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 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> include file as follows:
typedef | struct { | |||
u_int |
bsize ; |
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u_int |
ffactor ; |
|||
u_int |
nelem ; |
|||
u_int |
cachesize ; |
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u_int32_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.
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.
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 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
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 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.
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.
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