utf-8

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 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.

   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 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 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 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.

   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 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.

   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 5.04 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)