|
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 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 in not.
To delete a filter pass undef to it.
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.
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 = "/tmp/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.
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 = "/tmp/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.
DB_File, GDBM_File, NDBM_File, ODBM_File and SDBM_File.
Paul Marquess
|
|
