Computer Science
UTF-8(7) Linux Programmer's Manual UTF-8(7)
NAME
UTF-8 - an ASCII compatible multibyte Unicode encoding
DESCRIPTION
The Unicode character set occupies a 16-bit code space.
The most obvious Unicode encoding (known as UCS-2) con-
sists of a sequence of 16-bit words. Such strings can con-
tain 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 rea-
sons, 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 char-
acter set under Unix-style operating systems.
PROPERTIES
The UTF-8 encoding has the following nice properties:
* 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.
* 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
'/'.
* The lexicographic sorting order of UCS-4 strings is pre-
served.
* 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 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 resyn-
chronization 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. 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.
ENCODING
The following byte sequences are used to represent a char-
acter. 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.
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)
Linux 1995-11-26 1
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