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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
   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 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).

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 4.09 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/.



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