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Hash::Util(3pm) Perl Programmers Reference Guide Hash::Util(3pm)
NAME
Hash::Util - A selection of general-utility hash subroutines
SYNOPSIS
use Hash::Util qw(lock_keys unlock_keys
lock_value unlock_value
lock_hash unlock_hash
hash_seed);
%hash = (foo => 42, bar => 23);
lock_keys(%hash);
lock_keys(%hash, @keyset);
unlock_keys(%hash);
lock_value (%hash, ’foo’);
unlock_value(%hash, ’foo’);
lock_hash (%hash);
unlock_hash(%hash);
my $hashes_are_randomised = hash_seed() != 0;
DESCRIPTION
"Hash::Util" contains special functions for manipulating hashes that don’t really
warrant a keyword.
By default "Hash::Util" does not export anything.
Restricted hashes
5.8.0 introduces the ability to restrict a hash to a certain set of keys. No keys
outside of this set can be added. It also introduces the ability to lock an indi-
vidual key so it cannot be deleted and the value cannot be changed.
This is intended to largely replace the deprecated pseudo-hashes.
lock_keys
unlock_keys
lock_keys(%hash);
lock_keys(%hash, @keys);
Restricts the given %hash’s set of keys to @keys. If @keys is not given it
restricts it to its current keyset. No more keys can be added. delete() and
exists() will still work, but will not alter the set of allowed keys. Note: the
current implementation prevents the hash from being bless()ed while it is in a
locked state. Any attempt to do so will raise an exception. Of course you can
still bless() the hash before you call lock_keys() so this shouldn’t be a prob-
lem.
unlock_keys(%hash);
Removes the restriction on the %hash’s keyset.
lock_value
unlock_value
lock_value (%hash, $key);
unlock_value(%hash, $key);
Locks and unlocks an individual key of a hash. The value of a locked key can-
not be changed.
%hash must have already been locked for this to have useful effect.
lock_hash
unlock_hash
lock_hash(%hash);
lock_hash() locks an entire hash, making all keys and values readonly. No
value can be changed, no keys can be added or deleted.
unlock_hash(%hash);
unlock_hash() does the opposite of lock_hash(). All keys and values are made
read/write. All values can be changed and keys can be added and deleted.
hash_seed
my $hash_seed = hash_seed();
hash_seed() returns the seed number used to randomise hash ordering. Zero
means the "traditional" random hash ordering, non-zero means the new even more
random hash ordering introduced in Perl 5.8.1.
Note that the hash seed is sensitive information: by knowing it one can craft a
denial-of-service attack against Perl code, even remotely, see "Algorithmic
Complexity Attacks" in perlsec for more information. Do not disclose the hash
seed to people who don’t need to know it. See also "PERL_HASH_SEED_DEBUG" in
perlrun.
CAVEATS
Note that the trapping of the restricted operations is not atomic: for example
eval { %hash = (illegal_key => 1) }
leaves the %hash empty rather than with its original contents.
AUTHOR
Michael G Schwern <schwern AT pobox.com> on top of code by Nick Ing-Simmons and Jef-
frey Friedl.
SEE ALSO
Scalar::Util, List::Util, Hash::Util, and "Algorithmic Complexity Attacks" in
perlsec.
perl v5.8.6 2001-09-21 Hash::Util(3pm)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
2008-11-21 09:03 @38.103.63.58 CrawledBy CCBot/1.0 (+http://www.commoncrawl.org/bot.html)