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HASH(3)								       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 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   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 exam-
	      ple, 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	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 routines described	in dbm(3), and
       ndbm(3)	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)

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

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

BUGS
       Only big	and little endian byte order is	supported.

4.4 Berkeley Distribution	  1994-08-18			       HASH(3)

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

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