an ASCII compatible multibyte 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 part of many
16-bit characters—bytes such as '\0' or '/', which
have a special meaning in filenames and other C library
function arguments. In addition, the majority of UNIX tools
expect 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, and so on. The
ISO 10646 Universal Character Set (UCS), a superset of
Unicode, occupies an even larger code
space—31 bits—and the obvious UCS-4
encoding for it (a sequence of 32-bit words) has the same
encoding of Unicode and UCS does not have these problems and
is the common way in which Unicode is used on UNIX-style
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 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'
The lexicographic sorting order of UCS-4 strings is
All possible 2^31 UCS codes can be encoded using
The bytes 0xc0, 0xc1, 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 0xc2
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.
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:
0x00000000 − 0x0000007F:
10xxxxxx 10xxxxxx 10xxxxxx
10xxxxxx 10xxxxxx 10xxxxxx
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 UCS code
values 0xd800–0xdfff (UTF-16 surrogates) as well as
0xfffe and 0xffff (UCS noncharacters) should not appear in
conforming UTF-8 streams.
The Unicode character 0xa9 = 1010 1001 (the copyright sign)
is encoded in UTF-8 as
10101001 = 0xc2 0xa9
0x2260 = 0010 0010 0110 0000 (the "not equal"
symbol) is encoded as:
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.
software that has to be aware of the used character encoding
should always set the locale with for example
can then test the expression
"UTF-8") == 0
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.
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 nonspacing combining
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.
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.
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
ISO/IEC 10646-1:2000, Unicode 3.1, RFC 3629, Plan
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