| NAME | DESCRIPTION | SEE ALSO | COLOPHON | The Linux Programming Interface |
UTF-8(7) Linux Programmer's Manual UTF-8(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 parts of many 16-bit characters 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 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 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 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 0xc0 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 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 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
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
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.
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, 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 encoding.
ISO/IEC 10646-1:2000, Unicode 3.1, RFC 2279, Plan 9.
nl_langinfo(3), setlocale(3), charsets(7), unicode(7)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
GNU 2001-05-11 UTF-8(7)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface