hash.3

早期freebsd实现

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.\"	@(#)hash.3	8.5 (Berkeley) 2/21/94
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.TH HASH 3 "February 21, 1994"
.UC 7
.SH NAME
hash \- hash database access method
.SH SYNOPSIS
.nf
.ft B
#include <sys/types.h>
#include <db.h>
.ft R
.fi
.SH DESCRIPTION
The routine
.IR 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
.IR dbopen (3),
this manual page describes only the hash specific information.
.PP
The hash data structure is an extensible, dynamic hashing scheme.
.PP
The access method specific data structure provided to
.I dbopen
is defined in the <db.h> include file as follows:
.sp
typedef struct {
.RS
u_int bsize;
.br
u_int ffactor;
.br
u_int nelem;
.br
u_int cachesize;
.br
u_int32_t (*hash)(const void *, size_t);
.br
int lorder;
.RE
} HASHINFO;
.PP
The elements of this structure are as follows:
.TP
bsize
.I 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.
.TP
ffactor
.I 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.
.TP
nelem
.I 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.
.TP
cachesize
A suggested maximum size, in bytes, of the memory cache.
This value is
.B only
advisory, and the access method will allocate more memory rather
than fail.
.TP
hash
.I 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.
.TP
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
.I 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.
.PP
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.
.PP
If a hash function is specified,
.I 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.
.PP
Backward compatible interfaces to the routines described in
.IR dbm (3),
and
.IR ndbm (3)
are provided, however these interfaces are not compatible with
previous file formats.
.SH "SEE ALSO"
.IR btree (3),
.IR dbopen (3),
.IR mpool (3),
.IR recno (3)
.sp
.IR "Dynamic Hash Tables" ,
Per-Ake Larson, Communications of the ACM, April 1988.
.sp
.IR "A New Hash Package for UNIX" ,
Margo Seltzer, USENIX Proceedings, Winter 1991.
.SH BUGS
Only big and little endian byte order is supported.