hash(3) Library Functions Manual hash(3)
NAME
hash - hash database access method
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/types.h>
#include <db.h>
DESCRIPTION
Note well: This page documents interfaces provided up until glibc 2.1.
Since glibc 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 de-
fined 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 allo-
cate more memory rather than fail.
hash is a user-defined hash function. Since no hash function per-
forms 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 argu-
ments (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 ig-
nored 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.
ERRORS
The hash access method routines may fail and set errno for any of the
errors specified for the library routine dbopen(3).
BUGS
Only big and little endian byte order are supported.
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.
4.4 Berkeley Distribution 2024-05-02 hash(3)
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