1UTF-8(7)                   Linux Programmer's Manual                  UTF-8(7)
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NAME

6       UTF-8 - an ASCII compatible multi-byte Unicode encoding
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DESCRIPTION

9       The  Unicode  3.0  character set occupies a 16-bit code space. The most
10       obvious Unicode encoding (known as UCS-2) consists  of  a  sequence  of
11       16-bit  words. Such strings can contain as parts of many 16-bit charac‐
12       ters bytes like '\0' or '/' which have a special meaning  in  filenames
13       and  other C library function parameters.  In addition, the majority of
14       UNIX tools expects ASCII files and can't read 16-bit words  as  charac‐
15       ters  without  major  modifications.  For these reasons, UCS-2 is not a
16       suitable external encoding of Unicode in filenames, text  files,  envi‐
17       ronment  variables, etc. The ISO 10646 Universal Character Set (UCS), a
18       superset of Unicode, occupies even a 31-bit code space and the  obvious
19       UCS-4  encoding  for it (a sequence of 32-bit words) has the same prob‐
20       lems.
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22       The UTF-8 encoding of Unicode and UCS does not have these problems  and
23       is the common way in which Unicode is used on Unix-style operating sys‐
24       tems.
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PROPERTIES

27       The UTF-8 encoding has the following nice properties:
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29       * UCS characters 0x00000000 to 0x0000007f (the classic US-ASCII charac‐
30         ters) are encoded simply as bytes 0x00 to 0x7f (ASCII compatibility).
31         This means that files and strings  which  contain  only  7-bit  ASCII
32         characters have the same encoding under both ASCII and UTF-8.
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34       * All  UCS  characters > 0x7f are encoded as a multi-byte sequence con‐
35         sisting only of bytes in the range 0x80 to 0xfd, so no ASCII byte can
36         appear  as  part  of another character and there are no problems with
37         e.g. '\0' or '/'.
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39       * The lexicographic sorting order of UCS-4 strings is preserved.
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41       * All possible 2^31 UCS codes can be encoded using UTF-8.
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43       * The bytes 0xfe and 0xff are never used in the UTF-8 encoding.
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45       * The first byte of a multi-byte sequence  which  represents  a  single
46         non-ASCII UCS character is always in the range 0xc0 to 0xfd and indi‐
47         cates how long this multi-byte sequence is. All further  bytes  in  a
48         multi-byte  sequence  are in the range 0x80 to 0xbf. This allows easy
49         resynchronization and makes the encoding stateless and robust against
50         missing bytes.
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52       * UTF-8 encoded UCS characters may be up to six bytes long, however the
53         Unicode standard specifies no characters above 0x10ffff,  so  Unicode
54         characters can only be up to four bytes long in UTF-8.
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ENCODING

57       The  following  byte  sequences  are used to represent a character. The
58       sequence to be used depends on the UCS code number of the character:
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60       0x00000000 - 0x0000007F:
61           0xxxxxxx
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63       0x00000080 - 0x000007FF:
64           110xxxxx 10xxxxxx
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66       0x00000800 - 0x0000FFFF:
67           1110xxxx 10xxxxxx 10xxxxxx
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69       0x00010000 - 0x001FFFFF:
70           11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
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72       0x00200000 - 0x03FFFFFF:
73           111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
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75       0x04000000 - 0x7FFFFFFF:
76           1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
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78       The xxx bit positions are filled with the bits of  the  character  code
79       number  in binary representation. Only the shortest possible multi-byte
80       sequence which can represent the code number of the  character  can  be
81       used.
82
83       The UCS code values 0xd800–0xdfff (UTF-16 surrogates) as well as 0xfffe
84       and 0xffff (UCS non-characters) should not appear in  conforming  UTF-8
85       streams.
86

EXAMPLE

88       The  Unicode character 0xa9 = 1010 1001 (the copyright sign) is encoded
89       in UTF-8 as
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91              11000010 10101001 = 0xc2 0xa9
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93       and character 0x2260 = 0010 0010 0110 0000 (the "not equal" symbol)  is
94       encoded as:
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96              11100010 10001001 10100000 = 0xe2 0x89 0xa0
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APPLICATION NOTES

99       Users have to select a UTF-8 locale, for example with
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101              export LANG=en_GB.UTF-8
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103       in order to activate the UTF-8 support in applications.
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105       Application  software that has to be aware of the used character encod‐
106       ing should always set the locale with for example
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108              setlocale(LC_CTYPE, "")
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110       and programmers can then test the expression
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112              strcmp(nl_langinfo(CODESET), "UTF-8") == 0
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114       to determine whether a UTF-8  locale  has  been  selected  and  whether
115       therefore  all plaintext standard input and output, terminal communica‐
116       tion, plaintext file content, filenames and environment  variables  are
117       encoded in UTF-8.
118
119       Programmers accustomed to single-byte encodings such as US-ASCII or ISO
120       8859 have to be aware that two assumptions made so far  are  no  longer
121       valid  in  UTF-8  locales.  Firstly, a single byte does not necessarily
122       correspond any more to a single character. Secondly, since modern  ter‐
123       minal  emulators  in  UTF-8  mode  also  support Chinese, Japanese, and
124       Korean double-width characters as well as non-spacing combining charac‐
125       ters,  outputting  a  single character does not necessarily advance the
126       cursor by one position as it did in ASCII.  Library functions  such  as
127       mbsrtowcs(3)  and  wcswidth(3) should be used today to count characters
128       and cursor positions.
129
130       The official ESC sequence to switch from an ISO  2022  encoding  scheme
131       (as  used  for  instance  by  VT100  terminals)  to  UTF-8  is  ESC % G
132       ("\x1b%G"). The corresponding return sequence from UTF-8 to ISO 2022 is
133       ESC % @ ("\x1b%@"). Other ISO 2022 sequences (such as for switching the
134       G0 and G1 sets) are not applicable in UTF-8 mode.
135
136       It can be hoped that in the  foreseeable  future,  UTF-8  will  replace
137       ASCII  and  ISO  8859 at all levels as the common character encoding on
138       POSIX systems, leading to a significantly richer environment  for  han‐
139       dling plain text.
140

SECURITY

142       The Unicode and UCS standards require that producers of UTF-8 shall use
143       the shortest form possible, e.g., producing a  two-byte  sequence  with
144       first  byte 0xc0 is non-conforming.  Unicode 3.1 has added the require‐
145       ment that conforming programs must not  accept  non-shortest  forms  in
146       their input. This is for security reasons: if user input is checked for
147       possible security violations, a program might check only for the  ASCII
148       version  of  "/../" or ";" or NUL and overlook that there are many non-
149       ASCII ways to represent these things in a non-shortest UTF-8 encoding.
150

STANDARDS

152       ISO/IEC 10646-1:2000, Unicode 3.1, RFC 2279, Plan 9.
153

AUTHOR

155       Markus Kuhn <mgk25@cl.cam.ac.uk>
156

SEE ALSO

158       nl_langinfo(3), setlocale(3), charsets(7), unicode(7)
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162GNU                               2001-05-11                          UTF-8(7)
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