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

       UTF-8 - an ASCII compatible multibyte Unicode encoding

       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  con‐
       tain—as  part  of  many  16-bit characters—bytes such as '\0' or '/', which have a special
       meaning in filenames and other C library function arguments.  In addition, the majority of
       UNIX tools expect ASCII files and can't read 16-bit words as characters without major mod‐
       ifications.  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 Charac‐
       ter 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 systems.

       The UTF-8 encoding has the following nice properties:

       * UCS characters 0x00000000 to 0x0000007f (the classic US-ASCII  characters)  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 char‐
         acter 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 multibyte sequence are in the range 0x80 to 0xbf.  This allows
         easy resynchronization and makes the  encoding  stateless  and  robust  against  missing

       * 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.

       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:

       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 rep‐
       resentation.   Only  the shortest possible multibyte sequence which can represent 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.

       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 encoding 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 communication, 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  terminal emulators in UTF-8 mode also support Chinese, Japanese, and Korean
       double-width characters as well as nonspacing combining characters,  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 char‐
       acters 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.

       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 nonconform‐
       ing.  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 encoding.

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

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

       This page is part of release 3.74 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.

GNU                                         2014-06-13                                   UTF-8(7)

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