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 DBM
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 DBM 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.¶
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 application. 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 something 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 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'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";
# 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 terminating 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;
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.
SEE ALSO¶
DB_File, GDBM_File, NDBM_File, ODBM_File and SDBM_File.
AUTHOR¶
Paul Marquess