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UTF-8(7)                   Linux Programmer's Manual                  UTF-8(7)



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 arguments.  In addition, the
       majority of Unix tools expects ASCII files and can't read 16-bit words as characters  without  major  modifica-
       tions.  For these reasons, UCS-2 is not a suitable external encoding of Unicode in filenames, text files, envi-
       ronment 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 greater than 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,
         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 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 charac-
         ters 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.

   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  assump-
       tions 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 com-
       mon 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,  for  exam-
       ple,  producing a two-byte sequence with first byte 0xc0 is non-conforming.  Unicode 3.1 has added the require-
       ment 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-short-
       est UTF-8 encoding.

   Standards
       ISO/IEC 10646-1:2000, Unicode 3.1, RFC 2279, Plan 9.

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

COLOPHON
       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



GNU                               2001-05-11                          UTF-8(7)