "An Example: the NULL termination problem"
.IX Subsection "An Example: the NULL termination problem"
DBM Filters are useful for a class of problems where you \fIalways\fR
want to make the same transformation to all keys, all values or both.
.PP
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 \fIall\fR keys and values are NULL terminated. Unfortunately
when Perl writes to DBM databases it doesn\*(Aqt 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:
.PP
.Vb 1
\&    $hash{"$key\e0"} = "$value\e0";
.Ve
.PP
Similarly the NULL needs to be taken into account when you are considering
the length of existing keys/values.
.PP
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\*(Aqm
sure you have already guessed, this is a problem that DBM Filters can
fix very easily.
.PP
.Vb 3
\&    use v5.36;
\&    use SDBM_File;
\&    use Fcntl;
\&
\&    my %hash;
\&    my $filename = "filt";
\&    unlink $filename;
\&
\&    my $db = tie(%hash, \*(AqSDBM_File\*(Aq, $filename, O_RDWR|O_CREAT, 0640)
\&      or die "Cannot open $filename: $!\en";
\&
\&    # Install DBM Filters
\&    $db\->filter_fetch_key  ( sub { s/\e0$//    } );
\&    $db\->filter_store_key  ( sub { $_ .= "\e0" } );
\&    $db\->filter_fetch_value( 
\&        sub { no warnings \*(Aquninitialized\*(Aq; s/\e0$// } );
\&    $db\->filter_store_value( sub { $_ .= "\e0" } );
\&
\&    $hash{"abc"} = "def";
\&    my $x = $hash{"ABC"};
\&    # ...
\&    undef $db;
\&    untie %hash;
.Ve
.PP
The code above uses SDBM_File, but it will work with any of the DBM
modules.
.PP
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.
.SS "Another Example: Key is a C int"
.IX Subsection "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:
.PP
.Vb 1
\&    $hash{12345} = "something";
.Ve
.PP
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 \f(CW\*(C`pack\*(C'\fR when writing, and \f(CW\*(C`unpack\*(C'\fR
when reading.
.PP
Here is a DBM Filter that does it:
.PP
.Vb 5
\&    use v5.36;
\&    use DB_File;
\&    my %hash;
\&    my $filename = "filt";
\&    unlink $filename;
\&
\&
\&    my $db = tie %hash, \*(AqDB_File\*(Aq, $filename, O_CREAT|O_RDWR, 0666,
\&        $DB_HASH or die "Cannot open $filename: $!\en";
\&
\&    $db\->filter_fetch_key  ( sub { $_ = unpack("i", $_) } );
\&    $db\->filter_store_key  ( sub { $_ = pack ("i", $_) } );
\&    $hash{123} = "def";
\&    # ...
\&    undef $db;
\&    untie %hash;
.Ve
.PP
The code above uses DB_File, but again it will work with any of the
DBM modules.
.PP
This time only two filters have been used; we only need to manipulate
the contents of the key, so it wasn\*(Aqt necessary to install any value
filters.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
DB_File, GDBM_File, NDBM_File, ODBM_File and SDBM_File.
.SH AUTHOR
.IX Header "AUTHOR"
Paul Marquess