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
              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 quan-
              tity to be used as the hash value.

       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.

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.

BUGS

       Only big and little endian byte order is supported.

4.4 Berkeley Distribution         1994-08-18                           hash(3)