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HASH(3)		       FreeBSD 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   The bsize element 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
	     The ffactor element 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   The nelem element is an estimate of the final size	of the hash
	     table.  If	not set	or set too low,	hash tables will expand	grace-
	     fully as keys are entered,	although a slight performance degrada-
	     tion may be noticed.  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    The hash element 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
	     quantity 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
	     already exists, the specified value is ignored and	the value
	     specified 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 bsize, ffactor, lorder and nelem arguments 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
     errors 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.

FreeBSD	10.1			August 18, 1994			  FreeBSD 10.1

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

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