UTF-8(7) — Linux manual page


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

NAME         top

       UTF-8 - an ASCII compatible multibyte Unicode encoding

DESCRIPTION         top

       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 '\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 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.

       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

       * 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.

       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:

       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 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.

       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 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
       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.

       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.

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

SEE ALSO         top

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

COLOPHON         top

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