hash - hash database access method
#include <sys/types.h>
#include <db.h>
Note well: This page documents interfaces provided in glibc up until
version 2.1. Since version 2.2, glibc no longer provides these interfaces.
Probably, you are looking for the APIs provided by the
libdb library
instead.
The routine
dbopen(3) 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(3) is
defined in the
<db.h> include file as follows:
typedef struct {
unsigned int bsize;
unsigned int ffactor;
unsigned int nelem;
unsigned int cachesize;
uint32_t (*hash)(const void *, size_t);
int lorder;
} HASHINFO;
The elements of this structure are as follows:
- 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
- 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
- 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
- is the 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
- 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. A 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
- is 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
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 attempts to determine if the
hash function specified is the same as the one with which the database was
created, and fails 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).
Only big and little endian byte order are supported.
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.