hash - hash database access method
#include <sys/types.h> #include <db.h>
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 hashspecific
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> include file as follows:
typedef struct
{
u_int bsize;
u_int ffactor;
u_int nelem;
u_int cachesize;
u_int32_t (*hash)(const void *, size_t);
int lorder;
} HASHINFO;
The elements of this structure are as follows: 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 for tables with large data items. 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. 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. A 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. A user-defined hash function. Because 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 arguments (a pointer to a byte string and a
length) and return a 32-bit quantity to be used as the
hash value. 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 option 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 database was created, and will fail
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.
Only big and little endian byte order is supported.
The hash access method routines may fail and set errno for
any of the errors specified for the library routine
dbopen(3).
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.
hash(3)
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