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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 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.
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.