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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> 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   The bsize element 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
             The ffactor element 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   The nelem element 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    The hash element 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 par-ticular particular
             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.

     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 speci-fied specified
             fied 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 bsize,
     ffactor, lorder and nelem arguments 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 older dbm and ndbm routines are provided, however these inter-faces interfaces
     faces 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)

     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.

BSD                                            August 18, 1994                                           BSD