1PERLDBMFILTER(1) Perl Programmers Reference Guide PERLDBMFILTER(1)
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6 perldbmfilter - Perl DBM Filters
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9 $db = tie %hash, 'DBM', ...
10 $old_filter = $db->filter_store_key ( sub { ... } );
12 $old_filter = $db->filter_store_value( sub { ... } );
13 $old_filter = $db->filter_fetch_key ( sub { ... } );
14 $old_filter = $db->filter_fetch_value( sub { ... } );
15
17 The four "filter_*" methods shown above are available in all the DBM
18 modules that ship with Perl, namely DB_File, GDBM_File, NDBM_File,
19 ODBM_File and SDBM_File.
20 Each of the methods works identically, and is used to install (or
22 uninstall) a single DBM Filter. The only difference between them is the
23 place that the filter is installed.
24 To summarise:
26 filter_store_key
28 If a filter has been installed with this method, it will be
29 invoked every time you write a key to a DBM database.
30 filter_store_value
32 If a filter has been installed with this method, it will be
33 invoked every time you write a value to a DBM database.
34 filter_fetch_key
36 If a filter has been installed with this method, it will be
37 invoked every time you read a key from a DBM database.
38 filter_fetch_value
40 If a filter has been installed with this method, it will be
41 invoked every time you read a value from a DBM database.
42 You can use any combination of the methods from none to all four.
44 All filter methods return the existing filter, if present, or "undef"
46 if not.
47 To delete a filter pass "undef" to it.
49 The Filter
51 When each filter is called by Perl, a local copy of $_ will contain the
52 key or value to be filtered. Filtering is achieved by modifying the
53 contents of $_. The return code from the filter is ignored.
54 An Example: the NULL termination problem.
56 DBM Filters are useful for a class of problems where you always want to
57 make the same transformation to all keys, all values or both.
58 For example, consider the following scenario. You have a DBM database
60 that you need to share with a third-party C application. The C
61 application assumes that all keys and values are NULL terminated.
62 Unfortunately when Perl writes to DBM databases it doesn't use NULL
63 termination, so your Perl application will have to manage NULL
64 termination itself. When you write to the database you will have to use
65 something like this:
66 $hash{"$key\0"} = "$value\0";
68 Similarly the NULL needs to be taken into account when you are
70 considering the length of existing keys/values.
71 It would be much better if you could ignore the NULL terminations issue
73 in the main application code and have a mechanism that automatically
74 added the terminating NULL to all keys and values whenever you write to
75 the database and have them removed when you read from the database. As
76 I'm sure you have already guessed, this is a problem that DBM Filters
77 can fix very easily.
78 use v5.36;
80 use SDBM_File;
81 use Fcntl;
82 my %hash;
84 my $filename = "filt";
85 unlink $filename;
86 my $db = tie(%hash, 'SDBM_File', $filename, O_RDWR|O_CREAT, 0640)
88 or die "Cannot open $filename: $!\n";
89 # Install DBM Filters
91 $db->filter_fetch_key ( sub { s/\0$// } );
92 $db->filter_store_key ( sub { $_ .= "\0" } );
93 $db->filter_fetch_value(
94 sub { no warnings 'uninitialized'; s/\0$// } );
95 $db->filter_store_value( sub { $_ .= "\0" } );
96 $hash{"abc"} = "def";
98 my $a = $hash{"ABC"};
99 # ...
100 undef $db;
101 untie %hash;
102 The code above uses SDBM_File, but it will work with any of the DBM
104 modules.
105 Hopefully the contents of each of the filters should be self-
107 explanatory. Both "fetch" filters remove the terminating NULL, and both
108 "store" filters add a terminating NULL.
109 Another Example: Key is a C int.
111 Here is another real-life example. By default, whenever Perl writes to
112 a DBM database it always writes the key and value as strings. So when
113 you use this:
114 $hash{12345} = "something";
116 the key 12345 will get stored in the DBM database as the 5 byte string
118 "12345". If you actually want the key to be stored in the DBM database
119 as a C int, you will have to use "pack" when writing, and "unpack" when
120 reading.
121 Here is a DBM Filter that does it:
123 use v5.36;
125 use DB_File;
126 my %hash;
127 my $filename = "filt";
128 unlink $filename;
129 my $db = tie %hash, 'DB_File', $filename, O_CREAT|O_RDWR, 0666,
132 $DB_HASH or die "Cannot open $filename: $!\n";
133 $db->filter_fetch_key ( sub { $_ = unpack("i", $_) } );
135 $db->filter_store_key ( sub { $_ = pack ("i", $_) } );
136 $hash{123} = "def";
137 # ...
138 undef $db;
139 untie %hash;
140 The code above uses DB_File, but again it will work with any of the DBM
142 modules.
143 This time only two filters have been used; we only need to manipulate
145 the contents of the key, so it wasn't necessary to install any value
146 filters.
147
149 DB_File, GDBM_File, NDBM_File, ODBM_File and SDBM_File.
150
152 Paul Marquess
153perl v5.36.0 2022-08-30 PERLDBMFILTER(1)