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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  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
       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 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 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
         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 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. 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 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.

   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 3629, Plan 9.

SEE ALSO

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

COLOPHON

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