метод доступа к хэш-базе данных (hash database access method)
Имя (Name)
hash - hash database access method
Синопсис (Synopsis)
#include <sys/types.h>
#include <db.h>
Описание (Description)
Note well: This page documents interfaces provided in glibc up
until version 2.1. Since version 2.2, glibc no longer provides
these interfaces. Probably, you are looking for the APIs
provided by the libdb library instead.
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 attempts to determine
if the hash function specified is the same as the one with which
the database was created, and fails 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.
Ошибки (Error)
The hash access method routines may fail and set errno for any of
the errors specified for the library routine dbopen(3).
Ошибки (баги) (Bugs)
Only big and little endian byte order are supported.
Смотри также (See also)
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.
