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Perl Cookbook
book

Perl Cookbook

by Tom Christiansen, Nathan Torkington
August 1998
Intermediate to advanced content levelIntermediate to advanced
800 pages
39h 20m
English
O'Reilly Media, Inc.
Content preview from Perl Cookbook

Emptying a DBM File

Problem

You want to clear out a DBM file.

Solution

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

dbmopen(%HASH, $FILENAME, 0666)         or die "Can't open FILENAME: $!\n";
%HASH = ();
dbmclose %HASH;

or 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 with create mode:

unlink $FILENAME
    or die "Couldn't unlink $FILENAME to empty the database: $!\n";
dbmopen(%HASH, $FILENAME, 0666)
    or die "Couldn't create $FILENAME database: $!\n";

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.

See Also

The documentation for the standard DB_File module, also in Chapter 7 of Programming Perl; the unlink function in perlfunc (1); Section 14.1

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Publisher Resources

ISBN: 1565922433Supplemental ContentCatalog PageErrata