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       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.  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
       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)


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