HASH(3) UNIX System V (August 18, 1994) 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 {
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.
ERRORS
The hash access method routines may fail and set errno for
any of the errors specified for the library routine
dbopen(3).
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.
This version of berkeley db (1.85) is free software which is
not developed nor maintained by SGI. It is known to have
some bugs that are unlikely to get fixed (See NOTES below)
in particular, the following hash operations are known to
have problems, up to corrupting databases, and should be
avoided according to http://www.sleepycat.com/db.185.html:
o Overwriting or deleting overflow hash key/data pairs
(pairs with items larger than the page size).
o Intermixing hash cursor operations with deletes.
NOTES
The default hash function in this version of db is the
Fowler/Vo/Noll hash which gives better distributions (less
collisions) on average than the publicly released version.
This version of berkeley db is 1.85. A newer enhanced
version db-2.x requires licensing. The db 2.x version is
more stable, faster, and supports concurrent accesses plus
many other new features. Check out
http://www.sleepycat.com/ for details.
Page 3 (printed 4/30/98)