HASH(3) 1993 HASH(3)
NAME
hash - hash database access method
SYNOPSIS
#include <sys/types.h>
#include <db.h>
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> include file as follows:
typedef struct {
int bsize;
int cachesize;
int ffactor;
u_long (*hash)(const void *, size_t);
int lorder;
int nelem;
} 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.
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.
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.
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)
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HASH(3) 1993 HASH(3)
and return an u_long 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.
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.
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.
SEE ALSO
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.
BUGS
Only big and little endian byte order is supported.
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