utf8(7) - man - phpman

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 encod‐
       ing (known as UCS-2) consists of a sequence of 16-bit words.   Such  strings  can  contain—as
       part  of  many  16-bit  characters—bytes such as '\0' or '/', which have a special meaning in
       filenames and other C library function arguments.  In addition, the majority  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 su‐
       perset 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 systems.

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

       * UCS characters 0x00000000 to 0x0000007f (the classic US-ASCII characters) are encoded  sim‐
         ply  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 multibyte 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 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 de‐
       pends 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  repre‐
       sentation, most significant bit first (big-endian).  Only the shortest possible multibyte se‐
       quence 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. 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 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  en‐
       vironment 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 nonspacing 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.

   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 encoding.

   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 5.10 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                                          2019-03-06                                     UTF-8(7)