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Linux 2.6 - man page for hash (linux section 3)

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;

       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 3.55 of the Linux man-pages project.  A description of the
       project,    and	  information	 about	  reporting    bugs,	can    be    found     at

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

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