Recipe 14.3 Converting Between DBM Files

14.3.1 Problem

You have a file in one DBM format, but another program expects input in a different DBM format.

14.3.2 Solution

Read the keys and values from the initial DBM file and write them to a new file in the different DBM format as in Example 14-2.

Example 14-2. db2gdbm
  #!/usr/bin/perl -w
  # db2gdbm: converts DB to GDBM
  use strict;
  use DB_File;
  use GDBM_File;
  unless (@ARGV =  = 2) {
      die "usage: db2gdbm infile outfile\n";
  my ($infile, $outfile) = @ARGV;                     
  my (%db_in, %db_out);                               
  # open the files
  tie(%db_in, 'DB_File', $infile)
      or die "Can't tie $infile: $!";
  tie(%db_out, 'GDBM_File', $outfile, GDBM_WRCREAT, 0666)
      or die "Can't tie $outfile: $!";
  # copy (don't use %db_out = %db_in because it's slow on big databases)
  while (my($k, $v) = each %db_in) {
      $db_out{$k} = $v;
  # these unties happen automatically at program exit
  untie %db_in;
  untie %db_out;

Call the program as:

% db2gdbm /tmp/users.db /tmp/users.gdbm

14.3.3 Discussion

When multiple types of DBM file are used in the same program, you have to use tie, not the dbmopen interface. That's because with dbmopen you can use only one database format, which is why its use is deprecated.

Copying hashes by simple assignment, as in %new = %old, works on DBM files. However, it loads everything into memory first as a list, which doesn't matter with small hashes, but can be prohibitively expensive in the case of DBM files. For database hashes, use each to iterate through them instead.

14.3.4 See Also

The documentation for the standard modules GDBM_File, NDBM_File, SDBM_File, DB_File, some of which are in Chapter 32 of Programming Perl; Recipe 14.1