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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 containā€”as part of many 16-bit charactersā€”bytes such as '\0' or '/', which have a special meaning in filenames and other  C  library  function  arguā€
       ments.   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
       superset of Unicode, occupies an even larger code spaceā€”31 bitsā€”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 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 multiā€
         byte sequence is.  All further bytes in a multibyte sequence are in the range 0x80 to 0xbf.  This allows easy resynchronization and makes the  encoding  stateā€
         less 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 represent the code number of the character can be used.

       The  UCS  code values 0xd800ā€“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 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  conā€
       tent, 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 loā€
       cales.  Firstly, a single byte does not necessarily correspond any more to a single character.  Secondly, since modern terminal emulators in UTF-8 mode also supā€
       port 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  posiā€
       tions.

       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)

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