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NAME

       UTF-8 - an ASCII compatible multi-byte 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 parameters.  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 >  0x7f  are  encoded  as  a  multi-byte  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 e.g. ’\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 multi-byte sequence which represents a single
         non-ASCII UCS character is always in  the  range  0xc0  to  0xfd  and
         indicates  how long this multi-byte sequence is. All further bytes in
         a multi-byte 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  multi-byte
       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 non-characters) should not appear in conforming UTF-8
       streams.

EXAMPLES

       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  non-spacing  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, e.g., producing a two-byte sequence with
       first  byte  0xc0  is  non-conforming.   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.

AUTHOR

       Markus Kuhn <mgk25@cl.cam.ac.uk>

SEE ALSO

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