hash man page on ElementaryOS

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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
       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;
		 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 data‐
       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 3.54 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	  2012-04-23			       HASH(3)
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