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

6       UTF-8 - an ASCII compatible multibyte 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 arguments.  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,  and  so on.  The ISO 10646 Universal Character Set
18       (UCS), a superset of Unicode, occupies even a 31-bit code space and the
19       obvious UCS-4 encoding for it (a sequence of 32-bit words) has the same
20       problems.
21
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.
25
26   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.
33
34       * All  UCS  characters  greater  than  0x7f  are encoded as a multibyte
35         sequence consisting only of bytes in the range 0x80 to  0xfd,  so  no
36         ASCII  byte  can appear as part of another character and there are no
37         problems with, for example,  '\0' or '/'.
38
39       * The lexicographic sorting order of UCS-4 strings is preserved.
40
41       * All possible 2^31 UCS codes can be encoded using UTF-8.
42
43       * The bytes 0xc0, 0xc1, 0xfe and 0xff  are  never  used  in  the  UTF-8
44         encoding.
45
46       * The first byte of a multibyte sequence which represents a single non-
47         ASCII UCS character is always in the range 0xc2 to 0xfd and indicates
48         how  long  this multibyte sequence is.  All further bytes in a multi‐
49         byte sequence are in the range 0x80 to 0xbf.  This allows easy resyn‐
50         chronization  and  makes  the  encoding  stateless and robust against
51         missing bytes.
52
53       * UTF-8 encoded UCS characters may be up to six bytes long, however the
54         Unicode  standard  specifies no characters above 0x10ffff, so Unicode
55         characters can be only up to four bytes long in UTF-8.
56
57   Encoding
58       The following byte sequences are used to represent  a  character.   The
59       sequence to be used depends on the UCS code number of the character:
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61       0x00000000 - 0x0000007F:
62           0xxxxxxx
63
64       0x00000080 - 0x000007FF:
65           110xxxxx 10xxxxxx
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67       0x00000800 - 0x0000FFFF:
68           1110xxxx 10xxxxxx 10xxxxxx
69
70       0x00010000 - 0x001FFFFF:
71           11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
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73       0x00200000 - 0x03FFFFFF:
74           111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
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76       0x04000000 - 0x7FFFFFFF:
77           1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
78
79       The  xxx  bit  positions are filled with the bits of the character code
80       number in binary representation.  Only the shortest possible  multibyte
81       sequence  which  can  represent the code number of the character can be
82       used.
83
84       The UCS code values 0xd800–0xdfff (UTF-16 surrogates) as well as 0xfffe
85       and  0xffff  (UCS  noncharacters) should not appear in conforming UTF-8
86       streams.
87
88   Example
89       The Unicode character 0xa9 = 1010 1001 (the copyright sign) is  encoded
90       in UTF-8 as
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92              11000010 10101001 = 0xc2 0xa9
93
94       and  character 0x2260 = 0010 0010 0110 0000 (the "not equal" symbol) is
95       encoded as:
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97              11100010 10001001 10100000 = 0xe2 0x89 0xa0
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99   Application notes
100       Users have to select a UTF-8 locale, for example with
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102              export LANG=en_GB.UTF-8
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104       in order to activate the UTF-8 support in applications.
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106       Application software that has to be aware of the used character  encod‐
107       ing should always set the locale with for example
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109              setlocale(LC_CTYPE, "")
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111       and programmers can then test the expression
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113              strcmp(nl_langinfo(CODESET), "UTF-8") == 0
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115       to  determine  whether  a  UTF-8  locale  has been selected and whether
116       therefore all plaintext standard input and output, terminal  communica‐
117       tion,  plaintext  file content, filenames and environment variables are
118       encoded in UTF-8.
119
120       Programmers accustomed to single-byte encodings such as US-ASCII or ISO
121       8859  have  to  be aware that two assumptions made so far are no longer
122       valid in UTF-8 locales.  Firstly, a single byte  does  not  necessarily
123       correspond any more to a single character.  Secondly, since modern ter‐
124       minal emulators in UTF-8  mode  also  support  Chinese,  Japanese,  and
125       Korean  double-width characters as well as nonspacing combining charac‐
126       ters, outputting a single character does not  necessarily  advance  the
127       cursor  by  one position as it did in ASCII.  Library functions such as
128       mbsrtowcs(3) and wcswidth(3) should be used today to  count  characters
129       and cursor positions.
130
131       The  official  ESC  sequence to switch from an ISO 2022 encoding scheme
132       (as used for  instance  by  VT100  terminals)  to  UTF-8  is  ESC  %  G
133       ("\x1b%G").   The  corresponding return sequence from UTF-8 to ISO 2022
134       is ESC % @ ("\x1b%@").  Other ISO 2022 sequences (such as for switching
135       the G0 and G1 sets) are not applicable in UTF-8 mode.
136
137       It  can  be  hoped  that  in the foreseeable future, UTF-8 will replace
138       ASCII and ISO 8859 at all levels as the common  character  encoding  on
139       POSIX  systems,  leading to a significantly richer environment for han‐
140       dling plain text.
141
142   Security
143       The Unicode and UCS standards require that producers of UTF-8 shall use
144       the  shortest form possible, for example, producing a two-byte sequence
145       with first byte 0xc0 is  nonconforming.   Unicode  3.1  has  added  the
146       requirement that conforming programs must not accept non-shortest forms
147       in their input.  This is for security reasons: if user input is checked
148       for  possible  security  violations, a program might check only for the
149       ASCII version of "/../" or ";" or NUL and overlook that there are  many
150       non-ASCII ways to represent these things in a non-shortest UTF-8 encod‐
151       ing.
152
153   Standards
154       ISO/IEC 10646-1:2000, Unicode 3.1, RFC 3629, Plan 9.
155

SEE ALSO

157       nl_langinfo(3), setlocale(3), charsets(7), unicode(7)
158

COLOPHON

160       This page is part of release 3.53 of the Linux  man-pages  project.   A
161       description  of  the project, and information about reporting bugs, can
162       be found at http://www.kernel.org/doc/man-pages/.
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166GNU                               2012-04-30                          UTF-8(7)
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