HASH(3) OpenBSD Programmer's Manual HASH(3)NAMEhash - 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 {
unsigned int bsize;
unsigned int ffactor;
unsigned int nelem;
unsigned 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, the block size of the underlying filesystem. 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 the same as
bsize.
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, although these interfaces are not compatible with
previous file formats.
ERRORS
The hash access method routines may fail and set errno for any of the
errors specified for the library routine dbopen(3).
SEE ALSObtree(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.
BUGS
Only big and little endian byte order is supported.
OpenBSD 4.9 May 31, 2007 OpenBSD 4.9
