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UTF-8(7) Linux Programmer’s Manual UTF-8(7)
NAME
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 Uni-
code 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 parameters. In addition,
the majority of UNIX tools expects ASCII files and can’t read 16-bit words as char-
acters 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 com-
mon 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 > 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 e.g. ’\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 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:
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 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.
EXAMPLES
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 con-
tent, 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 charac-
ter. Secondly, since modern terminal emulators in UTF-8 mode also support Chinese,
Japanese, and Korean double-width characters as well as non-spacing combining char-
acters, 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 short-
est form possible, e.g., 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.
AUTHOR
Markus Kuhn <mgk25 AT cl.uk>
SEE ALSO
nl_langinfo(3), setlocale(3), charsets(7), unicode(7)
GNU 2001-05-11 UTF-8(7)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
2008-08-30 09:15 @38.103.63.61 CrawledBy CCBot/1.0 (+http://www.commoncrawl.org/bot.html)