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RedHat 9 (Linux i386) - man page for hash (redhat section 3)

HASH(3) 			     Library Functions Manual				  HASH(3)

NAME
       hash - hash database access method

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

DESCRIPTION
       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  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
	      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  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
	      A suggested maximum size, in bytes, of the memory cache.	This value is only  advi-
	      sory, and the access method will allocate more memory rather than fail.

       hash   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.  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  num-
	      ber  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 parameters 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.

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

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.

BUGS
       Only big and little endian byte order is supported.

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


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