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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>

	LHASH *lh_new(LHASH_HASH_FN_TYPE hash, LHASH_COMP_FN_TYPE compare);
	void lh_free(LHASH *table);

	void *lh_insert(LHASH *table, void *data);
	void *lh_delete(LHASH *table, void *data);
	void *lh_retrieve(LHASH	*table,	void *data);

	void lh_doall(LHASH *table, LHASH_DOALL_FN_TYPE	func);
	void lh_doall_arg(LHASH	*table,	LHASH_DOALL_ARG_FN_TYPE	func,
		 void *arg);

	int lh_error(LHASH *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 dynamic hash tables. The	hash table entries can
       be arbitrary structures.	Usually	they consist of	key and	value fields.

       lh_new()	creates	a new LHASH 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_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(f_name,o_type) \
		unsigned long f_name##_LHASH_HASH(const	void *);
	#define	IMPLEMENT_LHASH_HASH_FN(f_name,o_type) \
		unsigned long f_name##_LHASH_HASH(const	void *arg) { \
			o_type a = (o_type)arg;	\
			return f_name(a); }
	#define	LHASH_HASH_FN(f_name) f_name##_LHASH_HASH

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

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

	#define	DECLARE_LHASH_DOALL_ARG_FN(f_name,o_type,a_type) \
		void f_name##_LHASH_DOALL_ARG(const void *, const void *);
	#define	IMPLEMENT_LHASH_DOALL_ARG_FN(f_name,o_type,a_type) \
		void f_name##_LHASH_DOALL_ARG(const void *arg1,	const void *arg2) { \
			o_type a = (o_type)arg1; \
			a_type b = (a_type)arg2; \
			f_name(a,b); }
	#define	LHASH_DOALL_ARG_FN(f_name) f_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_hash, const STUFF *)
	static IMPLEMENT_LHASH_COMP_FN(STUFF_cmp, const	STUFF *);
	/* ... */
	int main(int argc, char	*argv[]) {
		/* Create the new hash table using the hash/compare wrappers */
		LHASH *hashtable = lh_new(LHASH_HASH_FN(STUFF_hash),
					  LHASH_COMP_FN(STUFF_cmp));
		/* ... */
	}

       lh_free() frees the LHASH structure table. Allocated hash table entries
       will not	be freed; consider using lh_doall() to deallocate any
       remaining entries in the	hash table (see	below).

       lh_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_insert() stores pointers, the data are not copied.

       lh_delete() deletes an entry from table.

       lh_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_doall() will,	for every entry	in the hash table, call	func with the
       data item as its	parameter.  For	lh_doall() and lh_doall_arg(),
       function	pointer	casting	should be avoided in the callbacks (see	NOTE)
       - instead, either declare the callbacks to match	the prototype required
       in lh_new() or use the declare/implement	macros to create type-safe
       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(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_doall(hashtable, LHASH_DOALL_FN(STUFF_cleanup));
	/* Then	the hash table itself can be deallocated */
	lh_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_doall_arg() is the same as lh_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(const STUFF *a, BIO *output_bio);
	/* Implement a prototype-compatible wrapper for	"STUFF_print" */
	static IMPLEMENT_LHASH_DOALL_ARG_FN(STUFF_print, const STUFF *,	BIO *)
		/* ... then later in the code ... */
	/* Print out the entire	hashtable to a particular BIO */
	lh_doall_arg(hashtable,	LHASH_DOALL_ARG_FN(STUFF_print), logging_bio);

       lh_error() can be used to determine if an error occurred	in the last
       operation. lh_error() is	a macro.

RETURN VALUES
       lh_new()	returns	NULL on	error, otherwise a pointer to the new LHASH
       structure.

       When a hash table entry is replaced, lh_insert()	returns	the value
       being replaced. NULL is returned	on normal operation and	on error.

       lh_delete() returns the entry being deleted.  NULL is returned if there
       is no such value	in the hash table.

       lh_retrieve() returns the hash table entry if it	has been found,	NULL
       otherwise.

       lh_error() returns 1 if an error	occurred in the	last operation,	0
       otherwise.

       lh_free(), lh_doall() and lh_doall_arg()	return no values.

NOTE
       The various LHASH macros	and callback types exist to make it possible
       to write	type-safe 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_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_new(), rather	it would be
       used in the function passed to lh_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.

0.9.8za				  2014-06-05			      lhash(3)

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

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