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

       hash - hash database access method

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

       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 per‐
                 formance 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 pro‐
       vided, however these interfaces are not compatible with previous file formats.

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

       Only big and little endian byte order are supported.

       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.

       This  page  is  part of release 3.74 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 http://www.kernel.org/doc/man-pages/.

4.4 Berkeley Distribution                   2012-04-23                                    HASH(3)

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