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HASH(3) Linux Programmer's Manual HASH(3)
hash - hash database access method
#include <sys/types.h>
#include <db.h>
The routine dbopen(3) 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(3) is defined in
the <db.h> include file as follows:
typedef struct {
unsigned int bsize;
unsigned int ffactor;
unsigned int nelem;
unsigned int cachesize;
uint32_t (*hash)(const void *, size_t);
int lorder;
} HASHINFO;
The elements of this structure are as follows:
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 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 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 is the 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 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. A 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 is 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 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).
Only big and little endian byte order are supported.
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 3.32 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/.
4.4 Berkeley Distribution 1994-08-18 HASH(3)
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