hash

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

NAME
       hash - hash database access method

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

DESCRIPTION
       Note  well:  This  page documents interfaces provided in glibc up until
       version 2.1.  Since version 2.2, glibc no longer provides these  inter-
       faces.   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  de-
       scribes 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 de-
       fined 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 allo-
                 cate more memory rather than fail.

       hash      is a user-defined hash function.  Since no hash function per-
                 forms  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 ar-
                 guments (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 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
       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.

COLOPHON
       This page is part of release 5.05 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
       https://www.kernel.org/doc/man-pages/.

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