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 charac-
ters bytes like '\0' or '/' which have a special meaning in filenames
and other C library function arguments. In addition, the majority of
UNIX tools expects ASCII files and can't read 16-bit words as charac-
ters without major modifications. For these reasons, UCS-2 is not a
suitable external encoding of Unicode in filenames, text files, envi-
ronment variables, and so on. 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
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 sys-
The UTF-8 encoding has the following nice properties:
* UCS characters 0x00000000 to 0x0000007f (the classic US-ASCII charac-
ters) 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 greater than 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, for example, '\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 0xc0, 0xc1, 0xfe and 0xff are never used in the UTF-8
* The first byte of a multibyte sequence which represents a single non-
ASCII UCS character is always in the range 0xc2 to 0xfd and indicates
how long this multibyte 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
* 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 be only 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:
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
The UCS code values 0xd800-0xdfff (UTF-16 surrogates) as well as 0xfffe
and 0xffff (UCS noncharacters) should not appear in conforming UTF-8
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
11100010 10001001 10100000 = 0xe2 0x89 0xa0
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 encod-
ing should always set the locale with for example
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 communica-
tion, 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 ter-
minal emulators in UTF-8 mode also support Chinese, Japanese, and
Korean double-width characters as well as nonspacing combining charac-
ters, outputting a single character does not necessarily advance the
POSIX systems, leading to a significantly richer environment for han-
dling plain text.
The Unicode and UCS standards require that producers of UTF-8 shall use
the shortest form possible, for example, producing a two-byte sequence
with first byte 0xc0 is nonconforming. 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
non-ASCII ways to represent these things in a non-shortest UTF-8 encod-
ISO/IEC 10646-1:2000, Unicode 3.1, RFC 3629, Plan 9.
nl_langinfo(3), setlocale(3), charsets(7), unicode(7)
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