DB_HASH(3)

NAME
db_hash - hash database access method

DESCRIPTION
specific details of the hashing access method.

The hash data structure is an extensible, dynamic hashing scheme. Backward compatible interfaces to the functions described in dbm(3), and ndbm(3) are provided, however these interfaces are not compatible with previous file formats.

ACCESS METHOD SPECIFIC INFORMATION
The hash access method specific data structure provided to db_open is typedef'd and named HASHINFO. A HASHINFO structure has at least the following fields, which may be initialized before calling db_open:

u_int 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.

u_int 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.

u_int 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.

u_int32_t (*hash)(const void *, size_t);
    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.

    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.

int 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.

u_int 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.

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.

DB OPERATIONS
The functions returned by db_open for the hash access method are as described in db_open(3).

ERRORS
The hash access method functions may fail and set errno for any of the errors specified for the library function db_open(3).

SEE ALSO
db_btree(3), db_lock(3), db_log(3), db_mpool(3), db_open(3), db_recno(3), db_txn(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.