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.\" ========================================================================
.\"
.IX Title "PERLDBMFILTER 1"
.TH PERLDBMFILTER 1 "2019-10-24" "perl v5.30.2" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.nh
.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 = $db\->filter_fetch_value( sub { ... } );
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
The four \f(CW\*(C`filter_*\*(C'\fR methods shown above are available in all the \s-1DBM\s0
modules that ship with Perl, namely DB_File, GDBM_File, NDBM_File,
ODBM_File and SDBM_File.
.PP
Each of the methods works identically, and is used to install (or
uninstall) a single \s-1DBM\s0 Filter. The only difference between them is the
place that the filter is installed.
.PP
To summarise:
.IP "\fBfilter_store_key\fR" 5
.IX Item "filter_store_key"
If a filter has been installed with this method, it will be invoked
every time you write a key to a \s-1DBM\s0 database.
.IP "\fBfilter_store_value\fR" 5
.IX Item "filter_store_value"
If a filter has been installed with this method, it will be invoked
every time you write a value to a \s-1DBM\s0 database.
.IP "\fBfilter_fetch_key\fR" 5
.IX Item "filter_fetch_key"
If a filter has been installed with this method, it will be invoked
every time you read a key from a \s-1DBM\s0 database.
.IP "\fBfilter_fetch_value\fR" 5
.IX Item "filter_fetch_value"
If a filter has been installed with this method, it will be invoked
every time you read a value from a \s-1DBM\s0 database.
.PP
You can use any combination of the methods from none to all four.
.PP
All filter methods return the existing filter, if present, or \f(CW\*(C`undef\*(C'\fR
if not.
.PP
To delete a filter pass \f(CW\*(C`undef\*(C'\fR to it.
.SS "The Filter"
.IX Subsection "The Filter"
When each filter is called by Perl, a local copy of \f(CW$_\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 \s-1NULL\s0 termination problem."
.IX Subsection "An Example: the NULL termination problem."
\&\s-1DBM\s0 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 \s-1DBM\s0 database
that you need to share with a third-party C application. The C application
assumes that \fIall\fR keys and values are \s-1NULL\s0 terminated. Unfortunately
when Perl writes to \s-1DBM\s0 databases it doesn't use \s-1NULL\s0 termination, so
your Perl application will have to manage \s-1NULL\s0 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 \s-1NULL\s0 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 \s-1NULL\s0 terminations issue
in the main application code and have a mechanism that automatically
added the terminating \s-1NULL\s0 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 \s-1DBM\s0 Filters can
fix very easily.
.PP
.Vb 4
\&    use strict;
\&    use warnings;
\&    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 $a = $hash{"ABC"};
\&    # ...
\&    undef $db;
\&    untie %hash;
.Ve
.PP
The code above uses SDBM_File, but it will work with any of the \s-1DBM\s0
modules.
.PP
Hopefully the contents of each of the filters should be
self-explanatory. Both \*(L"fetch\*(R" filters remove the terminating \s-1NULL,\s0
and both \*(L"store\*(R" filters add a terminating \s-1NULL.\s0
.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 \s-1DBM\s0 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 \s-1DBM\s0 database as the 5 byte string
\&\*(L"12345\*(R". If you actually want the key to be stored in the \s-1DBM\s0 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 \s-1DBM\s0 Filter that does it:
.PP
.Vb 6
\&    use strict;
\&    use warnings;
\&    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
\&\s-1DBM\s0 modules.
.PP
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.
.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