hash(3) — Linux manual page


hash(3)                 Library Functions Manual                 hash(3)

NAME         top

       hash - hash database access method

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

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

DESCRIPTION         top

       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.

              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.

              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         top

       The hash access method routines may fail and set errno for any of
       the errors specified for the library routine dbopen(3).

BUGS         top

       Only big and little endian byte order are supported.

SEE ALSO         top

       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.

4.4 Berkeley Distribution        (date)                          hash(3)

Pages that refer to this page: btree(3)dbopen(3)mpool(3)recno(3)