127 lines
4.2 KiB
Groff
127 lines
4.2 KiB
Groff
.\" Copyright (c) 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" This code is derived from software contributed to Berkeley by
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.\" Paul Borman at Krystal Technologies.
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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. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. 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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.\" @(#)utf2.4 8.1 (Berkeley) 6/4/93
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.\" $FreeBSD$
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.\"
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.Dd April 7, 2004
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.Dt UTF8 5
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.Os
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.Sh NAME
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.Nm utf8
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.Nd "UTF-8, a transformation format of ISO 10646"
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.Sh SYNOPSIS
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.Nm ENCODING
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.Qq UTF-8
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.Sh DESCRIPTION
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The
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.Nm UTF-8
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encoding represents UCS-4 characters as a sequence of octets, using
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between 1 and 6 for each character.
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It is backwards compatible with
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.Tn ASCII ,
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so 0x00-0x7f refer to the
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.Tn ASCII
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character set.
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The multibyte encoding of
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.No non- Ns Tn ASCII
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characters
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consist entirely of bytes whose high order bit is set.
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The actual
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encoding is represented by the following table:
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.Bd -literal
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[0x00000000 - 0x0000007f] [00000000.0bbbbbbb] -> 0bbbbbbb
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[0x00000080 - 0x000007ff] [00000bbb.bbbbbbbb] -> 110bbbbb, 10bbbbbb
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[0x00000800 - 0x0000ffff] [bbbbbbbb.bbbbbbbb] ->
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1110bbbb, 10bbbbbb, 10bbbbbb
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[0x00010000 - 0x001fffff] [00000000.000bbbbb.bbbbbbbb.bbbbbbbb] ->
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11110bbb, 10bbbbbb, 10bbbbbb, 10bbbbbb
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[0x00200000 - 0x03ffffff] [000000bb.bbbbbbbb.bbbbbbbb.bbbbbbbb] ->
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111110bb, 10bbbbbb, 10bbbbbb, 10bbbbbb, 10bbbbbb
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[0x04000000 - 0x7fffffff] [0bbbbbbb.bbbbbbbb.bbbbbbbb.bbbbbbbb] ->
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1111110b, 10bbbbbb, 10bbbbbb, 10bbbbbb, 10bbbbbb, 10bbbbbb
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.Ed
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.Pp
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If more than a single representation of a value exists (for example,
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0x00; 0xC0 0x80; 0xE0 0x80 0x80) the shortest representation is always
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used.
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Longer ones are detected as an error as they pose a potential
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security risk, and destroy the 1:1 character:octet sequence mapping.
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.Sh COMPATIBILITY
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The
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.Nm
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encoding supersedes the
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.Xr utf2 5
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encoding.
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The only differences between the two are that
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.Nm
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handles the full 31-bit character set of
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.Tn ISO
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10646
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whereas
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.Xr utf2 5
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is limited to a 16-bit character set,
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and that
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.Xr utf2 5
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accepts redundant,
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.No non- Ns Dq "shortest form"
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representations of characters.
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.Sh SEE ALSO
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.Xr euc 5 ,
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.Xr utf2 5
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.Rs
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.%A "Rob Pike"
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.%A "Ken Thompson"
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.%T "Hello World"
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.%J "Proceedings of the Winter 1993 USENIX Technical Conference"
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.%Q "USENIX Association"
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.%D "January 1993"
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.Re
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.Rs
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.%A "F. Yergeau"
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.%T "UTF-8, a transformation format of ISO 10646"
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.%O "RFC 2279"
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.%D "January 1998"
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.Re
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.Rs
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.%Q "The Unicode Consortium"
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.%T "The Unicode Standard, Version 3.0"
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.%D "2000"
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.%O "as amended by the Unicode Standard Annex #27: Unicode 3.1 and by the Unicode Standard Annex #28: Unicode 3.2"
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.Re
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.Sh STANDARDS
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The
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.Nm
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encoding is compatible with RFC 2279 and Unicode 3.2.
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