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:
bsize
Bsize
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
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.
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.
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)
Dynamic Hash Tables, Per-Ake Larson, Communications of the ACM, April 1988.
A New Hash Package for UNIX, Margo Seltzer, USENIX Proceedings, Winter 1991.
This page is part of release 2.70 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.
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