HASH(3) | Library Functions Manual | HASH(3) |
hash
— hash
database access method
#include
<sys/types.h>
#include <db.h>
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:
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.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.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.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.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.
August 18, 1994 | macOS 14.6 |