NAME
UTF-8 - an ASCII compatible multibyte 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 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 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.
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 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.
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 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.
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 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.
Standards
ISO/IEC 10646-1:2000, Unicode 3.1, RFC 2279, Plan 9.
SEE ALSO
nl_langinfo(3), setlocale(3), charsets(7), unicode(7)
COLOPHON
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