Ubuntu Manpage: UTF-8 - an ASCII compatible multibyte Unicode encoding

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 part of many 16-bit characters—bytes such as '\0' or '/', which have a special
meaning in filenames and other C library function arguments. In addition, the majority of
UNIX tools expect 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, and so on. The ISO 10646 Universal
Character Set (UCS), a superset of Unicode, occupies an even larger code space—31 bits—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 0xc0, 0xc1, 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 0xc2 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 be only 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, most significant bit first (big-endian). 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. According to RFC 3629 no
point above U+10FFFF should be used, which limits characters to four bytes.

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.

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 3629, Plan 9.

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