HASH(3) BSD 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 de-
fault, 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 ac-
cumulate 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 grace-
fully 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 al-
locate more memory rather than fail.
hash hash is a user defined hash function. Since no hash func-
tion 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 metada-
ta. 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 data-
base 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 er-
rors 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.
MirOS BSD #10-current August 18, 1994 1
