The lexicographic sorting order of UCS-4 strings is preserved.

All possible 2^31 UCS codes can be encoded using UTF-8.

The bytes 0xfe and 0xff are never used in the UTF-8 encoding.

The first byte of a multibyte sequence that represents a single non-ASCII UCS character is always in the range 0xc0 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, Unicode characters can only be up to three bytes long. Because Linux uses only the 16-bit Unicode subset of UCS, under Linux, UTF-8 multibyte sequences can only be one, two, or three bytes long.

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. Only the shortest possible multibyte sequence that can represent the code number of the character can be used.

EXAMPLES

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

STANDARDS

ISO 10646, Unicode 1.1, XPG4, Plan 9.

AUTHOR

Markus Kuhn (mskuhn@cip.informatik.uni-erlangen.de)

SEE ALSO

unicode(7)