Emptying a DBM File (Perl Cookbook, 2nd Edition)

14.2. Emptying a DBM File

14.2.1. Problem

14.2.2. Solution

Open the database and assign ( ) to it. Use tie:

use DB_File;

tie(%HASH, "DB_File", $FILENAME)        or die "Can't open FILENAME: $!\n";
%HASH = ( );
untie %HASH;

Alternatively, delete the file and reopen:

unlink $FILENAME
    or die "Couldn't unlink $FILENAME to empty the database: $!\n";
tie(%HASH => "DB_File", $FILENAME)
    or die "Couldn't create $FILENAME database: $!\n";

14.2.3. Discussion

It may be quicker to delete the file and create a new one than to reset it, but doing so opens you up to a race condition that trips up a careless program or makes it vulnerable to an attacker. The attacker could make a link pointing to the file /etc/precious with the same name as your file between the time when you deleted the file and when you recreated it. When the DBM library opens the file, it clobbers /etc/precious.

If you delete a DB_File database and recreate it, you'll lose any customizable settings like page size, fill-factor, and so on. This is another good reason to assign the empty list to the tied hash.

14.2.4. See Also

The documentation for the standard DB_File module, also in Chapter 32 of Programming Perl; the unlink function in perlfunc(1); Recipe 14.1