mirror of https://github.com/mkerrisk/man-pages
231 lines
7.3 KiB
Groff
231 lines
7.3 KiB
Groff
.\" Copyright (C) Markus Kuhn, 1996, 2001
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.\"
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.\" %%%LICENSE_START(GPLv2+_DOC_FULL)
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.\" This is free documentation; you can redistribute it and/or
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.\" modify it under the terms of the GNU General Public License as
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.\" published by the Free Software Foundation; either version 2 of
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.\" the License, or (at your option) any later version.
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.\"
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.\" The GNU General Public License's references to "object code"
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.\" and "executables" are to be interpreted as the output of any
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.\" document formatting or typesetting system, including
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.\" intermediate and printed output.
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.\"
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.\" This manual is distributed in the hope that it will be useful,
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.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
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.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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.\" GNU General Public License for more details.
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.\"
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.\" You should have received a copy of the GNU General Public
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.\" License along with this manual; if not, see
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.\" <http://www.gnu.org/licenses/>.
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.\" %%%LICENSE_END
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.\"
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.\" 1995-11-26 Markus Kuhn <mskuhn@cip.informatik.uni-erlangen.de>
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.\" First version written
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.\" 2001-05-11 Markus Kuhn <mgk25@cl.cam.ac.uk>
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.\" Update
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.\"
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.TH UTF-8 7 2019-03-06 "GNU" "Linux Programmer's Manual"
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.SH NAME
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UTF-8 \- an ASCII compatible multibyte Unicode encoding
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.SH DESCRIPTION
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The Unicode 3.0 character set occupies a 16-bit code space.
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The most obvious
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Unicode encoding (known as UCS-2)
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consists of a sequence of 16-bit words.
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Such strings can contain\(emas part of many 16-bit characters\(embytes
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such as \(aq\e0\(aq or \(aq/\(aq, which have a
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special meaning in filenames and other C library function arguments.
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In addition, the majority of UNIX tools expect ASCII files and can't
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read 16-bit words as characters without major modifications.
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For these reasons,
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UCS-2 is not a suitable external encoding of Unicode
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in filenames, text files, environment variables, and so on.
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The ISO 10646 Universal Character Set (UCS),
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a superset of Unicode, occupies an even larger code
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space\(em31\ bits\(emand the obvious
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UCS-4 encoding for it (a sequence of 32-bit words) has the same problems.
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.PP
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The UTF-8 encoding of Unicode and UCS
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does not have these problems and is the common way in which
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Unicode is used on UNIX-style operating systems.
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.SS Properties
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The UTF-8 encoding has the following nice properties:
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.TP 0.2i
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*
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UCS
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characters 0x00000000 to 0x0000007f (the classic US-ASCII
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characters) are encoded simply as bytes 0x00 to 0x7f (ASCII
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compatibility).
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This means that files and strings which contain only
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7-bit ASCII characters have the same encoding under both
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ASCII
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and
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UTF-8 .
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.TP
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*
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All UCS characters greater than 0x7f are encoded as a multibyte sequence
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consisting only of bytes in the range 0x80 to 0xfd, so no ASCII
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byte can appear as part of another character and there are no
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problems with, for example, \(aq\e0\(aq or \(aq/\(aq.
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.TP
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*
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The lexicographic sorting order of UCS-4 strings is preserved.
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.TP
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*
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All possible 2^31 UCS codes can be encoded using UTF-8.
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.TP
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*
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The bytes 0xc0, 0xc1, 0xfe, and 0xff are never used in the UTF-8 encoding.
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.TP
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*
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The first byte of a multibyte sequence which represents a single non-ASCII
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UCS character is always in the range 0xc2 to 0xfd and indicates how long
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this multibyte sequence is.
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All further bytes in a multibyte sequence
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are in the range 0x80 to 0xbf.
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This allows easy resynchronization and
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makes the encoding stateless and robust against missing bytes.
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.TP
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*
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UTF-8 encoded UCS characters may be up to six bytes long, however the
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Unicode standard specifies no characters above 0x10ffff, so Unicode characters
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can be only up to four bytes long in
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UTF-8.
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.SS Encoding
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The following byte sequences are used to represent a character.
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The sequence to be used depends on the UCS code number of the character:
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.TP 0.4i
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0x00000000 \- 0x0000007F:
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.RI 0 xxxxxxx
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.TP
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0x00000080 \- 0x000007FF:
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.RI 110 xxxxx
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.RI 10 xxxxxx
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.TP
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0x00000800 \- 0x0000FFFF:
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.RI 1110 xxxx
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.RI 10 xxxxxx
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.RI 10 xxxxxx
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.TP
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0x00010000 \- 0x001FFFFF:
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.RI 11110 xxx
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.RI 10 xxxxxx
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.RI 10 xxxxxx
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.RI 10 xxxxxx
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.TP
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0x00200000 \- 0x03FFFFFF:
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.RI 111110 xx
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.RI 10 xxxxxx
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.RI 10 xxxxxx
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.RI 10 xxxxxx
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.RI 10 xxxxxx
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.TP
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0x04000000 \- 0x7FFFFFFF:
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.RI 1111110 x
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.RI 10 xxxxxx
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.RI 10 xxxxxx
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.RI 10 xxxxxx
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.RI 10 xxxxxx
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.RI 10 xxxxxx
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.PP
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The
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.I xxx
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bit positions are filled with the bits of the character code number in
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binary representation, most significant bit first (big-endian).
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Only the shortest possible multibyte sequence
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which can represent the code number of the character can be used.
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.PP
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The UCS code values 0xd800\(en0xdfff (UTF-16 surrogates) as well as 0xfffe and
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0xffff (UCS noncharacters) should not appear in conforming UTF-8 streams. According
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to RFC 3629 no point above U+10FFFF should be used, which limits characters to four
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bytes.
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.SS Example
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The Unicode character 0xa9 = 1010 1001 (the copyright sign) is encoded
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in UTF-8 as
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.PP
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.RS
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11000010 10101001 = 0xc2 0xa9
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.RE
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.PP
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and character 0x2260 = 0010 0010 0110 0000 (the "not equal" symbol) is
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encoded as:
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.PP
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.RS
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11100010 10001001 10100000 = 0xe2 0x89 0xa0
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.RE
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.SS Application notes
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Users have to select a UTF-8 locale, for example with
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.PP
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.RS
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export LANG=en_GB.UTF-8
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.RE
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.PP
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in order to activate the UTF-8 support in applications.
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.PP
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Application software that has to be aware of the used character
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encoding should always set the locale with for example
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.PP
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.RS
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setlocale(LC_CTYPE, "")
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.RE
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.PP
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and programmers can then test the expression
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.PP
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.RS
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strcmp(nl_langinfo(CODESET), "UTF-8") == 0
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.RE
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.PP
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to determine whether a UTF-8 locale has been selected and whether
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therefore all plaintext standard input and output, terminal
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communication, plaintext file content, filenames, and environment
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variables are encoded in UTF-8.
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.PP
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Programmers accustomed to single-byte encodings such as US-ASCII or ISO 8859
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have to be aware that two assumptions made so far are no longer valid
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in UTF-8 locales.
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Firstly, a single byte does not necessarily correspond any
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more to a single character.
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Secondly, since modern terminal emulators in UTF-8
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mode also support Chinese, Japanese, and Korean
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double-width characters as well as nonspacing combining characters,
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outputting a single character does not necessarily advance the cursor
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by one position as it did in ASCII.
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Library functions such as
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.BR mbsrtowcs (3)
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and
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.BR wcswidth (3)
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should be used today to count characters and cursor positions.
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.PP
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The official ESC sequence to switch from an ISO 2022
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encoding scheme (as used for instance by VT100 terminals) to
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UTF-8 is ESC % G
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("\ex1b%G").
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The corresponding return sequence from
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UTF-8 to ISO 2022 is ESC % @ ("\ex1b%@").
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Other ISO 2022 sequences (such as
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for switching the G0 and G1 sets) are not applicable in UTF-8 mode.
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.SS Security
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The Unicode and UCS standards require that producers of UTF-8
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shall use the shortest form possible, for example, producing a two-byte
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sequence with first byte 0xc0 is nonconforming.
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Unicode 3.1 has added the requirement that conforming programs must not accept
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non-shortest forms in their input.
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This is for security reasons: if
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user input is checked for possible security violations, a program
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might check only for the ASCII
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version of "/../" or ";" or NUL and overlook that there are many
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non-ASCII ways to represent these things in a non-shortest UTF-8
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encoding.
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.SS Standards
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ISO/IEC 10646-1:2000, Unicode 3.1, RFC\ 3629, Plan 9.
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.\" .SH AUTHOR
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.\" Markus Kuhn <mgk25@cl.cam.ac.uk>
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.SH SEE ALSO
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.BR locale (1),
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.BR nl_langinfo (3),
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.BR setlocale (3),
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.BR charsets (7),
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.BR unicode (7)
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