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

NAME
       UTF-8 - an ASCII compatible multibyte 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 charac‐
       ters 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  charac‐
       ters  without  major  modifications.  For these reasons, UCS-2 is not a
       suitable external encoding of Unicode in filenames, text	 files,	 envi‐
       ronment	variables,  and	 so on.	 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 sys‐
       tems.

   Properties
       The UTF-8 encoding has the following nice properties:

       * UCS characters 0x00000000 to 0x0000007f (the classic US-ASCII charac‐
	 ters) 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
	 encoding.

       * 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 multi‐
	 byte sequence are in the range 0x80 to 0xbf.  This allows easy resyn‐
	 chronization  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.

   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  multibyte
       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  noncharacters) 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	encod‐
       ing 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  communica‐
       tion,  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 ter‐
       minal emulators in UTF-8	 mode  also  support  Chinese,	Japanese,  and
       Korean  double-width characters as well as nonspacing 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 han‐
       dling plain text.

   Security
       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 encod‐
       ing.

   Standards
       ISO/IEC 10646-1:2000, Unicode 3.1, RFC 3629, Plan 9.

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

COLOPHON
       This page is part of release 3.55 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

GNU				  2012-04-30			      UTF-8(7)
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