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HASH(3)			 BSD Library Functions Manual		       HASH(3)

NAME
     hash -- hash database access method

SYNOPSIS
     #include <sys/types.h>
     #include <db.h>

DESCRIPTION
     The routine dbopen() is the library interface to database files.  One of
     the supported file	formats	is hash	files.	The general description	of the
     database access methods is	in dbopen(3), this manual page describes only
     the hash specific information.

     The hash data structure is	an extensible, dynamic hashing scheme.

     The access	method specific	data structure provided	to dbopen() is defined
     in	the <db.h> include file	as follows:

     typedef struct {
	     u_int bsize;
	     u_int ffactor;
	     u_int nelem;
	     u_int cachesize;
	     u_int32_t (*hash)(const void *, size_t);
	     int lorder;
     } HASHINFO;

     The elements of this structure are	as follows:

     bsize   Bsize defines the hash table bucket size, and is, by default, 256
	     bytes.  It	may be preferable to increase the page size for	disk-
	     resident tables and tables	with large data	items.

     ffactor
	     Ffactor indicates a desired density within	the hash table.	 It is
	     an	approximation of the number of keys allowed to accumulate in
	     any one bucket, determining when the hash table grows or shrinks.
	     The default value is 8.

     nelem   Nelem is an estimate of the final size of the hash	table.	If not
	     set or set	too low, hash tables will expand gracefully as keys
	     are entered, although a slight performance	degradation may	be no-
	     ticed.  The default value is 1.

     cachesize
	     A suggested maximum size, in bytes, of the	memory cache.  This
	     value is only advisory, and the access method will	allocate more
	     memory rather than	fail.

     hash    Hash is a user defined hash function.  Since no hash function
	     performs equally well on all possible data, the user may find
	     that the built-in hash function does poorly on a particular data
	     set.  User	specified hash functions must take two arguments (a
	     pointer to	a byte string and a length) and	return a 32-bit	quan-
	     tity to be	used as	the hash value.

     lorder  The byte order for	integers in the	stored database	metadata.  The
	     number should represent the order as an integer; for example, big
	     endian order would	be the number 4,321.  If lorder	is 0 (no order
	     is	specified) the current host order is used.  If the file	al-
	     ready exists, the specified value is ignored and the value	speci-
	     fied when the tree	was created is used.

     If	the file already exists	(and the O_TRUNC flag is not specified), the
     values specified for the parameters bsize,	ffactor, lorder	and nelem are
     ignored and the values specified when the tree was	created	are used.

     If	a hash function	is specified, hash_open() will attempt to determine if
     the hash function specified is the	same as	the one	with which the data-
     base was created, and will	fail if	it is not.

     Backward compatible interfaces to the older dbm and ndbm routines are
     provided, however these interfaces	are not	compatible with	previous file
     formats.

ERRORS
     The hash access method routines may fail and set errno for	any of the er-
     rors specified for	the library routine dbopen(3).

SEE ALSO
     btree(3), dbopen(3), mpool(3), recno(3)

     Per-Ake Larson, Dynamic Hash Tables, Communications of the	ACM, April
     1988.

     Margo Seltzer, A New Hash Package for UNIX, USENIX	Proceedings, Winter
     1991.

BUGS
     Only big and little endian	byte order is supported.

BSD				August 18, 1994				   BSD

NAME | SYNOPSIS | DESCRIPTION | ERRORS | SEE ALSO | BUGS

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