freebsd-nq/lib/libc/locale/utf8.5

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