Originální popis anglicky:
hash - hash database access method
Návod, kniha: Library Functions Manual
#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
- 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.
- 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.
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.
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)
Dynamic Hash Tables, Per-Ake Larson, Communications of the ACM, April
1988.
A New Hash Package for UNIX, Margo Seltzer, USENIX Proceedings, Winter
1991.
Only big and little endian byte order is supported.
