hash
—
hash database access method
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;
uint32_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, 4096
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 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 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 interfaces
are not compatible with previous file formats.
The hash
access method routines may fail and set
errno for any of the errors specified for the library
routine
dbopen(3).
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.
Only big and little endian byte order is supported.