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 parts of many 16-bit characters bytes like '0' or '/' which have a special meaning in filenames and other C library function parameters. In addition, the majority of UNIX tools expects 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, etc. The ISO 10646 Universal Character Set (UCS), a superset of Unicode, occupies even a 31-bit code space and the obvious UCS-4 encoding for it (a sequence of 32-bit words) has the same problems.
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 __
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 multi-byte sequence which represents a single non-ASCII UCS character is always in the range 0xc0 to 0xfd and indicates how long this multi-byte sequence is. All further bytes in a multi-byte sequence are in the range 0x80 to 0xbf. This allows easy resynchronization and makes the encoding stateless and robust against missing bytes.
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:
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 multi-byte sequence which can represent the code number of the character can be used.
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
Users have to select a UTF-8 locale, for example with
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
and programmers can then test the expression
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 non-spacing 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 ( UTF-8 to ISO 2022 is ESC % @ ( __