HASH(3) 		   Linux Programmers Manual		      HASH(3)



NAME
       hash - hash database access method

SYNOPSIS
       #include 
       #include 

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  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;
		 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 3.05 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)