utf2.4

早期freebsd实现

.\" Copyright (c) 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" This code is derived from software contributed to Berkeley by
.\" Paul Borman at Krystal Technologies.
.\"
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.\" modification, are permitted provided that the following conditions
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.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
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.\" 3. All advertising materials mentioning features or use of this software
.\"    must display the following acknowledgement:
.\"	This product includes software developed by the University of
.\"	California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
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.\"	@(#)utf2.4	8.1 (Berkeley) 6/4/93
.\"
.Dd "June 4, 1993"
.Dt UTF2 4
.Os
.Sh NAME
.Nm UTF2
.Nd "Universal character set Transformation Format encoding of runes
.Sh SYNOPSIS
\fBENCODING "UTF2"\fP
.Sh DESCRIPTION
The
.Nm UTF2
encoding is based on a proposed X-Open multibyte
\s-1FSS-UCS-TF\s+1 (File System Safe Universal Character Set Transformation Format) encoding as used in
.Nm Plan 9 from Bell Labs.
Although it is capable of representing more than 16 bits,
the current implementation is limited to 16 bits as defined by the
Unicode Standard.
.Pp
.Nm UTF2
representation is backwards compatible with ASCII, so 0x00-0x7f refer to the
ASCII character set.  The multibyte encoding of runes between 0x0080 and 0xffff
consist entirely of bytes whose high order bit is set.  The actual
encoding is represented by the following table:
.Bd -literal
[0x0000 - 0x007f] [00000000.0bbbbbbb] -> 0bbbbbbb
[0x0080 - 0x03ff] [00000bbb.bbbbbbbb] -> 110bbbbb, 10bbbbbb
[0x0400 - 0xffff] [bbbbbbbb.bbbbbbbb] -> 1110bbbb, 10bbbbbb, 10bbbbbb
.Ed
.sp
If more than a single representation of a value exists (for example,
0x00; 0xC0 0x80; 0xE0 0x80 0x80) the shortest representation is always
used (but the longer ones will be correctly decoded).
.Pp
The final three encodings provided by X-Open:
.Bd -literal
[00000000.000bbbbb.bbbbbbbb.bbbbbbbb] ->
	11110bbb, 10bbbbbb, 10bbbbbb, 10bbbbbb

[000000bb.bbbbbbbb.bbbbbbbb.bbbbbbbb] ->
	111110bb, 10bbbbbb, 10bbbbbb, 10bbbbbb, 10bbbbbb

[0bbbbbbb.bbbbbbbb.bbbbbbbb.bbbbbbbb] ->
	1111110b, 10bbbbbb, 10bbbbbb, 10bbbbbb, 10bbbbbb, 10bbbbbb
.Ed
.sp
which provides for the entire proposed ISO-10646 31 bit standard are currently
not implemented.
.Sh "SEE ALSO"
.Xr mklocale 1 ,
.Xr setlocale 3