hash(3) 						     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 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 mem-
	      ory 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.

       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.

RESTRICTIONS

       Only big and little endian byte order is supported.

ERRORS

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

RELATED INFORMATION

       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.  delim off

																	   hash(3)