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NAME
hash - hash database access method
SYNOPSIS
#include <sys/types.h>
#include <db.h>
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.
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.
COLOPHON
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4.4 Berkeley Distribution 2017-09-15 HASH(3)