[Chapter 7] 7.2.1 AnyDBM_File - Provide Framework for Multiple DBMs
Author: Larry Wall, Tom Christiansen & Randal L. Schwartz
Date: 1999-07-02T02:00:40Z

#

| Chapter 7 The Standard Perl Library |

### 7.2.1 AnyDBM_File - Provide Framework for Multiple DBMs

    use AnyDBM_File;

This module is a "pure virtual base class" - it has nothing of its own. It's just there to inherit from the various DBM packages. By default it inherits from NDBM_File for compatibility with earlier versions of Perl. If it doesn't find NDBM_File, it looks for DB_File, GDBM_File, SDBM_File (which is always there - it comes with Perl), and finally ODBM_File.

Perl's dbmopen function (which now exists only for backward compatibility) actually just calls tie to bind a hash to AnyDBM_File. The effect is to bind the hash to one of the specific DBM classes that AnyDBM_File inherits from.

You can override the defaults and determine which class dbmopen will tie to. Do this by redefining @ISA:

    @AnyDBM_File::ISA = qw(DB_File GDBM_File NDBM_File);

Note, however, that an explicit use takes priority over the ordering of @ISA, so that:

    use GDBM_File;

will cause the next dbmopen to tie your hash to GDBM_File.

You can tie hash variables directly to the desired class yourself, without using dbmopen or AnyDBM_File. For example, by using multiple DBM implementations, you can copy a database from one format to another:

    use Fcntl;         # for O_* values
    use NDBM_File;
    use DB_File;
    tie %oldhash, "NDBM_File", $old_filename, O_RDWR;
    tie %newhash, "DB_File",   $new_filename, O_RDWR|O_CREAT|O_EXCL, 0644;
    while (($key,$val) = each %oldhash) {
        $newhash{$key} = $val;
    }

#### 7.2.1.1 DBM comparisons

Here's a table of the features that the different DBMish packages offer:

Feature | ODBM | NDBM | SDBM | GDBM | BSD-DB
--- | --- | --- | --- | --- | ---
Linkage comes with Perl | Yes | Yes | Yes | Yes | Yes
Source bundled with Perl | No | No | Yes | No | No
Source redistributable | No | No | Yes | GPL | Yes
Often comes with UNIX | Yes | Yes[1] | No | No | No
Builds OK on UNIX | N/A | N/A | Yes | Yes | Yes[2]
Code size | Varies[3] | Varies[3] | Small | Big | Big
Disk usage | Varies[3] | Varies[3] | Small | Big | OK[4]
Speed | Varies[3] | Varies[3] | Slow | OK | Fast
FTPable | No | No | Yes | Yes | Yes
Easy to build | N/A | N/A | Yes | Yes | OK[5]
Block size limits | 1k | 4k | 1k[6] | None | None
Byte-order independent | No | No | No | No | Yes
User-defined sort order | No | No | No | No | Yes
Wildcard lookups | No | No | No | No | Yes

> [1] On mixed-universe machines, may be in the BSD compatibility library, which is often shunned.

> [2] Providing you have an ANSI C compiler.

> [3] Depends on how much your vendor has "tweaked" it.

> [4] Can be trimmed if you compile for one access method.

> [5] See the DB_File library module. Requires symbolic links.

> [6] By default, but can be redefined (at the expense of compatibility with older files).

#### 7.2.1.2 See also

Relevant library modules include: DB_File, GDBM_File, NDBM_File, ODBM_File, and SDBM_File. Related manpages: dbm(3), ndbm(3). Tied variables are discussed extensively in Chapter 5, and the dbmopen entry in Chapter 3, Functions, may also be helpful. You can pick up the unbundled modules from the src/misc/ directory on your nearest CPAN site. Here are the most popular ones, but note that their version numbers may have changed by the time you read this:

    http://www.perl.com/CPAN/src/misc/db.1.85.tar.gz
    http://www.perl.com/CPAN/src/misc/gdbm-1.7.3.tar.gz

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