167 lines
5.3 KiB
Groff
167 lines
5.3 KiB
Groff
.\" $NetBSD: hash.3,v 1.13 2010/03/22 19:30:53 joerg Exp $
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.\"
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.\" Copyright (c) 1990, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)hash.3 8.6 (Berkeley) 8/18/94
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.\"
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.Dd September 5, 2008
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.Dt HASH 3
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.Os
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.Sh NAME
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.Nm hash
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.Nd hash database access method
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.Sh SYNOPSIS
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.In sys/types.h
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.In db.h
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.Sh DESCRIPTION
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The routine
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.Fn dbopen
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is the library interface to database files.
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One of the supported file formats is hash files.
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The general description of the database access methods is in
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.Xr dbopen 3 ,
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this manual page describes only the hash specific information.
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.Pp
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The hash data structure is an extensible, dynamic hashing scheme.
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.Pp
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The access method specific data structure provided to
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.Fn dbopen
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is defined in the
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.In db.h
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include file as follows:
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.Bd -literal
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typedef struct {
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u_int bsize;
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u_int ffactor;
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u_int nelem;
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u_int cachesize;
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uint32_t (*hash)(const void *, size_t);
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int lorder;
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} HASHINFO;
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.Ed
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.Pp
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The elements of this structure are as follows:
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.Bl -tag -width cachesizex
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.It Fa bsize
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.Fa bsize
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defines the hash table bucket size, and defaults to 4096 for in-memory tables.
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If
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.Fa bsize
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is 0 (no bucket size is specified) a bucket size is chosen based on the
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underlying file system I/O block size.
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It may be preferable to increase the page size for disk-resident
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tables and tables with large data items.
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.It Fa ffactor
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.Fa ffactor
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indicates a desired density within the hash table.
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It is an approximation of the number of keys allowed to accumulate in
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any one bucket, determining when the hash table grows or shrinks.
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The default value is 8.
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.It Fa nelem
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.Fa nelem
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is an estimate of the final size of the hash table.
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If not set or set too low, hash tables will expand gracefully as keys
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are entered, although a slight performance degradation may be
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noticed.
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The default value is 1.
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.It Fa cachesize
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A suggested maximum size, in bytes, of the memory cache.
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This value is
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.Em only
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advisory, and the access method will allocate more memory rather
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than fail.
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.It Fa hash
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.Fa hash
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is a user defined hash function.
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Since no hash function performs equally well on all possible data, the
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user may find that the built-in hash function does poorly on a
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particular data set.
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User specified hash functions must take two arguments (a pointer to a
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byte string and a length) and return a 32-bit quantity to be used as
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the hash value.
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.It Fa lorder
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The byte order for integers in the stored database metadata.
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The number should represent the order as an integer; for example,
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big endian order would be the number 4,321.
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If
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.Fa lorder
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is 0 (no order is specified) the current host order is used.
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If the file already exists, the specified value is ignored and the
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value specified when the tree was created is used.
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.El
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.Pp
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If the file already exists (and the
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.Dv O_TRUNC
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flag is not specified), the values specified for the parameters
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.Fa bsize ,
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.Fa ffactor ,
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.Fa lorder ,
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and
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.Fa nelem
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are ignored and the values specified when the tree was created are
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used.
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.Pp
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If a hash function is specified,
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.Fn hash_open
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will attempt to determine if the hash function specified is the same
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as the one with which the database was created, and will fail if it is
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not.
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.\".Pp
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.\"Backward compatible interfaces to the routines described in
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.\".Xr dbm 3 ,
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.\"and
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.\".Xr ndbm 3
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.\"are provided, however these interfaces are not compatible with
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.\"previous file formats.
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.Sh ERRORS
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The
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.Nm
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access method routines may fail and set
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.Va errno
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for any of the errors specified for the library routine
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.Xr dbopen 3 .
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.Sh SEE ALSO
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.Xr btree 3 ,
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.Xr dbopen 3 ,
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.Xr mpool 3 ,
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.Xr recno 3
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.Pp
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.Rs
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.%T "Dynamic Hash Tables"
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.%A Per-Ake Larson
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.%J Communications of the ACM
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.%D April 1988
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.Re
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.Rs
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.%T "A New Hash Package for UNIX"
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.%A Margo Seltzer
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.%J USENIX Proceedings
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.%D Winter 1991
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.Re
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.Sh BUGS
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Only big and little endian byte order is supported.
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