hash


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

DESCRIPTION
       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 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
                 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  meta-
                 data.   The  number should represent the order as an integer;

       base 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 3.23 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



4.4 Berkeley Distribution         1994-08-18                           HASH(3)
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