HASH(3) BSD Library Functions Manual HASH(3)NAMEhash — hash database access method
SYNOPSIS
#include <sys/types.h>
#include <db.h>
DESCRIPTION
The routine dbopen() 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() is defined
in the <db.h> header as follows:
typedef struct {
u_int bsize;
u_int ffactor;
u_int nelem;
u_int cachesize;
uint32_t (*hash)(const void *, size_t);
int lorder;
} HASHINFO;
The elements of this structure are as follows:
bsize bsize defines the hash table bucket size, and defaults to
4096 for in-memory tables. If bsize is 0 (no bucket size is
specified) a bucket size is chosen based on the underlying
file system I/O block size. It may be preferable to increase
the page size for disk-resident tables and tables with large
data items.
ffactor ffactor indicates a desired density within the hash table.
It is an approximation of the number of keys allowed to accu‐
mulate in any one bucket, determining when the hash table
grows or shrinks. The default value is 8.
nelem 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 A 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 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. 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 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 the parameters 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() will attempt to determine if
the hash function specified is the same as the one with which the data‐
base was created, and will fail if it is not.
ERRORS
The hash access method routines may fail and set errno for any of the
errors specified for the library routine dbopen(3).
SEE ALSObtree(3), dbopen(3), mpool(3), recno(3)
Per-Ake Larson, "Dynamic Hash Tables", Communications of the ACM, Issue
4, Volume 31, April 1988.
Margo Seltzer, "A New Hash Package for UNIX", Proceedings of the 1991
Winter USENIX Technical Conference, USENIX Association,
http://www.usenix.org/publications/library/proceedings/seltzer2.pdf,
173-184, January 1991.
BUGS
Only big and little endian byte order is supported.
BSD December 16, 2010 BSD
