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lhash(3)			    OpenSSL			      lhash(3)

NAME
       lh_new, lh_free,	lh_insert, lh_delete, lh_retrieve, lh_doall,
       lh_doall_arg, lh_error -	dynamic	hash table

SYNOPSIS
	#include <openssl/lhash.h>

	DECLARE_LHASH_OF(<type>);

	LHASH *lh_<type>_new();
	void lh_<type>_free(LHASH_OF(<type> *table);

	<type> *lh_<type>_insert(LHASH_OF(<type> *table, <type>	*data);
	<type> *lh_<type>_delete(LHASH_OF(<type> *table, <type>	*data);
	<type> *lh_retrieve(LHASH_OF<type> *table, <type> *data);

	void lh_<type>_doall(LHASH_OF(<type> *table, LHASH_DOALL_FN_TYPE func);
	void lh_<type>_doall_arg(LHASH_OF(<type> *table, LHASH_DOALL_ARG_FN_TYPE func,
		 <type2>, <type2> *arg);

	int lh_<type>_error(LHASH_OF(<type> *table);

	typedef	int (*LHASH_COMP_FN_TYPE)(const	void *,	const void *);
	typedef	unsigned long (*LHASH_HASH_FN_TYPE)(const void *);
	typedef	void (*LHASH_DOALL_FN_TYPE)(const void *);
	typedef	void (*LHASH_DOALL_ARG_FN_TYPE)(const void *, const void *);

DESCRIPTION
       This library implements type-checked dynamic hash tables. The hash
       table entries can be arbitrary structures. Usually they consist of key
       and value fields.

       lh_<type>_new() creates a new LHASH_OF(<type> structure to store
       arbitrary data entries, and provides the	'hash' and 'compare' callbacks
       to be used in organising	the table's entries.  The hash callback	takes
       a pointer to a table entry as its argument and returns an unsigned long
       hash value for its key field.  The hash value is	normally truncated to
       a power of 2, so	make sure that your hash function returns well mixed
       low order bits.	The compare callback takes two arguments (pointers to
       two hash	table entries),	and returns 0 if their keys are	equal, non-
       zero otherwise.	If your	hash table will	contain	items of some
       particular type and the hash and	compare	callbacks hash/compare these
       types, then the DECLARE_LHASH_HASH_FN and IMPLEMENT_LHASH_COMP_FN
       macros can be used to create callback wrappers of the prototypes
       required	by lh_<type>_new().  These provide per-variable	casts before
       calling the type-specific callbacks written by the application author.
       These macros, as	well as	those used for the "doall" callbacks, are
       defined as;

	#define	DECLARE_LHASH_HASH_FN(name, o_type) \
		unsigned long name##_LHASH_HASH(const void *);
	#define	IMPLEMENT_LHASH_HASH_FN(name, o_type) \
		unsigned long name##_LHASH_HASH(const void *arg) { \
			const o_type *a	= arg; \
			return name##_hash(a); }
	#define	LHASH_HASH_FN(name) name##_LHASH_HASH

	#define	DECLARE_LHASH_COMP_FN(name, o_type) \
		int name##_LHASH_COMP(const void *, const void *);
	#define	IMPLEMENT_LHASH_COMP_FN(name, o_type) \
		int name##_LHASH_COMP(const void *arg1,	const void *arg2) { \
			const o_type *a	= arg1;			   \
			const o_type *b	= arg2;	\
			return name##_cmp(a,b);	}
	#define	LHASH_COMP_FN(name) name##_LHASH_COMP

	#define	DECLARE_LHASH_DOALL_FN(name, o_type) \
		void name##_LHASH_DOALL(void *);
	#define	IMPLEMENT_LHASH_DOALL_FN(name, o_type) \
		void name##_LHASH_DOALL(void *arg) { \
			o_type *a = arg; \
			name##_doall(a); }
	#define	LHASH_DOALL_FN(name) name##_LHASH_DOALL

	#define	DECLARE_LHASH_DOALL_ARG_FN(name, o_type, a_type) \
		void name##_LHASH_DOALL_ARG(void *, void *);
	#define	IMPLEMENT_LHASH_DOALL_ARG_FN(name, o_type, a_type) \
		void name##_LHASH_DOALL_ARG(void *arg1,	void *arg2) { \
			o_type *a = arg1; \
			a_type *b = arg2; \
			name##_doall_arg(a, b);	}
	#define	LHASH_DOALL_ARG_FN(name) name##_LHASH_DOALL_ARG

	An example of a	hash table storing (pointers to) structures of type 'STUFF'
	could be defined as follows;

	/* Calculates the hash value of	'tohash' (implemented elsewhere) */
	unsigned long STUFF_hash(const STUFF *tohash);
	/* Orders 'arg1' and 'arg2' (implemented elsewhere) */
	int stuff_cmp(const STUFF *arg1, const STUFF *arg2);
	/* Create the type-safe	wrapper	functions for use in the LHASH internals */
	static IMPLEMENT_LHASH_HASH_FN(stuff, STUFF);
	static IMPLEMENT_LHASH_COMP_FN(stuff, STUFF);
	/* ... */
	int main(int argc, char	*argv[]) {
		/* Create the new hash table using the hash/compare wrappers */
		LHASH_OF(STUFF)	*hashtable = lh_STUFF_new(LHASH_HASH_FN(STUFF_hash),
					  LHASH_COMP_FN(STUFF_cmp));
		/* ... */
	}

       lh_<type>_free()	frees the LHASH_OF(<type> structure table. Allocated
       hash table entries will not be freed; consider using lh_<type>_doall()
       to deallocate any remaining entries in the hash table (see below).

       lh_<type>_insert() inserts the structure	pointed	to by data into	table.
       If there	already	is an entry with the same key, the old value is
       replaced. Note that lh_<type>_insert() stores pointers, the data	are
       not copied.

       lh_<type>_delete() deletes an entry from	table.

       lh_<type>_retrieve() looks up an	entry in table.	Normally, data is a
       structure with the key field(s) set; the	function will return a pointer
       to a fully populated structure.

       lh_<type>_doall() will, for every entry in the hash table, call func
       with the	data item as its parameter.  For lh_<type>_doall() and
       lh_<type>_doall_arg(), function pointer casting should be avoided in
       the callbacks (see NOTE)	- instead use the declare/implement macros to
       create type-checked wrappers that cast variables	prior to calling your
       type-specific callbacks.	 An example of this is illustrated here	where
       the callback is used to cleanup resources for items in the hash table
       prior to	the hashtable itself being deallocated:

	/* Cleans up resources belonging to 'a'	(this is implemented elsewhere)	*/
	void STUFF_cleanup_doall(STUFF *a);
	/* Implement a prototype-compatible wrapper for	"STUFF_cleanup"	*/
	IMPLEMENT_LHASH_DOALL_FN(STUFF_cleanup,	STUFF)
		/* ... then later in the code ... */
	/* So to run "STUFF_cleanup" against all items in a hash table ... */
	lh_STUFF_doall(hashtable, LHASH_DOALL_FN(STUFF_cleanup));
	/* Then	the hash table itself can be deallocated */
	lh_STUFF_free(hashtable);

       When doing this,	be careful if you delete entries from the hash table
       in your callbacks: the table may	decrease in size, moving the item that
       you are currently on down lower in the hash table - this	could cause
       some entries to be skipped during the iteration.	 The second best
       solution	to this	problem	is to set hash->down_load=0 before you start
       (which will stop	the hash table ever decreasing in size).  The best
       solution	is probably to avoid deleting items from the hash table	inside
       a "doall" callback!

       lh_<type>_doall_arg() is	the same as lh_<type>_doall() except that func
       will be called with arg as the second argument and func should be of
       type LHASH_DOALL_ARG_FN_TYPE (a callback	prototype that is passed both
       the table entry and an extra argument).	As with	lh_doall(), you	can
       instead choose to declare your callback with a prototype	matching the
       types you are dealing with and use the declare/implement	macros to
       create compatible wrappers that cast variables before calling your
       type-specific callbacks.	 An example of this is demonstrated here
       (printing all hash table	entries	to a BIO that is provided by the
       caller):

	/* Prints item 'a' to 'output_bio' (this is implemented	elsewhere) */
	void STUFF_print_doall_arg(const STUFF *a, BIO *output_bio);
	/* Implement a prototype-compatible wrapper for	"STUFF_print" */
	static IMPLEMENT_LHASH_DOALL_ARG_FN(STUFF, const STUFF,	BIO)
		/* ... then later in the code ... */
	/* Print out the entire	hashtable to a particular BIO */
	lh_STUFF_doall_arg(hashtable, LHASH_DOALL_ARG_FN(STUFF_print), BIO,
			   logging_bio);

       lh_<type>_error() can be	used to	determine if an	error occurred in the
       last operation. lh_<type>_error() is a macro.

RETURN VALUES
       lh_<type>_new() returns NULL on error, otherwise	a pointer to the new
       LHASH structure.

       When a hash table entry is replaced, lh_<type>_insert() returns the
       value being replaced. NULL is returned on normal	operation and on
       error.

       lh_<type>_delete() returns the entry being deleted.  NULL is returned
       if there	is no such value in the	hash table.

       lh_<type>_retrieve() returns the	hash table entry if it has been	found,
       NULL otherwise.

       lh_<type>_error() returns 1 if an error occurred	in the last operation,
       0 otherwise.

       lh_<type>_free(), lh_<type>_doall() and lh_<type>_doall_arg() return no
       values.

NOTE
       The various LHASH macros	and callback types exist to make it possible
       to write	type-checked code without resorting to function-prototype
       casting - an evil that makes application	code much harder to
       audit/verify and	also opens the window of opportunity for stack
       corruption and other hard-to-find bugs.	It also, apparently, violates
       ANSI-C.

       The LHASH code regards table entries as constant	data.  As such,	it
       internally represents lh_insert()'d items with a	"const void *" pointer
       type.  This is why callbacks such as those used by lh_doall() and
       lh_doall_arg() declare their prototypes with "const", even for the
       parameters that pass back the table items' data pointers	- for
       consistency, user-provided data is "const" at all times as far as the
       LHASH code is concerned.	 However, as callers are themselves providing
       these pointers, they can	choose whether they too	should be treating all
       such parameters as constant.

       As an example, a	hash table may be maintained by	code that, for reasons
       of encapsulation, has only "const" access to the	data being indexed in
       the hash	table (ie. it is returned as "const" from elsewhere in their
       code) - in this case the	LHASH prototypes are appropriate as-is.
       Conversely, if the caller is responsible	for the	life-time of the data
       in question, then they may well wish to make modifications to table
       item passed back	in the lh_doall() or lh_doall_arg() callbacks (see the
       "STUFF_cleanup" example above).	If so, the caller can either cast the
       "const" away (if	they're	providing the raw callbacks themselves)	or use
       the macros to declare/implement the wrapper functions without "const"
       types.

       Callers that only have "const" access to	data they're indexing in a
       table, yet declare callbacks without constant types (or cast the
       "const" away themselves), are therefore creating	their own risks/bugs
       without being encouraged	to do so by the	API.  On a related note, those
       auditing	code should pay	special	attention to any instances of
       DECLARE/IMPLEMENT_LHASH_DOALL_[ARG_]_FN macros that provide types
       without any "const" qualifiers.

BUGS
       lh_<type>_insert() returns NULL both for	success	and error.

INTERNALS
       The following description is based on the SSLeay	documentation:

       The lhash library implements a hash table described in the
       Communications of the ACM in 1991.  What	makes this hash	table
       different is that as the	table fills, the hash table is increased (or
       decreased) in size via OPENSSL_realloc().  When a 'resize' is done,
       instead of all hashes being redistributed over twice as many 'buckets',
       one bucket is split.  So	when an	'expand' is done, there	is only	a
       minimal cost to redistribute some values.  Subsequent inserts will
       cause more single 'bucket' redistributions but there will never be a
       sudden large cost due to	redistributing all the 'buckets'.

       The state for a particular hash table is	kept in	the LHASH structure.
       The decision to increase	or decrease the	hash table size	is made
       depending on the	'load' of the hash table.  The load is the number of
       items in	the hash table divided by the size of the hash table.  The
       default values are as follows.  If (hash->up_load < load) => expand.
       if (hash->down_load > load) => contract.	 The up_load has a default
       value of	1 and down_load	has a default value of 2.  These numbers can
       be modified by the application by just playing with the up_load and
       down_load variables.  The 'load'	is kept	in a form which	is multiplied
       by 256.	So hash->up_load=8*256;	will cause a load of 8 to be set.

       If you are interested in	performance the	field to watch is
       num_comp_calls.	The hash library keeps track of	the 'hash' value for
       each item so when a lookup is done, the 'hashes'	are compared, if there
       is a match, then	a full compare is done,	and hash->num_comp_calls is
       incremented.  If	num_comp_calls is not equal to num_delete plus
       num_retrieve it means that your hash function is	generating hashes that
       are the same for	different values.  It is probably worth	changing your
       hash function if	this is	the case because even if your hash table has
       10 items	in a 'bucket', it can be searched with 10 unsigned long
       compares	and 10 linked list traverses.  This will be much less
       expensive that 10 calls to your compare function.

       lh_strhash() is a demo string hashing function:

	unsigned long lh_strhash(const char *c);

       Since the LHASH routines	would normally be passed structures, this
       routine would not normally be passed to lh_<type>_new(),	rather it
       would be	used in	the function passed to lh_<type>_new().

SEE ALSO
       lh_stats(3)

HISTORY
       The lhash library is available in all versions of SSLeay	and OpenSSL.
       lh_error() was added in SSLeay 0.9.1b.

       This manpage is derived from the	SSLeay documentation.

       In OpenSSL 0.9.7, all lhash functions that were passed function
       pointers	were changed for better	type safety, and the function types
       LHASH_COMP_FN_TYPE, LHASH_HASH_FN_TYPE, LHASH_DOALL_FN_TYPE and
       LHASH_DOALL_ARG_FN_TYPE became available.

       In OpenSSL 1.0.0, the lhash interface was revamped for even better type
       checking.

1.0.1e				  2013-02-11			      lhash(3)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | NOTE | BUGS | INTERNALS | SEE ALSO | HISTORY

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