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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 performance degradation may be noticed.
       The default value is 1.

  cachesize
       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 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 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 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 provided, however these interfaces are not compatible with
  previous file formats.

RESTRICTIONS
  Only big and little endian byte order is supported.

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

RELATED INFORMATION
  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.

Index for Section 3

Alphabetical listing for H

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