HASH(3)                   OpenBSD Programmer's 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 de-
                   fault, 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 ac-
                   cumulate 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 grace-
                   fully 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 al-
                   locate more memory rather than fail.

           hash    hash is a user defined hash function.  Since no hash func-
                   tion 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 metada-
                   ta.  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 creat-
                   ed 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, although these interfaces are not compatible with
     previous file formats.

ERRORS
     The hash access method routines may fail and set errno for any of the er-
     rors 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.

OpenBSD 2.6                     August 18, 1994                              2