2fe8fb192f
There is important information about booting non-ack images in docs/UPDATING. ack/aout-format images can't be built any more, and booting clang/ELF-format ones is a little different. Updating to the new boot monitor is recommended. Changes in this commit: . drop boot monitor -> allowing dropping ack support . facility to copy ELF boot files to /boot so that old boot monitor can still boot fairly easily, see UPDATING . no more ack-format libraries -> single-case libraries . some cleanup of OBJECT_FMT, COMPILER_TYPE, etc cases . drop several ack toolchain commands, but not all support commands (e.g. aal is gone but acksize is not yet). . a few libc files moved to netbsd libc dir . new /bin/date as minix date used code in libc/ . test compile fix . harmonize includes . /usr/lib is no longer special: without ack, /usr/lib plays no kind of special bootstrapping role any more and bootstrapping is done exclusively through packages, so releases depend even less on the state of the machine making them now. . rename nbsd_lib* to lib* . reduce mtree
172 lines
5.4 KiB
Groff
172 lines
5.4 KiB
Groff
.\" $NetBSD: hash.3,v 1.14 2010/12/16 11:57:20 jruoho 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 December 16, 2010
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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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header as follows:
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.Bd -literal -offset indent
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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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.%N Issue 4
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.%V Volume 31
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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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.%I USENIX Association
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.%B Proceedings of the 1991 Winter USENIX Technical Conference
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.%D January 1991
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.%P 173-184
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.%U http://www.usenix.org/publications/library/proceedings/seltzer2.pdf
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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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