UTF-8 - an ASCII compatible multi-byte Unicode encoding
DESCRIPTION
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 aq\0aq or aq/aq 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.
Properties
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 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, for example, aq\0aq or aq/aq.
*
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.
Encoding
The following byte sequences are used to represent a character.
The sequence to be used depends on the UCS code number of the character:
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.
Example
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
Application Notes
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 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.
Security
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 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
non-ASCII
ways to represent these things in a non-shortest
UTF-8
encoding.
Standards
ISO/IEC 10646-1:2000, Unicode 3.1, RFC 2279, Plan 9.
This page is part of release 3.14 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/.
