3 perldbmfilter - Perl DBM Filters
7 $db = tie %hash, 'DBM', ...
9 $old_filter = $db->filter_store_key ( sub { ... } );
10 $old_filter = $db->filter_store_value( sub { ... } );
11 $old_filter = $db->filter_fetch_key ( sub { ... } );
12 $old_filter = $db->filter_fetch_value( sub { ... } );
16 The four C<filter_*> methods shown above are available in all the DBM
17 modules that ship with Perl, namely DB_File, GDBM_File, NDBM_File,
18 ODBM_File and SDBM_File.
20 Each of the methods work identically, and are used to install (or
21 uninstall) a single DBM Filter. The only difference between them is the
22 place that the filter is installed.
28 =item B<filter_store_key>
30 If a filter has been installed with this method, it will be invoked
31 every time you write a key to a DBM database.
33 =item B<filter_store_value>
35 If a filter has been installed with this method, it will be invoked
36 every time you write a value to a DBM database.
39 =item B<filter_fetch_key>
41 If a filter has been installed with this method, it will be invoked
42 every time you read a key from a DBM database.
44 =item B<filter_fetch_value>
46 If a filter has been installed with this method, it will be invoked
47 every time you read a value from a DBM database.
51 You can use any combination of the methods from none to all four.
53 All filter methods return the existing filter, if present, or C<undef>
56 To delete a filter pass C<undef> to it.
60 When each filter is called by Perl, a local copy of C<$_> will contain
61 the key or value to be filtered. Filtering is achieved by modifying
62 the contents of C<$_>. The return code from the filter is ignored.
64 =head2 An Example -- the NULL termination problem.
66 DBM Filters are useful for a class of problems where you I<always>
67 want to make the same transformation to all keys, all values or both.
69 For example, consider the following scenario. You have a DBM database
70 that you need to share with a third-party C application. The C application
71 assumes that I<all> keys and values are NULL terminated. Unfortunately
72 when Perl writes to DBM databases it doesn't use NULL termination, so
73 your Perl application will have to manage NULL termination itself. When
74 you write to the database you will have to use something like this:
76 $hash{"$key\0"} = "$value\0";
78 Similarly the NULL needs to be taken into account when you are considering
79 the length of existing keys/values.
81 It would be much better if you could ignore the NULL terminations issue
82 in the main application code and have a mechanism that automatically
83 added the terminating NULL to all keys and values whenever you write to
84 the database and have them removed when you read from the database. As I'm
85 sure you have already guessed, this is a problem that DBM Filters can
94 my $filename = "filt";
97 my $db = tie(%hash, 'SDBM_File', $filename, O_RDWR|O_CREAT, 0640)
98 or die "Cannot open $filename: $!\n";
100 # Install DBM Filters
101 $db->filter_fetch_key ( sub { s/\0$// } );
102 $db->filter_store_key ( sub { $_ .= "\0" } );
103 $db->filter_fetch_value(
104 sub { no warnings 'uninitialized'; s/\0$// } );
105 $db->filter_store_value( sub { $_ .= "\0" } );
107 $hash{"abc"} = "def";
108 my $a = $hash{"ABC"};
113 The code above uses SDBM_File, but it will work with any of the DBM
116 Hopefully the contents of each of the filters should be
117 self-explanatory. Both "fetch" filters remove the terminating NULL,
118 and both "store" filters add a terminating NULL.
121 =head2 Another Example -- Key is a C int.
123 Here is another real-life example. By default, whenever Perl writes to
124 a DBM database it always writes the key and value as strings. So when
127 $hash{12345} = "something";
129 the key 12345 will get stored in the DBM database as the 5 byte string
130 "12345". If you actually want the key to be stored in the DBM database
131 as a C int, you will have to use C<pack> when writing, and C<unpack>
134 Here is a DBM Filter that does it:
140 my $filename = "filt";
144 my $db = tie %hash, 'DB_File', $filename, O_CREAT|O_RDWR, 0666, $DB_HASH
145 or die "Cannot open $filename: $!\n";
147 $db->filter_fetch_key ( sub { $_ = unpack("i", $_) } );
148 $db->filter_store_key ( sub { $_ = pack ("i", $_) } );
154 The code above uses DB_File, but again it will work with any of the
157 This time only two filters have been used -- we only need to manipulate
158 the contents of the key, so it wasn't necessary to install any value
163 L<DB_File>, L<GDBM_File>, L<NDBM_File>, L<ODBM_File> and L<SDBM_File>.