NAME

UTF-8 - ASCII 兼容的多字節 Unicode 編碼  

描述

The Unicode 字符集使用的是 16 位（雙字節）碼。最普遍的 Unicode 編碼方法（ UCS-2) 由一個 16 位雙字序列組成。這樣的字符串中包括了的一些如‘\0’或‘/’這樣的在文件名中或者是在 C 庫函數中具有特殊意義的字符。另外，如果沒有做重大的修正的話，大部分操作 ASCII 碼文件的 UNIX 工具不能夠正確識別 16 位的字符。因此， UCS-2 對于 Unicode 的文件名、文本文件、環境變量等等來說并不是一種合適的外部編碼方式。 ISO 10646 Universal Character Set (UCS), 是 Unicode 的超集，甚至使用了 31 位編碼方式，另外還有使用 32 編碼的 UCS-4 也有同樣上述的問題。 UCS-4 而用 UTF-8 對 Unicode UCS 編碼就不會存在這樣的問題。所以，UTF-8 很明顯的是在 UNIX 類操作系統下的 Unicode 字符集的解決方案。  

屬性

UTF-8 編碼具有以下優良屬性：

*

UCS 字符從 0x00000000 到 0x0000007f （傳統的 US-ASCII 字符）簡單地編碼為字節 0x00 到 0x7f （與 ASCII 碼兼容）。這意味著只包含 7 位 ASCII 字符的文件和字符串在 ASCII 和 UTF-8. 編碼方式下是完全一樣的。

*

所有大于 0x7f 的 UCS 字符被編碼成為多字節序列。該序列全部是由 0x80 到 0fd 的字符組成，這樣就不會有標準 ASCII 字符會被作為某個字的一個部分這種現象出現，對于‘\0’和‘’這樣的特殊字符來說也就不會有問題了。

*

保留了 UCS-4 字典中的字節串的排列順序。

*

所有 2^32 次方的 UCS 碼都能夠使用 UTF-8 來進行編碼。

*

0xfe 和 0xff 兩個字符在 UTF-8 中不會被用到。

*

表示非 ASCII 碼的 UCS 多字節串的開始字符總是 0xc0 到 0xfd 之間的字符，并會指出該串的長度。多字節串的其他字符都是 0x80 到 0xbf 之間的字符。這使得再同步非常簡單，并令編碼是無態的，丟字節現象也不容易發生。

*

用 UTF-8 編碼的 UCS 字符可以增加到 6 個字節的長度。而 Unicode 只能增加到 3 個字節長。由于 Linux 只使用 16 位的 Unicode ， UCS 的子集。所以在 Linux 下， UTF-8 多字節串長度最多不會超過三個字節。

編碼方式

下面的字節串用來表示一個字符。用什么串依照該字符在 UCS 編碼中的序號來定：

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

這里 xxx 的位置二進制位形式的字符編碼填入。只用最短的那個足夠表達一個字符編碼數的多字節串。  

舉例說明

Unicode 字符 0xa9 = 1010 1001 (版權所有的符號) 在 UTF-8 中被編碼為：

11000010 10101001 = 0xc2 0xa9

字符0x2260 = 0010 0010 0110 0000 (“不等于”符號)被編碼為：

11100010 10001001 10100000 = 0xe2 0x89 0xa0

遵循標準

ISO 10646, Unicode 1.1, XPG4, Plan 9.  

作者

Markus Kuhn  

參考

unicode(7)

#p#

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 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.  

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 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 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., 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@cl.cam.ac.uk>  

SEE ALSO

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