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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  parts  of  many 16-bit characters bytes like '\0' or '/' which have a special
       meaning in filenames and other C library function arguments.  In addition, the majority of
       UNIX  tools  expects  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, etc.  The ISO 10646 Universal Character Set
       (UCS), a superset of Unicode, occupies even a 31-bit code  space  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.

   Properties
       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
         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 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 0xc0 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
         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 only  be  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.   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.

   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 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
       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.

       It can be hoped that in the foreseeable future, UTF-8 will replace ASCII and ISO  8859  at
       all  levels  as the common character encoding on POSIX systems, leading to a significantly
       richer environment for handling plain text.

   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 encoding.

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

SEE ALSO

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

COLOPHON

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