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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: 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 for tables with
       large data items.  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.  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.	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.  A user-defined  hash  function.
       Because	no  hash  function performs equally well on all possible data,
       the user may find that the built-in hash function does poorly on a par‐
       ticular 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.  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 option 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).

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.

								       hash(3)
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