UTF-8 - an ASCII compatible multi-byte Unicode encoding
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 of Unicode and UCS does not have these problems and
is the common way in which Unicode is used on Unix-style operating systems.
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 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.
* UTF-8 encoded UCS characters may be up to six bytes long, however the
Unicode standard specifies no characters above 0x10ffff, so Unicode
characters can only be up to four bytes long in UTF-8.
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 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.
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
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.
Application software that has to be aware of the used 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
emulators in UTF-8 mode also support Chinese, Japanese, and
Korean double-width characters as well as non-spacing combining charac-
ters, 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 significantly richer environment for handling
plain text.
The Unicode and UCS standards require that producers of UTF-8 shall use
the shortest form possible, e.g., producing a two-byte sequence with
first byte 0xc0 is non-conforming. Unicode 3.1 has added the requirement
that conforming 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 nonASCII
ways to represent these things in a non-shortest UTF-8 encoding.
ISO/IEC 10646-1:2000, Unicode 3.1, RFC 2279, Plan 9.
Markus Kuhn <mgk25@cl.cam.ac.uk>
nl_langinfo(3), setlocale(3), charsets(7), unicode(7)
GNU 2001-05-11 UTF-8(7)
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