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> header as
follows:
typedef struct {
u_int bsize;
u_int ffactor;
u_int nelem;
u_int cachesize;
uint32_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 defaults to 4096 for in-memory tables. If
bsize is 0 (no bucket size is specified) a bucket
size is chosen based on the underlying file system I/O block size. 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.
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, Issue 4, Volume
31, April 1988.
Margo Seltzer, A New
Hash Package for UNIX, Proceedings of the 1991 Winter
USENIX Technical Conference, USENIX Association,
http://www.usenix.org/publications/library/proceedings/seltzer2.pdf,
173-184, January
1991.
BUGS
Only big and little endian byte order is supported.