hash(3)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
{
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: 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 for tables with
large data items. 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. 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. 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. A user-defined hash function.
Because 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 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. 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 option 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.
RESTRICTIONS
Only big and little endian byte order is supported.
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)
Dynamic Hash Tables, Per-Ake Larson, Communications of the ACM, April
1988.
A New Hash Package for UNIX, Margo Seltzer, USENIX Proceedings, Winter
1991.
hash(3)