From Linux Man Pages
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
'\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 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 > 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.
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 out-
put, 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 assump-
tions 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, e.g., produc-
ing a two-byte sequence with first byte 0xc0 is non-conforming. Unicode 3.1 has added the requirement that con-
forming 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.
RELATED
nl_langinfo(3), setlocale(3), charsets(7), unicode(7)
CATEGORY