Hash::Util - A selection of general-utility hash subroutines

- NAME
- SYNOPSIS
- DESCRIPTION

- Restricted hashes

- CAVEATS
- AUTHOR
- SEE ALSO

# 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 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.

        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 Complexity Attacks in the perlsec manpage 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 the perlrun manpage.

# 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@pobox.com> on top of code by Nick Ing-Simmons and Jeffrey Friedl.

# SEE ALSO

the Scalar::Util manpage, the List::Util manpage, the Hash::Util manpage, and Algorithmic Complexity Attacks in the perlsec manpage.

Hash::Util - A selection of general-utility hash subroutines