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man:utf-8

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 part of many 16-bit charac-
     ters--bytes such as '\0' or '/', which have a special meaning in  file-
     names  and other C library function arguments.  In addition, the major-
     ity of UNIX tools expect 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,  and  so on.  The ISO 10646 Universal Character
     Set (UCS),  a  superset  of  Unicode,  occupies  an  even  larger  code
     space--31 bits--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,  most  significant  bit  first  (big-
     endian).   Only the shortest possible multibyte sequence which can rep-
     resent 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. According to RFC 3629 no point above U+10FFFF should be  used,
     which limits characters to four bytes.
 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.
 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

     locale(1), nl_langinfo(3), setlocale(3), charsets(7), unicode(7)

COLOPHON

     This page is part of release 4.16 of the Linux  man-pages  project.   A
     description  of  the project, information about reporting bugs, and the
     latest    version    of    this    page,    can     be     found     at
     https://www.kernel.org/doc/man-pages/.

GNU 2016-07-17 UTF-8(7)

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