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

       UTF-8 - an ASCII compatible multi-byte 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  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 modifi-
       cations. For these reasons, UCS-2 is not a suitable external encoding of Unicode in  file-
       names,  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.

       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

       * 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 charac-
	 ter 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  miss-
	 ing 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.

       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 rep-
       resentation.  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.

       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

       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 char-
       acters 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.

       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-short-
       est  forms in their input. This is for security reasons: if user input is checked for pos-
       sible 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.

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

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

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

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