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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
      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 for 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
      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
      A user-defined hash function. Because 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 option 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).


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.

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