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  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

   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

   * 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.

   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  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.

   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.

   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

   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 4.09 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


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