NAME

       hash - hash database access method

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/types.h>
       #include <db.h>

DESCRIPTION

       Note  well:  This page documents interfaces provided up until glibc 2.1.  Since glibc 2.2,
       glibc no longer provides these  interfaces.   Probably,  you  are  looking  for  the  APIs
       provided by the libdb library instead.

       The  routine  dbopen(3)  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(3) is defined in the <db.h>
       include file as follows:

           typedef struct {
               unsigned int       bsize;
               unsigned int       ffactor;
               unsigned int       nelem;
               unsigned int       cachesize;
               uint32_t         (*hash)(const void *, size_t);
               int         lorder;
           } HASHINFO;

       The elements of this structure are as follows:

       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   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     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 is the 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      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.  A 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    is  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 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 attempts to determine if the hash function
       specified is the same as the one with which the database was created, and fails 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).

BUGS

       Only big and little endian byte order are supported.

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.