PERLDBMFILTER(1)Perl Programmers Reference Guide PERLDBMFILTER(1)
perldbmfilter - Perl DBM Filters
$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 { ... } );
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 differ-
ence 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.
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PERLDBMFILTER(1)Perl Programmers Reference Guide PERLDBMFILTER(1)
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 appli-
cation. 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 some-
thing 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 termi-
nations 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 = "filt";
unlink $filename;
my $db = tie(%hash, 'SDBM_File', $filename, O_RDWR|O_CREAT, 0640)
or die "Cannot open $filename: $!\n";
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PERLDBMFILTER(1)Perl Programmers Reference Guide PERLDBMFILTER(1)
# 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 terminat-
ing 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} = "something";
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 = "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;
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PERLDBMFILTER(1)Perl Programmers Reference Guide PERLDBMFILTER(1)
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.
DB_File, GDBM_File, NDBM_File, ODBM_File and SDBM_File.
Paul Marquess
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