Section: Linux Programmer's Manual (7)
UTF-8 - 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 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:
characters 0x00000000 to 0x0000007f (the classic US-ASCII
characters) are encoded simply as bytes 0x00 to 0x7f (ASCII
This means that files and strings which contain only
7-bit ASCII characters have the same encoding under both
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 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
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:
- 0x00000080 - 0x000007FF:
- 0x00000800 - 0x0000FFFF:
- 0x00010000 - 0x001FFFFF:
- 0x00200000 - 0x03FFFFFF:
- 0x04000000 - 0x7FFFFFFF:
bit positions are filled with the bits of the character code number in
binary representation, most significant bit first (big-endian).
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. According
to RFC 3629 no point above U+10FFFF should be used, which limits characters to four
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
encoding 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
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 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
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
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 UTF-8
ISO/IEC 10646-1:2000, Unicode 3.1, RFC 3629, Plan 9.
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