# utf-8

## 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/IEC 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:
                  0 _xxxxxxx_
    
           0x00000080 - 0x000007FF:
                  110 _xxxxx_ 10 _xxxxxx_
    
           0x00000800 - 0x0000FFFF:
                  1110 _xxxx_ 10 _xxxxxx_ 10 _xxxxxx_
    
           0x00010000 - 0x001FFFFF:
                  11110 _xxx_ 10 _xxxxxx_ 10 _xxxxxx_ 10 _xxxxxx_
    
           0x00200000 - 0x03FFFFFF:
                  111110 _xx_ 10 _xxxxxx_ 10 _xxxxxx_ 10 _xxxxxx_ 10 _xxxxxx_
    
           0x04000000 - 0x7FFFFFFF:
                  1111110 _x_ 10 _xxxxxx_ 10 _xxxxxx_ 10 _xxxxxx_ 10 _xxxxxx_ 10 _xxxxxx_
    
           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/IEC 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/IEC 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/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.
    


* * *