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 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.
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, 173-184, January 1991, http://www.usenix.org/publications/library/proceedings/seltzer2.pdf.