Perl ships supporting four types of DBM files (which ones will be
compiled into your perl depends on your platforms availability when
you built perl). These are: NDBM, DB_File, GDBM, and SDBM. DB_File is
the most flexible (so we will use it here), but NDBM is widespread (was
used with perl 4) and SDBM libraries ship with perl. You always have at
least one possibility to use.
Tieing a hash to a DBM is quite simple -- you merely 'use' the
appropriate DB module, declare a hash to tie, and call the tie()
function to complete the process of associating the hash with the DBM
file.
#!/usr/bin/perl -w
use strict;
use DB_File; # or NDBM_File, GDBM_File, SDBM_File, AnyDBM_File
my %dbase;
my $file = 'phones.db';
tie(%dbase, 'DB_File', $file) || die "can't open $file: $!";
$dbase{andrew} = '555-1234'; # not really
$dbase{john} = '555-4321';
foreach my $key (keys %dbase) {
print "$key: $dbase{$key}\n";
}
untie(%dbase);
__END__
The call to tie() takes 3 arguments: the hash to tie; the classname of
the DB module, and the filename for the database (additional arguments
can set flags and permissions on the database file see the
documentation for details).
The above example defines a hash and then tie()'s it to a DB_File DBM
file. We then enter two key/value pairs (names and phone numbers) and
iterate over the hash. The big deal is, if you run this program and
then comment out the two lines assigning to the hash, you'll find that
it still prints out the same data because the data persists in the DBM
file 'phones.db'. We now have a persistent hash -- it works just like
any other hash, but it is stored on disk as well.
One thing that I need to mention is how we iterate over the hash. In
the above example we simply used the keys() function to iterate over
the list of keys in the hash. If this were a large database, this could
take more memory than you desire. A better way to iterate over the hash
is by using the each() function, which returns key/value pairs
sequentially (without building the entire list in memory):
while( my($k, $v) = each(%dbase) ) {
print "$k: $v\n";
}
You can use this mechanism for all sorts of persistent data. Netscape
maintains its history list of recently visited websites between
invocations by using such a mechanism. You could build an address book
application using DBM files, or you could store configuration
information for your application.
The DB_File module also allows you to tie() a hash to a BTREE structure
(which can maintain an order on your keys) or to tie a hash to a flat
text file using line numbers as keys for each line (record) in the file.
For further information see the following documentation:
perldoc -f tie
perldoc AnyDBM_File
perldoc DB_File
Next Week: Sorting in Perl
