utf-8

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

NAME
       UTF-8 - an ASCII compatible multibyte Unicode encoding

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--as part of many 16-bit charac-
       ters--bytes such as '\0' or '/', which have a special meaning in  file-
       names  and other C library function arguments.  In addition, the major-
       ity 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 10646 Universal Character
       Set (UCS),  a  superset  of  Unicode,  occupies  an  even  larger  code
       space--31 bits--and  the  obvious  UCS-4 encoding for it (a sequence of
       32-bit words) has the same problems.

       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 sys-
       tems.

   Properties
       The UTF-8 encoding has the following nice properties:

       * UCS characters 0x00000000 to 0x0000007f (the classic US-ASCII charac-
         ters) 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 .

       * 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,  '\0' or '/'.

       * The lexicographic sorting order of UCS-4 strings is preserved.

       * All possible 2^31 UCS codes can be encoded using UTF-8.

       * The bytes 0xc0, 0xc1, 0xfe, and 0xff are  never  used  in  the  UTF-8
         encoding.

       * 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 multi-
         byte sequence are in the range 0x80 to 0xbf.  This allows easy resyn-
         chronization  and  makes  the  encoding  stateless and robust against
         missing bytes.

       * 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.

   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:

       0x00000000 - 0x0000007F:
           0xxxxxxx

       0x00000080 - 0x000007FF:
           110xxxxx 10xxxxxx

       0x00000800 - 0x0000FFFF:
           1110xxxx 10xxxxxx 10xxxxxx

       0x00010000 - 0x001FFFFF:
           11110xxx 10xxxxxx 10xxxxxx 10xxxxxx

       0x00200000 - 0x03FFFFFF:
           111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx

       0x04000000 - 0x7FFFFFFF:
           1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx

       The  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 rep-
       resent the code number of the character can be used.

       The UCS code values 0xd800-0xdfff (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.

   Example
       The  Unicode character 0xa9 = 1010 1001 (the copyright sign) is encoded
       in UTF-8 as

              11000010 10101001 = 0xc2 0xa9

       and character 0x2260 = 0010 0010 0110 0000 (the "not equal" symbol)  is
       encoded as:

              11100010 10001001 10100000 = 0xe2 0x89 0xa0

   Application notes
       Users have to select a UTF-8 locale, for example with

              export LANG=en_GB.UTF-8

       in order to activate the UTF-8 support in applications.

       Application  software that has to be aware of the used character encod-
       ing should always set the locale with for example

              setlocale(LC_CTYPE, "")

       and programmers can then test the expression

              strcmp(nl_langinfo(CODESET), "UTF-8") == 0

       to determine whether a UTF-8  locale  has  been  selected  and  whether
       therefore  all plaintext standard input and output, terminal communica-
       tion, plaintext file content, filenames and environment  variables  are
       encoded in UTF-8.

       Programmers accustomed to single-byte encodings such as US-ASCII or ISO
       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 ter-
       minal  emulators  in  UTF-8  mode  also  support Chinese, Japanese, and
       Korean double-width characters as well as nonspacing combining  charac-
       ters,  outputting  a  single character does not necessarily advance the
       cursor by one position as it did in ASCII.  Library functions  such  as
       mbsrtowcs(3)  and  wcswidth(3) should be used today to count characters
       and cursor positions.

       The official ESC sequence to switch from an ISO  2022  encoding  scheme
       (as  used  for  instance  by  VT100  terminals)  to  UTF-8  is  ESC % G
       ("\x1b%G").  The corresponding return sequence from UTF-8 to  ISO  2022
       is ESC % @ ("\x1b%@").  Other ISO 2022 sequences (such as for switching
       the G0 and G1 sets) are not applicable in UTF-8 mode.

   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 encod-
       ing.

   Standards
       ISO/IEC 10646-1:2000, Unicode 3.1, RFC 3629, Plan 9.

SEE ALSO
       locale(1), nl_langinfo(3), setlocale(3), charsets(7), unicode(7)

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