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           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-
           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
           * 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.
           The following byte sequences are used to represent  a  character.   The
           sequence to be used depends on the UCS code number of the character:
               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
           The UCS code values 0xd800-0xdfff (UTF-16 surrogates) as well as 0xfffe
           and  0xffff  (UCS  noncharacters) should not appear in conforming UTF-8
           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-
           dling plain text.
           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-
           ISO/IEC 10646-1:2000, Unicode 3.1, RFC 3629, Plan 9.


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

    GNU 2014-02-26 UTF-8(7)


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