HASH(3) BSD Library Functions Manual 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 The bsize element 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
The ffactor element 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 The nelem element 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 The hash element 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 par-ticular particular
ticular 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 speci-fied specified
fied 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 bsize,
ffactor, lorder and nelem arguments 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 older dbm and ndbm routines are provided, however these inter-faces interfaces
faces 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)
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.
BSD August 18, 1994 BSD
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