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Hash::Util(3)	       Perl Programmers Reference Guide		 Hash::Util(3)

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

SYNOPSIS
	 use Hash::Util qw(
			    hash_seed all_keys
			    lock_keys unlock_keys
			    lock_value unlock_value
			    lock_hash unlock_hash
			    lock_keys_plus hash_locked
			    hidden_keys legal_keys
			  );

	 %hash = (foo => 42, bar => 23);
	 # Ways to restrict a hash
	 lock_keys(%hash);
	 lock_keys(%hash, @keyset);
	 lock_keys_plus(%hash, @additional_keys);

	 # Ways to inspect the properties of a restricted hash
	 my @legal = legal_keys(%hash);
	 my @hidden = hidden_keys(%hash);
	 my $ref = all_keys(%hash,@keys,@hidden);
	 my $is_locked = hash_locked(%hash);

	 # Remove restrictions on the hash
	 unlock_keys(%hash);

	 # Lock individual values in a hash
	 lock_value  (%hash, 'foo');
	 unlock_value(%hash, 'foo');

	 # Ways to change the restrictions on both keys and values
	 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
       ability to ensure that an individual 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.

	   Note that if any of the values of the hash have been locked they
	   will not be unlocked after this sub executes.

	   Both routines return a reference to the hash operated on.

       lock_keys_plus
	     lock_keys_plus(%hash,@additional_keys)

	   Similar to "lock_keys()", with the difference being that the
	   optional key list specifies keys that may or may not be already in
	   the hash. Essentially this is an easier way to say

	     lock_keys(%hash,@additional_keys,keys %hash);

	   Returns a reference to %hash

       lock_value
       unlock_value
	     lock_value	 (%hash, $key);
	     unlock_value(%hash, $key);

	   Locks and unlocks the value for an individual key of a hash.	 The
	   value of a locked key cannot be changed.

	   Unless %hash has already been locked the key/value could be deleted
	   regardless of this setting.

	   Returns a reference to the %hash.

       lock_hash
       unlock_hash
	       lock_hash(%hash);

	   lock_hash() locks an entire hash, making all keys and values
	   read-only.  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 val‐
	   ues are made writable.  All values can be changed and keys can be
	   added and deleted.

	   Returns a reference to the %hash.

       lock_hash_recurse
       unlock_hash_recurse
	       lock_hash_recurse(%hash);

	   lock_hash() locks an entire hash and any hashes it references
	   recursively, making all keys and values read-only. No value can be
	   changed, no keys can be added or deleted.

	   Only recurses into hashes that are referenced by another hash. Thus
	   a Hash of Hashes (HoH) will all be restricted, but a Hash of Arrays
	   of Hashes (HoAoH) will only have the top hash restricted.

	       unlock_hash_recurse(%hash);

	   unlock_hash_recurse() does the opposite of lock_hash_recurse().
	   All keys and values are made writable.  All values can be changed
	   and keys can be added and deleted. Identical recursion restrictions
	   apply as to lock_hash_recurse().

	   Returns a reference to the %hash.

       hash_unlocked
	     hash_unlocked(%hash) and print "Hash is unlocked!\n";

	   Returns true if the hash and its keys are unlocked.

       b<legal_keys>
	     my @keys = legal_keys(%hash);

	   Returns the list of the keys that are legal in a restricted hash.
	   In the case of an unrestricted hash this is identical to calling
	   keys(%hash).

       hidden_keys
	     my @keys = hidden_keys(%hash);

	   Returns the list of the keys that are legal in a restricted hash
	   but do not have a value associated to them. Thus if 'foo' is a
	   "hidden" key of the %hash it will return false for both "defined"
	   and "exists" tests.

	   In the case of an unrestricted hash this will return an empty list.

	   NOTE this is an experimental feature that is heavily dependent on
	   the current implementation of restricted hashes. Should the imple‐
	   mentation change, this routine may become meaningless, in which
	   case it will return an empty list.

       all_keys
	     all_keys(%hash,@keys,@hidden);

	   Populates the arrays @keys with the all the keys that would pass an
	   "exists" tests, and populates @hidden with the remaining legal keys
	   that have not been utilized.

	   Returns a reference to the hash.

	   In the case of an unrestricted hash this will be equivalent to

	     $ref = do {
		 @keys = keys %hash;
		 @hidden = ();
		 \%hash
	     };

	   NOTE this is an experimental feature that is heavily dependent on
	   the current implementation of restricted hashes. Should the imple‐
	   mentation change this routine may become meaningless in which case
	   it will behave identically to how it would behave on an unre‐
	   stricted hash.

       hash_seed
	       my $hash_seed = hash_seed();

	   hash_seed() returns the seed number used to randomise hash order‐
	   ing.	 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.

       hv_store
	     my $sv = 0;
	     hv_store(%hash,$key,$sv) or die "Failed to alias!";
	     $hash{$key} = 1;
	     print $sv; # prints 1

	   Stores an alias to a variable in a hash instead of copying the
	   value.

       Operating on references to hashes.

       Most subroutines documented in this module have equivalent versions
       that operate on references to hashes instead of native hashes.  The
       following is a list of these subs. They are identical except in name
       and in that instead of taking a %hash they take a $hashref, and addi‐
       tionally are not prototyped.

       lock_ref_keys
       unlock_ref_keys
       lock_ref_keys_plus
       lock_ref_value
       unlock_ref_value
       lock_hashref
       unlock_hashref
       lock_hashref_recurse
       unlock_hashref_recurse
       hash_ref_unlocked
       legal_ref_keys
       hidden_ref_keys

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.

BUGS
       The interface exposed by this module is very close to the current
       implementation of restricted hashes. Over time it is expected that this
       behavior will be extended and the interface abstracted further.

AUTHOR
       Michael G Schwern <schwern@pobox.com> on top of code by Nick Ing-Sim‐
       mons and Jeffrey Friedl.

       hv_store() is from Array::RefElem, Copyright 2000 Gisle Aas.

       Additional code by Yves Orton.

SEE ALSO
       Scalar::Util, List::Util, Hash::Util, and "Algorithmic Complexity
       Attacks" in perlsec.

perl v5.8.8			  2008-09-19			 Hash::Util(3)
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