UTF-8 - an ASCII compatible multi-byte 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 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 Uni
code 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 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 > 0x7f are encoded as a multi-byte
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 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 resyn
chronization and makes the encoding stateless and robust
against missing bytes.
long, however the Unicode standard specifies no charac
ters above 0x10ffff, so Unicode characters can only be
up to four bytes long in UTF-8.
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 multi-byte 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 non-characters) should not
appear in conforming UTF-8 streams.
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
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.
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 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 emula
tors in UTF-8 mode also support Chinese, Japanese, and
Korean double-width characters as well as non-spacing com
bining 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 ("\x1b%G"). The corresponding return
sequence from UTF-8 to ISO 2022 is ESC % @ ("\x1b%@").
Other ISO 2022 sequences (such as for switching the G0 and
G1 sets) are not applicable in UTF-8 mode.
It can be hoped that in the foreseeable future, UTF-8 will
replace ASCII and ISO 8859 at all levels as the common
character encoding on POSIX systems, leading to a signifi
cantly richer environment for handling plain text.
SECURITY
The Unicode and UCS standards require that producers of
UTF-8 shall use the shortest form possible, e.g., produc
ing a two-byte sequence with first byte 0xc0 is non-con
forming. Unicode 3.1 has added the requirement that con
forming 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 2279, Plan 9.
Markus Kuhn <mgk25@cl.cam.ac.uk>
SEE ALSO
nl_langinfo(3), setlocale(3), charsets(7), unicode(7)
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