Hash::Util(3p) Perl Programmers Reference Guide Hash::Util(3p)
Hash::Util - A selection of general-utility hash subroutines
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;
"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 individual 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 problem.
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Hash::Util(3p) Perl Programmers Reference Guide Hash::Util(3p)
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 cannot 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 Com-
plexity 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.
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 con-
tents.
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Hash::Util(3p) Perl Programmers Reference Guide Hash::Util(3p)
Michael G Schwern <schwern@pobox.com> on top of code by Nick
Ing-Simmons and Jeffrey Friedl.
Scalar::Util, List::Util, Hash::Util, and "Algorithmic Com-
plexity Attacks" in perlsec.
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