UTF-8

UTF-8 ? an ASCII compatible multibyte Unicode encoding

The Unicode 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 of Unicode and UCS does not have these problems and is the way to go for using the Unicode character set under Unix-style operating systems.

The UTF-8 encoding has the following nice properties:

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UCS characters 0x00000000 to 0x0000007f (the classical 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.

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All UCS characters > 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 e.g. ’\0’ or ’/’.

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

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

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

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The first byte of a multibyte sequence which 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.

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UTF-8 encoded UCS characters may be up to six bytes long, however Unicode characters can only be up to three bytes long. As 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.

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 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 "not equal" symbol) is encoded as:

11100010 10001001 10100000 = 0xe2 0x89 0xa0

ISO 10646, Unicode 1.1, XPG4, Plan 9.

Markus Kuhn <mskuhn@cip.informatik.uni-erlangen.de>

unicode(7)