UTF-8(7)                  (2019-03-06)                   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 characters-bytes such as aq\0aq or aq/aq,
          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 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 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,  aq\0aq or aq/aq.

          * 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

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            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 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 char-
          acter code number in binary representation, most significant
          bit first (big-endian).  Only the shortest possible multi-
          byte sequence which can represent the code number of the
          character can be used.

          The UCS code values 0xd800en0xdfff (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:

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               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 char-
          acter 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 nonspacing combining
          characters, outputting a single character does not necessar-
          ily 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 posi-
          tions.

          The official ESC sequence to switch from an ISO 2022 encod-
          ing 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 non-
          conforming.  Unicode 3.1 has added the requirement that con-
          forming 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

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          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/.

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