1 .\" Copyright (C) Markus Kuhn, 1996, 2001
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4 .\" This is free documentation; you can redistribute it and/or
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24 .\" 1995-11-26 Markus Kuhn <mskuhn@cip.informatik.uni-erlangen.de>
25 .\" First version written
26 .\" 2001-05-11 Markus Kuhn <mgk25@cl.cam.ac.uk>
29 .TH UTF-8 7 2019-03-06 "GNU" "Linux Programmer's Manual"
31 UTF-8 \- an ASCII compatible multibyte Unicode encoding
33 The Unicode 3.0 character set occupies a 16-bit code space.
35 Unicode encoding (known as UCS-2)
36 consists of a sequence of 16-bit words.
37 Such strings can contain\(emas part of many 16-bit characters\(embytes
38 such as \(aq\e0\(aq or \(aq/\(aq, which have a
39 special meaning in filenames and other C library function arguments.
40 In addition, the majority of UNIX tools expect ASCII files and can't
41 read 16-bit words as characters without major modifications.
43 UCS-2 is not a suitable external encoding of Unicode
44 in filenames, text files, environment variables, and so on.
45 The ISO 10646 Universal Character Set (UCS),
46 a superset of Unicode, occupies an even larger code
47 space\(em31\ bits\(emand the obvious
48 UCS-4 encoding for it (a sequence of 32-bit words) has the same problems.
50 The UTF-8 encoding of Unicode and UCS
51 does not have these problems and is the common way in which
52 Unicode is used on UNIX-style operating systems.
54 The UTF-8 encoding has the following nice properties:
58 characters 0x00000000 to 0x0000007f (the classic US-ASCII
59 characters) are encoded simply as bytes 0x00 to 0x7f (ASCII
61 This means that files and strings which contain only
62 7-bit ASCII characters have the same encoding under both
68 All UCS characters greater than 0x7f are encoded as a multibyte sequence
69 consisting only of bytes in the range 0x80 to 0xfd, so no ASCII
70 byte can appear as part of another character and there are no
71 problems with, for example, \(aq\e0\(aq or \(aq/\(aq.
74 The lexicographic sorting order of UCS-4 strings is preserved.
77 All possible 2^31 UCS codes can be encoded using UTF-8.
80 The bytes 0xc0, 0xc1, 0xfe, and 0xff are never used in the UTF-8 encoding.
83 The first byte of a multibyte sequence which represents a single non-ASCII
84 UCS character is always in the range 0xc2 to 0xfd and indicates how long
85 this multibyte sequence is.
86 All further bytes in a multibyte sequence
87 are in the range 0x80 to 0xbf.
88 This allows easy resynchronization and
89 makes the encoding stateless and robust against missing bytes.
92 UTF-8 encoded UCS characters may be up to six bytes long, however the
93 Unicode standard specifies no characters above 0x10ffff, so Unicode characters
94 can be only up to four bytes long in
97 The following byte sequences are used to represent a character.
98 The sequence to be used depends on the UCS code number of the character:
100 0x00000000 \- 0x0000007F:
103 0x00000080 \- 0x000007FF:
107 0x00000800 \- 0x0000FFFF:
112 0x00010000 \- 0x001FFFFF:
118 0x00200000 \- 0x03FFFFFF:
125 0x04000000 \- 0x7FFFFFFF:
135 bit positions are filled with the bits of the character code number in
136 binary representation, most significant bit first (big-endian).
137 Only the shortest possible multibyte sequence
138 which can represent the code number of the character can be used.
140 The UCS code values 0xd800\(en0xdfff (UTF-16 surrogates) as well as 0xfffe and
141 0xffff (UCS noncharacters) should not appear in conforming UTF-8 streams. According
142 to RFC 3629 no point above U+10FFFF should be used, which limits characters to four
145 The Unicode character 0xa9 = 1010 1001 (the copyright sign) is encoded
149 11000010 10101001 = 0xc2 0xa9
152 and character 0x2260 = 0010 0010 0110 0000 (the "not equal" symbol) is
156 11100010 10001001 10100000 = 0xe2 0x89 0xa0
158 .SS Application notes
159 Users have to select a UTF-8 locale, for example with
162 export LANG=en_GB.UTF-8
165 in order to activate the UTF-8 support in applications.
167 Application software that has to be aware of the used character
168 encoding should always set the locale with for example
171 setlocale(LC_CTYPE, "")
174 and programmers can then test the expression
177 strcmp(nl_langinfo(CODESET), "UTF-8") == 0
180 to determine whether a UTF-8 locale has been selected and whether
181 therefore all plaintext standard input and output, terminal
182 communication, plaintext file content, filenames, and environment
183 variables are encoded in UTF-8.
185 Programmers accustomed to single-byte encodings such as US-ASCII or ISO 8859
186 have to be aware that two assumptions made so far are no longer valid
188 Firstly, a single byte does not necessarily correspond any
189 more to a single character.
190 Secondly, since modern terminal emulators in UTF-8
191 mode also support Chinese, Japanese, and Korean
192 double-width characters as well as nonspacing combining characters,
193 outputting a single character does not necessarily advance the cursor
194 by one position as it did in ASCII.
195 Library functions such as
199 should be used today to count characters and cursor positions.
201 The official ESC sequence to switch from an ISO 2022
202 encoding scheme (as used for instance by VT100 terminals) to
205 The corresponding return sequence from
206 UTF-8 to ISO 2022 is ESC % @ ("\ex1b%@").
207 Other ISO 2022 sequences (such as
208 for switching the G0 and G1 sets) are not applicable in UTF-8 mode.
210 The Unicode and UCS standards require that producers of UTF-8
211 shall use the shortest form possible, for example, producing a two-byte
212 sequence with first byte 0xc0 is nonconforming.
213 Unicode 3.1 has added the requirement that conforming programs must not accept
214 non-shortest forms in their input.
215 This is for security reasons: if
216 user input is checked for possible security violations, a program
217 might check only for the ASCII
218 version of "/../" or ";" or NUL and overlook that there are many
219 non-ASCII ways to represent these things in a non-shortest UTF-8
222 ISO/IEC 10646-1:2000, Unicode 3.1, RFC\ 3629, Plan 9.
224 .\" Markus Kuhn <mgk25@cl.cam.ac.uk>