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1 | =head1 NAME |
2 | ||
3 | perldbmfilter - Perl DBM Filters | |
4 | ||
5 | =head1 SYNOPSIS | |
6 | ||
7 | $db = tie %hash, 'DBM', ... | |
8 | ||
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 = $db->filter_fetch_value( sub { ... } ) ; | |
13 | ||
14 | =head1 DESCRIPTION | |
15 | ||
16 | The four C<filter_*> methods shown above are available in all the DBM | |
17 | modules that ship with Perl, namely DB_File, GDBM_File, NDBM_File, | |
18 | ODBM_File and SDBM_File. | |
19 | ||
20 | Each of the methods work identically, and are used to install (or | |
21 | uninstall) a single DBM Filter. The only difference between them is the | |
22 | place that the filter is installed. | |
23 | ||
24 | To summarise: | |
25 | ||
26 | =over 5 | |
27 | ||
28 | =item B<filter_store_key> | |
29 | ||
30 | If a filter has been installed with this method, it will be invoked | |
31 | every time you write a key to a DBM database. | |
32 | ||
33 | =item B<filter_store_value> | |
34 | ||
35 | If a filter has been installed with this method, it will be invoked | |
36 | every time you write a value to a DBM database. | |
37 | ||
38 | ||
39 | =item B<filter_fetch_key> | |
40 | ||
41 | If a filter has been installed with this method, it will be invoked | |
42 | every time you read a key from a DBM database. | |
43 | ||
44 | =item B<filter_fetch_value> | |
45 | ||
46 | If a filter has been installed with this method, it will be invoked | |
47 | every time you read a value from a DBM database. | |
48 | ||
49 | =back | |
50 | ||
51 | You can use any combination of the methods from none to all four. | |
52 | ||
53 | All filter methods return the existing filter, if present, or C<undef> | |
54 | in not. | |
55 | ||
56 | To delete a filter pass C<undef> to it. | |
57 | ||
58 | =head2 The Filter | |
59 | ||
60 | When each filter is called by Perl, a local copy of C<$_> 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. | |
63 | ||
64 | =head2 An Example -- the NULL termination problem. | |
65 | ||
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. | |
68 | ||
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: | |
75 | ||
76 | $hash{"$key\0"} = "$value\0" ; | |
77 | ||
78 | Similarly the NULL needs to be taken into account when you are considering | |
79 | the length of existing keys/values. | |
80 | ||
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 | |
86 | fix very easily. | |
87 | ||
88 | use strict ; | |
89 | use warnings ; | |
90 | use SDBM_File ; | |
91 | use Fcntl ; | |
92 | ||
93 | my %hash ; | |
94 | my $filename = "/tmp/filt" ; | |
95 | unlink $filename ; | |
96 | ||
97 | my $db = tie(%hash, 'SDBM_File', $filename, O_RDWR|O_CREAT, 0640) | |
98 | or die "Cannot open $filename: $!\n" ; | |
99 | ||
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" } ) ; | |
106 | ||
107 | $hash{"abc"} = "def" ; | |
108 | my $a = $hash{"ABC"} ; | |
109 | # ... | |
110 | undef $db ; | |
111 | untie %hash ; | |
112 | ||
113 | The code above uses SDBM_File, but it will work with any of the DBM | |
114 | modules. | |
115 | ||
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. | |
119 | ||
120 | ||
121 | =head2 Another Example -- Key is a C int. | |
122 | ||
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 | |
125 | you use this: | |
126 | ||
127 | $hash{12345} = "something" ; | |
128 | ||
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> | |
132 | when reading. | |
133 | ||
134 | Here is a DBM Filter that does it: | |
135 | ||
136 | use strict ; | |
137 | use warnings ; | |
138 | use DB_File ; | |
139 | my %hash ; | |
140 | my $filename = "/tmp/filt" ; | |
141 | unlink $filename ; | |
142 | ||
143 | ||
144 | my $db = tie %hash, 'DB_File', $filename, O_CREAT|O_RDWR, 0666, $DB_HASH | |
145 | or die "Cannot open $filename: $!\n" ; | |
146 | ||
147 | $db->filter_fetch_key ( sub { $_ = unpack("i", $_) } ) ; | |
148 | $db->filter_store_key ( sub { $_ = pack ("i", $_) } ) ; | |
149 | $hash{123} = "def" ; | |
150 | # ... | |
151 | undef $db ; | |
152 | untie %hash ; | |
153 | ||
154 | The code above uses DB_File, but again it will work with any of the | |
155 | DBM modules. | |
156 | ||
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 | |
159 | filters. | |
160 | ||
161 | =head1 SEE ALSO | |
162 | ||
163 | L<DB_File>, L<GDBM_File>, L<NDBM_File>, L<ODBM_File> and L<SDBM_File>. | |
164 | ||
165 | =head1 AUTHOR | |
166 | ||
167 | Paul Marquess | |
168 |