man > utf8(7)

UTF-8(7)                          Miscellaneous Information 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/IEC 10646 Universal Character Set (UCS), a
       superset of Unicode, occupies an even larger code space—31 bits—and the obvious UCS-4  encod‐
       ing 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 charac‐
          ter 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/IEC 8859 have to be
       aware that two assumptions made so far are no longer valid in UTF-8 locales.  Firstly, a sin‐
       gle 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 func‐
       tions such as mbsrtowcs(3) and wcswidth(3) should be used today to count characters and  cur‐
       sor positions.

       The  official  ESC  sequence  to switch from an ISO/IEC 2022 encoding scheme (as used for in‐
       stance by VT100 terminals) to UTF-8 is ESC % G ("\x1b%G").  The corresponding return sequence
       from UTF-8 to ISO/IEC 2022 is ESC % @ ("\x1b%@").  Other ISO/IEC 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)

Linux man-pages 6.7                          2024-03-14                                     UTF-8(7)