Advanced Perl Programming

Record-Oriented Approach

In this section, we will study three modules that essentially depend on the DBM library. DBM is a disk-based hash table library, originally written by Ken Thompson for the Seventh Edition Unix system. This library has since spawned many variants: SDBM (Simple DBM, a public-domain module bundled with Perl), NDBM (New DBM, which is packaged with some operating systems), and GDBM (from the Free Software Foundation). All these libraries can be accessed from equivalent Perl modules, which use Perl’s tie facility to provide transparent access to the disk-based table. Performance and portability are the only criteria for selecting one of these systems. Be warned that the files produced by these approaches are not interchangeable.

DBM

We use SDBM here, because it is bundled with Perl. The SDBM_File module provides a wrapper over this extension:

use Fcntl;
use SDBM_File;
tie (%capital, 'SDBM_File', 'capitals.dat', O_RDWR|O_CREAT, 0666) 
     || die $!;
$capital{USA}      = "Washington D.C.";
$capital{Colombia} = "Bogota";
untie %capital;

The tie statement associates the in-memory hash variable, %capital, with the disk-based hash file, capitals.dat. Read and write accesses to %capital are automatically translated to corresponding accesses to the file. untie breaks this association and flushes any pending changes to the disk. O_RDWR and O_CREAT, “constants” imported from Fcntl, specify that capitals.dat is to be opened for reading and writing, and to create it if it doesn’t exist. ...