hash -- hash database access method
#include <sys/types.h>
#include <db.h>
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 The bsize element 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
The ffactor element 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 The nelem element 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 The hash element 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 bsize, ffactor, lorder and nelem arguments 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 older dbm and ndbm routines are
provided, however these interfaces are not compatible with previous file
formats.
The hash access method routines may fail and set errno for any of the
errors specified for the library routine dbopen(3).
btree(3), dbopen(3), mpool(3), recno(3)
Per-Ake Larson, Dynamic Hash Tables, Communications of the ACM, April
1988.
Margo Seltzer, A New Hash Package for UNIX, USENIX Proceedings, Winter
1991.
Only big and little endian byte order is supported.
FreeBSD 5.2.1 August 18, 1994 FreeBSD 5.2.1 [ Back ] |
