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NAME    [Toc]    [Back]

     hash -- hash database access method

SYNOPSIS    [Toc]    [Back]

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

DESCRIPTION    [Toc]    [Back]

     The routine dbopen() 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() is defined
     in the <db.h> include file as follows:

     typedef struct {
	     u_int bsize;
	     u_int ffactor;
	     u_int nelem;
	     u_int cachesize;
	     u_int32_t (*hash)(const void *, size_t);
	     int lorder;
     } HASHINFO;

     The elements of this structure are as follows:

     bsize   The bsize element 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.

	     The ffactor element 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   The nelem element 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.

	     A 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    The hash element 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.	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  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 the bsize, ffactor, lorder and nelem arguments 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 older dbm and ndbm routines are
     provided, however these interfaces are not compatible with previous file

ERRORS    [Toc]    [Back]

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

SEE ALSO    [Toc]    [Back]

     btree(3), dbopen(3), mpool(3), recno(3)

     Per-Ake Larson, Dynamic Hash Tables, Communications of the ACM, April

     Margo Seltzer, A New Hash Package for UNIX, USENIX Proceedings, Winter

BUGS    [Toc]    [Back]

     Only big and little endian byte order is supported.

FreeBSD 5.2.1			August 18, 1994 		 FreeBSD 5.2.1
[ Back ]
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