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


6       UTF-8 - an ASCII compatible multibyte Unicode encoding


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 part of many 16-bit charac‐
12       ters—bytes such as '\0' or '/', which have a special meaning  in  file‐
13       names  and other C library function arguments.  In addition, the major‐
14       ity of UNIX tools expect ASCII files and can't  read  16-bit  words  as
15       characters  without  major  modifications.  For these reasons, UCS-2 is
16       not a suitable external encoding of Unicode in filenames,  text  files,
17       environment  variables,  and  so on.  The ISO 10646 Universal Character
18       Set (UCS),  a  superset  of  Unicode,  occupies  an  even  larger  code
19       space—31 bits—and  the  obvious  UCS-4  encoding  for it (a sequence of
20       32-bit words) has the same problems.
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.
26   Properties
27       The UTF-8 encoding has the following nice properties:
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 .
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 '/'.
39       * The lexicographic sorting order of UCS-4 strings is preserved.
41       * All possible 2^31 UCS codes can be encoded using UTF-8.
43       * The bytes 0xc0, 0xc1, 0xfe, and 0xff are  never  used  in  the  UTF-8
44         encoding.
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.
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.
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:
61       0x00000000 - 0x0000007F:
62           0xxxxxxx
64       0x00000080 - 0x000007FF:
65           110xxxxx 10xxxxxx
67       0x00000800 - 0x0000FFFF:
68           1110xxxx 10xxxxxx 10xxxxxx
70       0x00010000 - 0x001FFFFF:
71           11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
73       0x00200000 - 0x03FFFFFF:
74           111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
76       0x04000000 - 0x7FFFFFFF:
77           1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
79       The  xxx  bit  positions are filled with the bits of the character code
80       number in binary  representation,  most  significant  bit  first  (big-
81       endian).   Only the shortest possible multibyte sequence which can rep‐
82       resent the code number of the character can be used.
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. According to RFC 3629 no point above U+10FFFF should be  used,
87       which limits characters to four bytes.
89   Example
90       The  Unicode character 0xa9 = 1010 1001 (the copyright sign) is encoded
91       in UTF-8 as
93              11000010 10101001 = 0xc2 0xa9
95       and character 0x2260 = 0010 0010 0110 0000 (the "not equal" symbol)  is
96       encoded as:
98              11100010 10001001 10100000 = 0xe2 0x89 0xa0
100   Application notes
101       Users have to select a UTF-8 locale, for example with
103              export LANG=en_GB.UTF-8
105       in order to activate the UTF-8 support in applications.
107       Application  software that has to be aware of the used character encod‐
108       ing should always set the locale with for example
110              setlocale(LC_CTYPE, "")
112       and programmers can then test the expression
114              strcmp(nl_langinfo(CODESET), "UTF-8") == 0
116       to determine whether a UTF-8  locale  has  been  selected  and  whether
117       therefore  all plaintext standard input and output, terminal communica‐
118       tion, plaintext file content, filenames and environment  variables  are
119       encoded in UTF-8.
121       Programmers accustomed to single-byte encodings such as US-ASCII or ISO
122       8859 have to be aware that two assumptions made so far  are  no  longer
123       valid  in  UTF-8  locales.  Firstly, a single byte does not necessarily
124       correspond any more to a single character.  Secondly, since modern ter‐
125       minal  emulators  in  UTF-8  mode  also  support Chinese, Japanese, and
126       Korean double-width characters as well as nonspacing combining  charac‐
127       ters,  outputting  a  single character does not necessarily advance the
128       cursor by one position as it did in ASCII.  Library functions  such  as
129       mbsrtowcs(3)  and  wcswidth(3) should be used today to count characters
130       and cursor positions.
132       The official ESC sequence to switch from an ISO  2022  encoding  scheme
133       (as  used  for  instance  by  VT100  terminals)  to  UTF-8  is  ESC % G
134       ("\x1b%G").  The corresponding return sequence from UTF-8 to  ISO  2022
135       is ESC % @ ("\x1b%@").  Other ISO 2022 sequences (such as for switching
136       the G0 and G1 sets) are not applicable in UTF-8 mode.
138   Security
139       The Unicode and UCS standards require that producers of UTF-8 shall use
140       the  shortest form possible, for example, producing a two-byte sequence
141       with first byte 0xc0 is  nonconforming.   Unicode  3.1  has  added  the
142       requirement that conforming programs must not accept non-shortest forms
143       in their input.  This is for security reasons: if user input is checked
144       for  possible  security  violations, a program might check only for the
145       ASCII version of "/../" or ";" or NUL and overlook that there are  many
146       non-ASCII ways to represent these things in a non-shortest UTF-8 encod‐
147       ing.
149   Standards
150       ISO/IEC 10646-1:2000, Unicode 3.1, RFC 3629, Plan 9.


153       locale(1), nl_langinfo(3), setlocale(3), charsets(7), unicode(7)


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163GNU                               2016-07-17                          UTF-8(7)