hash(3) - NetBSD Manual Pages

HASH(3)                 NetBSD 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> header as follows:

           typedef struct {
                   u_int bsize;
                   u_int ffactor;
                   u_int nelem;
                   u_int cachesize;
                   uint32_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 defaults to
                 4096 for in-memory tables.  If bsize is 0 (no bucket size is
                 specified) a bucket size is chosen based on the underlying
                 file system I/O block size.  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 accu-
                 mulate 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 allo-
                 cate 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 argu-
                 ments (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 data-
     base was created, and will fail if it is not.


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)

     Per-Ake Larson, "Dynamic Hash Tables", Communications of the ACM, Issue
     4, Volume 31, April 1988.

     Margo Seltzer, "A New Hash Package for UNIX", Proceedings of the 1991
     Winter USENIX Technical Conference, USENIX Association,
     http://www.usenix.org/publications/library/proceedings/seltzer2.pdf,
     173-184, January 1991.


BUGS

     Only big and little endian byte order is supported.

NetBSD 9.1                     December 16, 2010                    NetBSD 9.1