anydbm_file(3pm) [mojave man page]
AnyDBM_File(3pm) Perl Programmers Reference Guide AnyDBM_File(3pm) NAME
AnyDBM_File - provide framework for multiple DBMs NDBM_File, DB_File, GDBM_File, SDBM_File, ODBM_File - various DBM implementations SYNOPSIS
use AnyDBM_File; DESCRIPTION
This module is a "pure virtual base class"--it has nothing of its own. It's just there to inherit from one of the various DBM packages. It prefers ndbm for compatibility reasons with Perl 4, then Berkeley DB (See DB_File), GDBM, SDBM (which is always there--it comes with Perl), and finally ODBM. This way old programs that used to use NDBM via dbmopen() can still do so, but new ones can reorder @ISA: BEGIN { @AnyDBM_File::ISA = qw(DB_File GDBM_File NDBM_File) } use AnyDBM_File; Having multiple DBM implementations makes it trivial to copy database formats: use Fcntl; use NDBM_File; use DB_File; tie %newhash, 'DB_File', $new_filename, O_CREAT|O_RDWR; tie %oldhash, 'NDBM_File', $old_filename, 1, 0; %newhash = %oldhash; DBM Comparisons Here's a partial table of features the different packages offer: odbm ndbm sdbm gdbm bsd-db ---- ---- ---- ---- ------ Linkage comes w/ perl yes yes yes yes yes Src comes w/ perl no no yes no no Comes w/ many unix os yes yes[0] no no no Builds ok on !unix ? ? yes yes ? Code Size ? ? small big big Database Size ? ? small big? ok[1] Speed ? ? slow ok fast FTPable no no yes yes yes Easy to build N/A N/A yes yes ok[2] Size limits 1k 4k 1k[3] none none Byte-order independent no no no no yes Licensing restrictions ? ? no yes no [0] on mixed universe machines, may be in the bsd compat library, which is often shunned. [1] Can be trimmed if you compile for one access method. [2] See DB_File. Requires symbolic links. [3] By default, but can be redefined. SEE ALSO
dbm(3), ndbm(3), DB_File(3), perldbmfilter perl v5.18.2 2013-11-04 AnyDBM_File(3pm)
Check Out this Related Man Page
PERLDBMFILTER(1) Perl Programmers Reference Guide PERLDBMFILTER(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 works identically, and is used to install (or uninstall) a single DBM Filter. The only difference 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" if not. To delete a filter pass "undef" to it. 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 "} = "$value "; 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 terminations 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 = "filt"; unlink $filename; my $db = tie(%hash, 'SDBM_File', $filename, O_RDWR|O_CREAT, 0640) or die "Cannot open $filename: $! "; # Install DBM Filters $db->filter_fetch_key ( sub { s/ $// } ); $db->filter_store_key ( sub { $_ .= " " } ); $db->filter_fetch_value( sub { no warnings 'uninitialized'; s/ $// } ); $db->filter_store_value( sub { $_ .= " " } ); $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 terminating 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 = "filt"; unlink $filename; my $db = tie %hash, 'DB_File', $filename, O_CREAT|O_RDWR, 0666, $DB_HASH or die "Cannot open $filename: $! "; $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 necessary to install any value filters. SEE ALSO
DB_File, GDBM_File, NDBM_File, ODBM_File and SDBM_File. AUTHOR
Paul Marquess perl v5.16.3 2013-03-04 PERLDBMFILTER(1)