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 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.
4.4 Berkeley Distribution 1994-08-18 HASH(3)
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