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NAME | SYNOPSIS | DESCRIPTION | ERRORS | BUGS | SEE ALSO | COLOPHON |
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HASH(3) Linux Programmer's Manual HASH(3)
hash - hash database access method
#include <sys/types.h>
#include <db.h>
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 accumu‐
late 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 par‐
ticular 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 meta‐
data. 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.
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 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
4.4 Berkeley Distribution 2017-09-15 HASH(3)
Pages that refer to this page: btree(3), dbopen(3), mpool(3), recno(3)
Copyright and license for this manual page
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