# DB_File.pm -- Perl 5 interface to Berkeley DB
#
# written by Paul Marquess (Paul.Marquess@btinternet.com)
-# last modified 6th March 1999
-# version 1.66
+# last modified 6th June 1999
+# version 1.67
#
-# Copyright (c) 1995-9 Paul Marquess. All rights reserved.
+# Copyright (c) 1995-1999 Paul Marquess. All rights reserved.
# This program is free software; you can redistribute it and/or
# modify it under the same terms as Perl itself.
use Carp;
-$VERSION = "1.66" ;
+$VERSION = "1.67" ;
#typedef enum { DB_BTREE, DB_HASH, DB_RECNO } DBTYPE;
$DB_BTREE = new DB_File::BTREEINFO ;
$a = $X->shift;
$X->unshift(list);
+ # DBM Filters
+ $old_filter = $db->filter_store_key ( sub { ... } ) ;
+ $old_filter = $db->filter_store_value( sub { ... } ) ;
+ $old_filter = $db->filter_fetch_key ( sub { ... } ) ;
+ $old_filter = $db->filter_fetch_value( sub { ... } ) ;
+
untie %hash ;
untie @array ;
=back
+=head1 DBM FILTERS
+
+A DBM Filter is a piece of code that is be used when you I<always>
+want to make the same transformation to all keys and/or values in a
+DBM database.
+
+There are four methods associated with DBM Filters. All work identically,
+and each is used to install (or uninstall) a single DBM Filter. Each
+expects a single parameter, namely a reference to a sub. The only
+difference between them is the place that the filter is installed.
+
+To summarise:
+
+=over 5
+
+=item B<filter_store_key>
+
+If a filter has been installed with this method, it will be invoked
+every time you write a key to a DBM database.
+
+=item B<filter_store_value>
+
+If a filter has been installed with this method, it will be invoked
+every time you write a value to a DBM database.
+
+
+=item B<filter_fetch_key>
+
+If a filter has been installed with this method, it will be invoked
+every time you read a key from a DBM database.
+
+=item B<filter_fetch_value>
+
+If a filter has been installed with this method, it will be invoked
+every time you read a value from a DBM database.
+
+=back
+
+You can use any combination of the methods, from none, to all four.
+
+All filter methods return the existing filter, if present, or C<undef>
+in not.
+
+To delete a filter pass C<undef> to it.
+
+=head2 The Filter
+
+When each filter is called by Perl, a local copy of C<$_> will contain
+the key or value to be filtered. Filtering is achieved by modifying
+the contents of C<$_>. The return code from the filter is ignored.
+
+=head2 An Example -- the NULL termination problem.
+
+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 I<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 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" ;
+
+ # Install DBM Filters
+ $db->filter_fetch_key ( sub { s/\0$// } ) ;
+ $db->filter_store_key ( sub { $_ .= "\0" } ) ;
+ $db->filter_fetch_value( sub { s/\0$// } ) ;
+ $db->filter_store_value( sub { $_ .= "\0" } ) ;
+
+ $hash{"abc"} = "def" ;
+ my $a = $hash{"ABC"} ;
+ # ...
+ undef $db ;
+ untie %hash ;
+
+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.
+
+
+=head2 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} = "soemthing" ;
+
+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 C<pack> when writing, and C<unpack>
+when reading.
+
+Here is a DBM Filter that does it:
+
+ use strict ;
+ 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 ;
+
+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.
+
=head1 HINTS AND TIPS
The vast majority of problems that are reported in this area boil down
to the fact that C strings are NULL terminated, whilst Perl strings are
-not.
+not. See L<DBM FILTERS> for a generic way to work around this problem.
Here is a real example. Netscape 2.0 keeps a record of the locations you
visit along with the time you last visited them in a DB_HASH database.
Although it might seem like a real pain, it is really worth the effort
of having a C<use strict> in all your scripts.
+=head1 REFERENCES
+
+Articles that are either about B<DB_File> or make use of it.
+
+=over 5
+
+=item 1.
+
+I<Full-Text Searching in Perl>, Tim Kientzle (tkientzle@ddj.com),
+Dr. Dobb's Journal, Issue 295, January 1999, pp 34-41
+
+=back
+
=head1 HISTORY
Moved to the Changes file.
This version of B<DB_File> will work with either version 1.x or 2.x of
Berkeley DB, but is limited to the functionality provided by version 1.
-The official web site for Berkeley DB is
-F<http://www.sleepycat.com/db>. The ftp equivalent is
-F<ftp.sleepycat.com:/pub>. Both versions 1 and 2 of Berkeley DB are
-available there.
+The official web site for Berkeley DB is F<http://www.sleepycat.com>.
+Both versions 1 and 2 of Berkeley DB are available there.
Alternatively, Berkeley DB version 1 is available at your nearest CPAN
archive in F<src/misc/db.1.85.tar.gz>.
=head1 COPYRIGHT
-Copyright (c) 1995-9 Paul Marquess. All rights reserved. This program
+Copyright (c) 1995-1999 Paul Marquess. All rights reserved. This program
is free software; you can redistribute it and/or modify it under the
same terms as Perl itself.
projects / p5sagit/p5-mst-13.2.git / blobdiff