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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 = "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 = "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
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