168 lines
5.3 KiB
Groff
168 lines
5.3 KiB
Groff
|
.\" $NetBSD: hash.3,v 1.13 2010/03/22 19:30:53 joerg Exp $
|
||
|
|
.\"
|
||
|
|
.\" Copyright (c) 1990, 1993
|
||
|
|
.\" The Regents of the University of California. All rights reserved.
|
||
|
|
.\"
|
||
|
|
.\" Redistribution and use in source and binary forms, with or without
|
||
|
|
.\" modification, are permitted provided that the following conditions
|
||
|
|
.\" are met:
|
||
|
|
.\" 1. Redistributions of source code must retain the above copyright
|
||
|
|
.\" notice, this list of conditions and the following disclaimer.
|
||
|
|
.\" 2. Redistributions in binary form must reproduce the above copyright
|
||
|
|
.\" notice, this list of conditions and the following disclaimer in the
|
||
|
|
.\" documentation and/or other materials provided with the distribution.
|
||
|
|
.\" 3. Neither the name of the University nor the names of its contributors
|
||
|
|
.\" may be used to endorse or promote products derived from this software
|
||
|
|
.\" without specific prior written permission.
|
||
|
|
.\"
|
||
|
|
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
|
||
|
|
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
|
||
|
|
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
|
||
|
|
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
|
||
|
|
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
|
||
|
|
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
|
||
|
|
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
|
||
|
|
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
|
||
|
|
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
|
||
|
|
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
|
||
|
|
.\" SUCH DAMAGE.
|
||
|
|
.\"
|
||
|
|
.\" @(#)hash.3 8.6 (Berkeley) 8/18/94
|
||
|
|
.\"
|
||
|
|
.Dd September 5, 2008
|
||
|
|
.Dt HASH 3
|
||
|
|
.Os
|
||
|
|
.Sh NAME
|
||
|
|
.Nm hash
|
||
|
|
.Nd hash database access method
|
||
|
|
.Sh SYNOPSIS
|
||
|
|
.In sys/types.h
|
||
|
|
.In db.h
|
||
|
|
.Sh DESCRIPTION
|
||
|
|
The routine
|
||
|
|
.Fn 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
|
||
|
|
.Xr 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
|
||
|
|
.Fn dbopen
|
||
|
|
is defined in the
|
||
|
|
.In db.h
|
||
|
|
include file as follows:
|
||
|
|
.Bd -literal
|
||
|
|
typedef struct {
|
||
|
|
u_int bsize;
|
||
|
|
u_int ffactor;
|
||
|
|
u_int nelem;
|
||
|
|
u_int cachesize;
|
||
|
|
uint32_t (*hash)(const void *, size_t);
|
||
|
|
int lorder;
|
||
|
|
} HASHINFO;
|
||
|
|
.Ed
|
||
|
|
.Pp
|
||
|
|
The elements of this structure are as follows:
|
||
|
|
.Bl -tag -width cachesizex
|
||
|
|
.It Fa bsize
|
||
|
|
.Fa bsize
|
||
|
|
defines the hash table bucket size, and defaults to 4096 for in-memory tables.
|
||
|
|
If
|
||
|
|
.Fa 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.
|
||
|
|
.It Fa ffactor
|
||
|
|
.Fa 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.
|
||
|
|
.It Fa nelem
|
||
|
|
.Fa 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.
|
||
|
|
.It Fa cachesize
|
||
|
|
A suggested maximum size, in bytes, of the memory cache.
|
||
|
|
This value is
|
||
|
|
.Em only
|
||
|
|
advisory, and the access method will allocate more memory rather
|
||
|
|
than fail.
|
||
|
|
.It Fa hash
|
||
|
|
.Fa 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.
|
||
|
|
.It Fa 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
|
||
|
|
.Fa 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.
|
||
|
|
.El
|
||
|
|
.Pp
|
||
|
|
If the file already exists (and the
|
||
|
|
.Dv O_TRUNC
|
||
|
|
flag is not specified), the values specified for the parameters
|
||
|
|
.Fa bsize ,
|
||
|
|
.Fa ffactor ,
|
||
|
|
.Fa lorder ,
|
||
|
|
and
|
||
|
|
.Fa nelem
|
||
|
|
are ignored and the values specified when the tree was created are
|
||
|
|
used.
|
||
|
|
.Pp
|
||
|
|
If a hash function is specified,
|
||
|
|
.Fn 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
|
||
|
|
.\".Xr dbm 3 ,
|
||
|
|
.\"and
|
||
|
|
.\".Xr ndbm 3
|
||
|
|
.\"are provided, however these interfaces are not compatible with
|
||
|
|
.\"previous file formats.
|
||
|
|
.Sh ERRORS
|
||
|
|
The
|
||
|
|
.Nm
|
||
|
|
access method routines may fail and set
|
||
|
|
.Va errno
|
||
|
|
for any of the errors specified for the library routine
|
||
|
|
.Xr dbopen 3 .
|
||
|
|
.Sh SEE ALSO
|
||
|
|
.Xr btree 3 ,
|
||
|
|
.Xr dbopen 3 ,
|
||
|
|
.Xr mpool 3 ,
|
||
|
|
.Xr recno 3
|
||
|
|
.Pp
|
||
|
|
.Rs
|
||
|
|
.%T "Dynamic Hash Tables"
|
||
|
|
.%A Per-Ake Larson
|
||
|
|
.%J Communications of the ACM
|
||
|
|
.%D April 1988
|
||
|
|
.Re
|
||
|
|
.Rs
|
||
|
|
.%T "A New Hash Package for UNIX"
|
||
|
|
.%A Margo Seltzer
|
||
|
|
.%J USENIX Proceedings
|
||
|
|
.%D Winter 1991
|
||
|
|
.Re
|
||
|
|
.Sh BUGS
|
||
|
|
Only big and little endian byte order is supported.
|