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NAME    [Toc]    [Back]

       UTF-8 - an ASCII compatible multi-byte Unicode encoding

DESCRIPTION    [Toc]    [Back]

       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    [Toc]    [Back]

       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    [Toc]    [Back]

       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 multi-byte
       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 non-characters) should not appear in  conforming  UTF-8

EXAMPLES    [Toc]    [Back]

       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    [Toc]    [Back]

       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 charac-
       ters,  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    [Toc]    [Back]

       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 nonASCII
 ways to represent these things in a non-shortest UTF-8  encoding.

STANDARDS    [Toc]    [Back]

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

AUTHOR    [Toc]    [Back]

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

SEE ALSO    [Toc]    [Back]

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

GNU				  2001-05-11			      UTF-8(7)
[ Back ]
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