


PERLDBMFILTER(1) Perl Programmers Reference GuidePERLDBMFILTER(1)


NAME
       perldbmfilter - Perl DBM Filters

SYNOPSIS
           $db = tie %hash, 'DBM', ...

           $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 { ... } ) ;


DESCRIPTION
       The four "filter_*" methods shown above are available in
       all the DBM modules that ship with Perl, namely DB_File,
       GDBM_File, NDBM_File, ODBM_File and SDBM_File.

       Each of the methods work identically, and are used to
       install (or uninstall) a single DBM Filter. The only dif-
       ference between them is the place that the filter is
       installed.

       To summarise:

       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.

       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.

       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.

       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.

       You can use any combination of the methods from none to
       all four.

       All filter methods return the existing filter, if present,
       or "undef" in not.

       To delete a filter pass "undef" to it.






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PERLDBMFILTER(1) Perl Programmers Reference GuidePERLDBMFILTER(1)


       The Filter

       When each filter is called by Perl, a local copy of "$_"
       will contain the key or value to be filtered. Filtering is
       achieved by modifying the contents of "$_". The return
       code from the filter is ignored.

       An Example -- the NULL termination problem.

       DBM Filters are useful for a class of problems where you
       always want to make the same transformation to all keys,
       all values or both.

       For example, 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 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 ter-
       minations 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 warnings ;
           use SDBM_File ;
           use Fcntl ;

           my %hash ;
           my $filename = "/tmp/filt" ;
           unlink $filename ;

           my $db = tie(%hash, 'SDBM_File', $filename, O_RDWR|O_CREAT, 0640)
             or die "Cannot open $filename: $!\n" ;

           # Install DBM Filters
           $db->filter_fetch_key  ( sub { s/\0$//    } ) ;
           $db->filter_store_key  ( sub { $_ .= "\0" } ) ;
           $db->filter_fetch_value(
               sub { no warnings 'uninitialized' ;s/\0$// } ) ;
           $db->filter_store_value( sub { $_ .= "\0" } ) ;




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PERLDBMFILTER(1) Perl Programmers Reference GuidePERLDBMFILTER(1)


           $hash{"abc"} = "def" ;
           my $a = $hash{"ABC"} ;
           # ...
           undef $db ;
           untie %hash ;

       The code above uses SDBM_File, but it will work with any
       of the DBM modules.

       Hopefully the contents of each of the filters should be
       self-explanatory. Both "fetch" filters remove the termi-
       nating NULL, and both "store" filters add a terminating
       NULL.

       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} = "something" ;

       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 "pack" when writing, and "unpack" when reading.

       Here is a DBM Filter that does it:

           use strict ;
           use warnings ;
           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 ;

       The code above uses DB_File, but again it will work with
       any of the DBM modules.

       This time only two filters have been used -- we only need
       to manipulate the contents of the key, so it wasn't neces-
       sary to install any value filters.





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PERLDBMFILTER(1) Perl Programmers Reference GuidePERLDBMFILTER(1)


SEE ALSO
       the DB_File manpage, the GDBM_File manpage, the NDBM_File
       manpage, the ODBM_File manpage and the SDBM_File manpage.

AUTHOR
       Paul Marquess



















































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