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utf-8(7) — Linux manual page

UTF-8(7) Miscellaneous Information Manual UTF-8(7)

NAME top

 UTF-8 - an ASCII compatible multibyte Unicode encoding

DESCRIPTION top

 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円' 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/IEC 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円' 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 =わ 0xc2 0xa9
 and character 0x2260 =わ 0010 0010 0110 0000 (the "not equal"
 symbol) is encoded as:
 11100010 10001001 10100000 =わ 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/IEC 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/IEC 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/IEC 2022 is ESC % @ ("\x1b%@"). Other ISO/IEC 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.

SEE ALSO top

 locale(1), nl_langinfo(3), setlocale(3), charsets(7), unicode(7)

COLOPHON top

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Linux man-pages 6.10 2024年06月15日 UTF-8(7)

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Pages that refer to this page: unicode_start(1), unicode_stop(1), locale(5), armscii-8(7), ascii(7), charsets(7), cp1251(7), cp1252(7), iso_8859-10(7), iso_8859-11(7), iso_8859-13(7), iso_8859-14(7), iso_8859-15(7), iso_8859-16(7), iso_8859-1(7), iso_8859-2(7), iso_8859-3(7), iso_8859-4(7), iso_8859-5(7), iso_8859-6(7), iso_8859-7(7), iso_8859-8(7), iso_8859-9(7), koi8-r(7), koi8-u(7), locale(7), man-pages(7), unicode(7), uri(7), setfont(8)

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