hash(3) — Linux manual page


HASH(3)                   Linux Programmer's Manual                  HASH(3)

NAME         top

       hash - hash database access method

SYNOPSIS         top

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

DESCRIPTION         top

       Note well: This page documents interfaces provided in glibc up until
       version 2.1.  Since version 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 accumu‐
                 late 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 par‐
                 ticular 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

       lorder    is the byte order for integers in the stored database meta‐
                 data.  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

       A New Hash Package for UNIX, Margo Seltzer, USENIX Proceedings,
       Winter 1991.

COLOPHON         top

       This page is part of release 5.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

4.4 Berkeley Distribution        2017-09-15                          HASH(3)

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