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NetBSD 6.1.5 - man page for hash (netbsd section 3)

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>
     header as follows:

	   typedef struct {
		   u_int bsize;
		   u_int ffactor;
		   u_int nelem;
		   u_int cachesize;
		   uint32_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 defaults to 4096 for in-memory
		 tables.  If bsize is 0 (no bucket size is specified) a bucket size is chosen
		 based on the underlying file system I/O block size.  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 approxima-
		 tion 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
		 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 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 database was created, and will fail if it is
     not.

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, Issue 4, Volume 31, April
     1988.

     Margo Seltzer, "A New Hash Package for UNIX", Proceedings of the 1991 Winter USENIX
     Technical Conference, USENIX Association,
     http://www.usenix.org/publications/library/proceedings/seltzer2.pdf, 173-184, January 1991.

BUGS
     Only big and little endian byte order is supported.

BSD					December 16, 2010				      BSD


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