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-.\" Copyright (C) Markus Kuhn, 1996, 2001
-.\"
-.\" SPDX-License-Identifier: GPL-2.0-or-later
-.\"
-.\" 1995-11-26 Markus Kuhn <mskuhn@cip.informatik.uni-erlangen.de>
-.\" First version written
-.\" 2001-05-11 Markus Kuhn <mgk25@cl.cam.ac.uk>
-.\" Update
-.\"
-.TH UTF-8 7 2024-03-14 "Linux man-pages 6.7"
-.SH NAME
-UTF-8 \- an ASCII compatible multibyte Unicode encoding
-.SH DESCRIPTION
-The Unicode 3.0 character set occupies a 16-bit code space.
-The most obvious
-Unicode encoding (known as UCS-2)
-consists of a sequence of 16-bit words.
-Such strings can contain\[em]as part of many 16-bit characters\[em]bytes
-such as \[aq]\e0\[aq] or \[aq]/\[aq], which have a
-special meaning in filenames and other C library function arguments.
-In addition, the majority of UNIX tools expect ASCII files and can't
-read 16-bit words as characters without major modifications.
-For these reasons,
-UCS-2 is not a suitable external encoding of Unicode
-in filenames, text files, environment variables, and so on.
-The ISO/IEC 10646 Universal Character Set (UCS),
-a superset of Unicode, occupies an even larger code
-space\[em]31\ bits\[em]and the obvious
-UCS-4 encoding for it (a sequence of 32-bit words) has the same problems.
-.P
-The UTF-8 encoding of Unicode and UCS
-does not have these problems and is the common way in which
-Unicode is used on UNIX-style operating systems.
-.SS Properties
-The UTF-8 encoding has the following nice properties:
-.IP \[bu] 3
-UCS
-characters 0x00000000 to 0x0000007f (the classic US-ASCII
-characters) are encoded simply as bytes 0x00 to 0x7f (ASCII
-compatibility).
-This means that files and strings which contain only
-7-bit ASCII characters have the same encoding under both
-ASCII
-and
-UTF-8.
-.IP \[bu]
-All UCS characters greater than 0x7f are encoded as a multibyte sequence
-consisting only of bytes in the range 0x80 to 0xfd, so no ASCII
-byte can appear as part of another character and there are no
-problems with, for example, \[aq]\e0\[aq] or \[aq]/\[aq].
-.IP \[bu]
-The lexicographic sorting order of UCS-4 strings is preserved.
-.IP \[bu]
-All possible 2\[ha]31 UCS codes can be encoded using UTF-8.
-.IP \[bu]
-The bytes 0xc0, 0xc1, 0xfe, and 0xff are never used in the UTF-8 encoding.
-.IP \[bu]
-The first byte of a multibyte sequence which represents a single non-ASCII
-UCS character is always in the range 0xc2 to 0xfd and indicates how long
-this multibyte sequence is.
-All further bytes in a multibyte sequence
-are in the range 0x80 to 0xbf.
-This allows easy resynchronization and
-makes the encoding stateless and robust against missing bytes.
-.IP \[bu]
-UTF-8 encoded UCS characters may be up to six bytes long, however the
-Unicode standard specifies no characters above 0x10ffff, so Unicode characters
-can be only up to four bytes long in
-UTF-8.
-.SS Encoding
-The following byte sequences are used to represent a character.
-The sequence to be used depends on the UCS code number of the character:
-.TP
-0x00000000 \- 0x0000007F:
-.RI 0 xxxxxxx
-.TP
-0x00000080 \- 0x000007FF:
-.RI 110 xxxxx
-.RI 10 xxxxxx
-.TP
-0x00000800 \- 0x0000FFFF:
-.RI 1110 xxxx
-.RI 10 xxxxxx
-.RI 10 xxxxxx
-.TP
-0x00010000 \- 0x001FFFFF:
-.RI 11110 xxx
-.RI 10 xxxxxx
-.RI 10 xxxxxx
-.RI 10 xxxxxx
-.TP
-0x00200000 \- 0x03FFFFFF:
-.RI 111110 xx
-.RI 10 xxxxxx
-.RI 10 xxxxxx
-.RI 10 xxxxxx
-.RI 10 xxxxxx
-.TP
-0x04000000 \- 0x7FFFFFFF:
-.RI 1111110 x
-.RI 10 xxxxxx
-.RI 10 xxxxxx
-.RI 10 xxxxxx
-.RI 10 xxxxxx
-.RI 10 xxxxxx
-.P
-The
-.I xxx
-bit positions are filled with the bits of the character code number in
-binary representation, most significant bit first (big-endian).
-Only the shortest possible multibyte sequence
-which can represent the code number of the character can be used.
-.P
-The UCS code values 0xd800\[en]0xdfff (UTF-16 surrogates) as well as 0xfffe and
-0xffff (UCS noncharacters) should not appear in conforming UTF-8 streams.
-According to RFC 3629 no point above U+10FFFF should be used,
-which limits characters to four bytes.
-.SS Example
-The Unicode character 0xa9 = 1010 1001 (the copyright sign) is encoded
-in UTF-8 as
-.P
-.RS
-11000010 10101001 = 0xc2 0xa9
-.RE
-.P
-and character 0x2260 = 0010 0010 0110 0000 (the "not equal" symbol) is
-encoded as:
-.P
-.RS
-11100010 10001001 10100000 = 0xe2 0x89 0xa0
-.RE
-.SS Application notes
-Users have to select a UTF-8 locale, for example with
-.P
-.RS
-export LANG=en_GB.UTF-8
-.RE
-.P
-in order to activate the UTF-8 support in applications.
-.P
-Application software that has to be aware of the used character
-encoding should always set the locale with for example
-.P
-.RS
-setlocale(LC_CTYPE, "")
-.RE
-.P
-and programmers can then test the expression
-.P
-.RS
-strcmp(nl_langinfo(CODESET), "UTF-8") == 0
-.RE
-.P
-to determine whether a UTF-8 locale has been selected and whether
-therefore all plaintext standard input and output, terminal
-communication, plaintext file content, filenames, and environment
-variables are encoded in UTF-8.
-.P
-Programmers accustomed to single-byte encodings
-such as US-ASCII or ISO/IEC\~8859
-have to be aware that two assumptions made so far are no longer valid
-in UTF-8 locales.
-Firstly, a single byte does not necessarily correspond any
-more to a single character.
-Secondly, since modern terminal emulators in UTF-8
-mode also support Chinese, Japanese, and Korean
-double-width characters as well as nonspacing combining characters,
-outputting a single character does not necessarily advance the cursor
-by one position as it did in ASCII.
-Library functions such as
-.BR mbsrtowcs (3)
-and
-.BR wcswidth (3)
-should be used today to count characters and cursor positions.
-.P
-The official ESC sequence to switch from an ISO/IEC\~2022
-encoding scheme (as used for instance by VT100 terminals) to
-UTF-8 is ESC % G
-("\ex1b%G").
-The corresponding return sequence from
-UTF-8 to ISO/IEC\~2022 is ESC % @ ("\ex1b%@").
-Other ISO/IEC\~2022 sequences (such as
-for switching the G0 and G1 sets) are not applicable in UTF-8 mode.
-.SS Security
-The Unicode and UCS standards require that producers of UTF-8
-shall use the shortest form possible, for example, producing a two-byte
-sequence with first byte 0xc0 is nonconforming.
-Unicode 3.1 has added the requirement that conforming programs must not accept
-non-shortest forms in their input.
-This is for security reasons: if
-user input is checked for possible security violations, a program
-might check only for the ASCII
-version of "/../" or ";" or NUL and overlook that there are many
-non-ASCII ways to represent these things in a non-shortest UTF-8
-encoding.
-.SS Standards
-ISO/IEC 10646-1:2000, Unicode 3.1, RFC\ 3629, Plan 9.
-.\" .SH AUTHOR
-.\" Markus Kuhn <mgk25@cl.cam.ac.uk>
-.SH SEE ALSO
-.BR locale (1),
-.BR nl_langinfo (3),
-.BR setlocale (3),
-.BR charsets (7),
-.BR unicode (7)