fbbd9655e5
Renumber cluase 4 to 3, per what everybody else did when BSD granted them permission to remove clause 3. My insistance on keeping the same numbering for legal reasons is too pedantic, so give up on that point. Submitted by: Jan Schaumann <jschauma@stevens.edu> Pull Request: https://github.com/freebsd/freebsd/pull/96
197 lines
5.1 KiB
Groff
197 lines
5.1 KiB
Groff
.\" 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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.\" $FreeBSD$
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.\"
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.Dd August 18, 1994
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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
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.Nm
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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
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.Nm
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specific information.
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.Pp
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The
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.Nm
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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 indent
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.It Va bsize
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The
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.Va bsize
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element
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defines the
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.Nm
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table bucket size, and is, by default, 4096 bytes.
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It may be preferable to increase the page size for disk-resident tables
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and tables with large data items.
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.It Va ffactor
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The
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.Va ffactor
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element
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indicates a desired density within the
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.Nm
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table.
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It is an approximation of the number of keys allowed to accumulate in any
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one bucket, determining when the
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.Nm
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table grows or shrinks.
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The default value is 8.
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.It Va nelem
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The
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.Va nelem
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element
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is an estimate of the final size of the
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.Nm
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table.
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If not set or set too low,
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.Nm
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tables will expand gracefully as keys
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are entered, although a slight performance degradation may be noticed.
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The default value is 1.
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.It Va 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 Va hash
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The
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.Va hash
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element
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is a user defined
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.Nm
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function.
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Since no
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.Nm
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function performs equally well on all possible data, the
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user may find that the built-in
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.Nm
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function does poorly on a particular
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data set.
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User specified
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.Nm
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functions must take two arguments (a pointer to a byte
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string and a length) and return a 32-bit quantity to be used as the
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.Nm
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value.
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.It Va 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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.Va 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
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values specified for the
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.Va bsize , ffactor , lorder
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and
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.Va nelem
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arguments
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are
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ignored and the values specified when the tree was created are used.
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.Pp
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If a
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.Nm
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function is specified,
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.Fn hash_open
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will attempt to determine if the
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.Nm
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function specified is the same as
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the one with which the database was created, and will fail if it is not.
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.Pp
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Backward compatible interfaces to the older
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.Em dbm
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and
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.Em ndbm
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routines 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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.Rs
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.%T "Dynamic Hash Tables"
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.%A Per-Ake Larson
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.%R "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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.%R "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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