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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(unsigned long (*hash)(/*void *a*/),
		 int (*compare)(/*void *a,void *b*/));
	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, void (*func)(/*void *b*/));
	void lh_doall_arg(LHASH *table, void (*func)(/*void *a,void *b*/),
		 void *arg);

	int lh_error(LHASH *table);

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. hash takes a pointer to the
       structure and returns an unsigned long hash value of 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. compare takes two
       arguments, and returns 0 if their keys are equal, non-zero otherwise.

       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.

       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 parameters.	 This function can be quite useful when used
       as follows:
	void cleanup(STUFF *a)
	 { STUFF_free(a); }
	lh_doall(hash,cleanup);
	lh_free(hash); This can be used to free all the entries. lh_free()
       then cleans up the 'buckets' that point to nothing. When doing this, be
       careful if you delete entries from the hash table in func: the table
       may decrease in size, moving item that you are currently on down lower
       in the hash table.  This could cause some entries to be skipped.	 The
       best solution to this problem is to set hash->down_load=0 before you
       start.  This will stop the hash table ever being decreased in size.

       lh_doall_arg() is the same as lh_doall() except that func will be
       called with arg as the second argument.

       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.

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 you 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.

3rd Berkeley Distribution	    0.9.6m			      lhash(3)
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