charsets 中文man页面

NAME

charsets - 程序员对字符集和国际化的观点  

描述

Linux 是一个国际性的操作系统。它的各种各样实用程序和设备驱动程序 (包括控制台驱动程序 ) 支持多种语言的字符集，包括带有附加符号的拉丁字母表字符，重音符，连字(字母结合), 和全部非拉丁文字母表（包括希腊语，古代斯拉夫语，阿拉伯语，和希伯来语。 )

这份手册以程序员的眼光去看待不同的字符集标准，以及它们是如何在 Linux 中调和在一起的。讨论的标准包括 ASCII，ISO 8859，KOI8-R ， Unicode，ISO 2022 和 ISO 4873 。  

ASCII

ASCII (,美国国家信息交换(用)标准(代)码) 是最初的 7-bit字符集, 原先是为美式英语设计的。当前它被 ECMA-6 标准所描述。

在英国使用一种 ASCII的变体（这变体是：用英国磅值的符号代替美国的 crosshatch/octothorpe/hash 的磅值符号）;当需要时，美国的（符号）和英国的变体（符号）可以用"US ASCII"和"UK ASCII" 作为区别。

因为 Linux 是为美国设计的硬件写的, 它生来就支持 US ASCII 。

ISO 8859

ISO 8859 是一系列 10 ８-bit 字符集,它包含美国 ASCII 的低位 (7 -bit ), 128 ～159 范围内的不可见控制字符，和 96 个定宽图形（字符）在 160-255 里。。LP 这些字符集中，最重要是 ISO 8859-1 ( Latin-1 )。它生来就被 Linux 控制台驱动程序支持， X11R6 的支持得也很好，并且是 HTML 的基础字符集。

Linux 下控制台也支持其他的 8859 字符集，通过用户模式实用程序( 例如 setfont(8)) 来修改键盘绑定和 EGA 图形表格，以及进行控制台驱动程序里的字体表格中的“user mapping（用户影射）”。

下面是每个集合简短的描述：

8859-1 (Latin-1)

Latin-1 覆盖大多数的西欧语言，比如阿尔巴尼亚, 加泰罗尼亚语, 丹麦, 荷兰,英语,法罗群岛,芬兰,法语,德语,加利西亚,爱尔兰,冰岛, 意大利，挪威，葡萄牙，西班牙和瑞典。缺少荷兰的 ij连字（i与j合字） ，法国的 oe（o与e合字）和旧风格的',,' 而德语中``（这样的）引号是可以的。

8859-2 (Latin-2)

Latin-2 支持大多数的拉丁文书写的斯拉夫语和中欧的语言：克罗地亚 , 捷克语, 德语, 匈牙利, 波兰，罗马尼亚，斯洛伐克，和斯洛文尼亚。

8859-3 (Latin-3)

Latin-3 是世界语,加里西亚 , 马耳他人, 和土耳其语作者受欢迎的（语言）。

8859-4 (Latin-4)

Latin-4 介绍了爱沙尼亚语，拉托维亚，和立陶宛的字符 。它是实质上过时的; 参见 8859-10 (Latin-6 ) 。

8859-5

古代斯拉夫语字母支持保加利亚语, 白俄罗斯语,马其顿语, 俄语, 塞尔维亚语和乌克兰语。乌克兰人读带有下挑笔的`geh'为`heh',和（当）需要用带有上挑笔的 ghe 写正确的ghe.参见下面的（关于）KOI8-R 的讨论。（译注：这些外国人书写习惯我们也不怎么需要理解吧，希望上面的解释不要把人搞糊涂了）

8859-6

支持阿拉伯语。 8859-6 字型表是分离字符格式的一种固定的字体，但是一个合适的显示引擎应该联合这些来使用合适的词首，中间字母，和***表格式。

8859-7

支持现代的希腊语。

8859-8

支持希伯来语。

8859-9 (Latin-5)

这是Latin-1 的一种变体，它用土耳其语的一些（字符）代替很少用的冰岛语。

8859-10 (Latin-6)

Latin 6 增加末因纽特(译：对于last Inuit 我不知道是否是对的) (格陵兰语) 和 Sami ( 拉普兰语 ) ，这些是 Lattin 4 中缺少的，来覆盖整个北欧地区（的字符集）。 RFC 1345 列出了初步的和不同的“ latin 6 "。 Skolt Sami 仍然比这些需要更多的重音符号。

8859-13 (Latin-7)

8859-14 (Latin-8)

8859-15

增加了欧洲符号和法国连字，它们是 Latin-1 里缺漏的。

KOI8-R

KOI8-R 是在俄国流行的一个非 ISO 字符集。下半部分是 US ASCII; 上部是比 ISO 8859-5 设计的更好的古斯拉夫字符集。

控制台为了支持 KOI8-R 字符集，在 Linux 下，可以利用用户模式实用程序修改键盘绑定和 EGA 图形表格，以及在控制台的驱动程序中使用字体表“user mapping（用户映射）”。  

UNICODE（统[单]一代码,宽[双]字节字符集）

Unicode（ ISO 10646 ) 是一个标准，它的目标是明白地表现在每种人类语言中的每种已知字符。Unicode 的编码是 32 位的 ( 旧些的版本使用了 16 位 ) 。在 Unicode 的一些信息可以在<http://www.unicode.com>获得。

Linux 使用８位的 Unicode 转移格式 (UTF-8 ) 表示 Unicode 。 UTF-8 是可变长的 Unicode 编码。使用１个字节给 7 bit 编码，使用２个字节给 １１ bit 编码，使用３个字节给 １６ bit 编码，使用４个字节给 ２１ bit 编码，使用５个字节给２６ bit 编码，使用６个字节给 ３１ bit 编码

让 0,1 , x 代表零，一，或任意的位。字节0xxxxxxx 代表Unicode 00000000 0xxxxxxx，这个符号和 ASCII 0xxxxxxx 编码的符号是一样。 这样， ASCII 没有改为 UTF-8，并且只用 ASCII 的人不会注意到任何变化：不在代码，并且不在文件大小。

字节 110xxxxx 是一个2 字节代码的开始， 110xxxxx 10yyyyyy 组装成 00000xxx xxyyyyyy 。字节 1110xxxx 是一个 ３ 字节代码的开始， 1110xxxx 10yyyyyy 10zzzzzz 被组装成 xxxxyyyy yyzzzzzz。（如果 UTF-8 使用 31-bit ISO 10646 编码，那么这个级数就会延伸到 6 字节编码）

对于 ISO-8859-1 的用户而言，这意味着带高位的字符编码成两个字节。这会令普通的文本文件增大１到２个百分点。不过没有变换问题, 因为 Unicode ISO-8859-1 符号的值等于他们的 ISO-8859-1 值 (用 8 个前导零做前缀) 。对于日语的用户，这意味着原来常用的 16 位编码将占 3 个字节，并且还要求有扩展的映射表。许多日本人因此比较喜欢 ISO 2022 。

注意 UTF-8 是自我同步的： 10xxxxxx 是一条尾巴, 任何其它的字节是编码的头。ASCII 字节出现在 UTF-8 流中唯一的可能是作为自己出现。特别是, 不会有 NULs 或 " /'s 嵌入在那些比较大的编码中。

因为编码中的 ASCII，特别是, NUL 和'/', 没有变化, 所以内核不会注意到在使用 UTF-8。它根本不在乎它正在处理的那字节代表什么东西。

Unicode 数据流的呈现通常是通过" subfont "表来操作，这个表是 Unicode 的一个子集到字符表格的映射。内核内部使用 Unicode 描述装载入显示内存的 subfont。这意味着在 UTF-8 中的一个模式能使用 512 个不同的符号。这对于日语，汉语和朝鲜语来说是不够的，但是它满足了大多数其它用途。  

ISO 2022 AND ISO 4873

ISO 2022 和 4873 标准描述了一个基于 VT100 实现的字体控制模型． Linux 内核和 xterm (1) ( 部分 ) 支持这个模型。它在日本和韩国很流行。

它有 4 个图形的字符集，称为 G0 ， G1 ， G2 和 G3 ，并且其中之一是当前的高位为０ 的编码的字符集(最初 G0 ),而他们之一是当前的高位为１的编码的字符集(最初 G1 )。每种图形的字符集有 94 或 96 个字符 ，并且是实际上是一个 7-bit字符集。它使用 040-0177 ( 041-0176 ) 或 0240-0377 ( 0241-0376 )编码中的一个。G0 大小总是为 94，并且使用 041-0176 之间的编码。

字符之间切换用转换（shift functions）功能 ^N (SO 或 LS1), ^O (SI 或 LS0), ESC n (LS2), ESC o (LS3), ESC N (SS2), ESC O (SS3), ESC ~ (LS1R), ESC } (LS2R), ESC | (LS3R). LSn 把字符集Gn标记为当前字符集，用于高位为０的编码。 LSnR 把字符集 Gn标记为当前字符集，用于高位为１的编码。 SSn 把字符集Gn (n=2 or 3) 标记为当前字符集，只用于下一个字符（不管它的高位的值是什么）

94 字符的集合用做 Gn字符集是用一个逃逸序列 ESC ( xx （用于 G0），ESC ) xx （用于 G1）， ESC * xx （用于 G2），ESC + xx （用于 G3），等代表的．这里的 xx 是一个符号或者是在 ISO 2375 国际注册编码字符集中的一对符号。例如，ESC ( @ 选用 ISO 646 字符集作为GO， ESC ( A 选用 UK 标准字符集(用磅代替数字记号), ESC ( B 选择 ASCII ( 用美元代替流通货币), ESC ( M 为非洲语言选择一个字符集， ESC ( ! A 选择古巴字符集, 等等. 等等.

94 字符的集合用做 Gn字符集是用一个逃逸序列 ESC - xx （对于 G1）, ESC . xx （对于 G2）或 ESC / xx （对于 G3）等表示．例如, ESC - G 选择希伯莱字母表作为 G1.

多字节的字符集用做 Gn 字符集是用一个逃逸序列 ESC $ xx 或者 ESC $ ( xx （对于 G0）， ESC $ ) xx （对于 G1），ESC $ * xx （对于 G2），ESC $ + xx （对于 G3）等来表示．例如, ESC $ ( C 为 G0选择韩国字符集. 日本字符集合由 ESC $ B选择更多临近的版本由ESC & @ ESC $ B选择.

ISO 4873 规定了一个范围比较窄的使用字符集，它的 G0是固定的 (总是 ASCII), 所以 G1, G2 和 G3只能被调用于高次序位编码集。尤其是，不再使用 ^N 和 ^O，ESC ( xx 仅用于 xx=B, 和 ESC ) xx, ESC * xx, ESC + xx 分别等价于 ESC - xx, ESC . xx, ESC / xx．

参考

console(4), console_ioctl(4), console_codes(4), ascii(7), iso_8859_1(7), unicode(7), utf-8(7)
 

#p#

NAME

charsets - programmer's view of character sets and internationalization  

DESCRIPTION

Linux is an international operating system. Various of its utilities and device drivers (including the console driver) support multilingual character sets including Latin-alphabet letters with diacritical marks, accents, ligatures, and entire non-Latin alphabets including Greek, Cyrillic, Arabic, and Hebrew.

This manual page presents a programmer's-eye view of different character-set standards and how they fit together on Linux. Standards discussed include ASCII, ISO 8859, KOI8-R, Unicode, ISO 2022 and ISO 4873. The primary emphasis is on character sets actually used as locale character sets, not the myriad others that can be found in data from other systems.

A complete list of charsets used in a officially supported locale in glibc 2.2.3 is: ISO-8859-{1,2,3,5,6,7,8,9,13,15}, CP1251, UTF-8, EUC-{KR,JP,TW}, KOI8-{R,U}, GB2312, GB18030, GBK, BIG5, BIG5-HKSCS and TIS-620 (in no particular order.) (Romanian may be switching to ISO-8859-16.)

ASCII

ASCII (American Standard Code For Information Interchange) is the original 7-bit character set, originally designed for American English. It is currently described by the ECMA-6 standard.

Various ASCII variants replacing the dollar sign with other currency symbols and replacing punctuation with non-English alphabetic characters to cover German, French, Spanish and others in 7 bits exist. All are deprecated; GNU libc doesn't support locales whose character sets aren't true supersets of ASCII. (These sets are also known as ISO-646, a close relative of ASCII that permitted replacing these characters.)

As Linux was written for hardware designed in the US, it natively supports ASCII.

ISO 8859

ISO 8859 is a series of 15 8-bit character sets all of which have US ASCII in their low (7-bit) half, invisible control characters in positions 128 to 159, and 96 fixed-width graphics in positions 160-255.

Of these, the most important is ISO 8859-1 (Latin-1). It is natively supported in the Linux console driver, fairly well supported in X11R6, and is the base character set of HTML.

Console support for the other 8859 character sets is available under Linux through user-mode utilities (such as setfont(8)) that modify keyboard bindings and the EGA graphics table and employ the "user mapping" font table in the console driver.

Here are brief descriptions of each set:

8859-1 (Latin-1)

Latin-1 covers most Western European languages such as Albanian, Catalan, Danish, Dutch, English, Faroese, Finnish, French, German, Galician, Irish, Icelandic, Italian, Norwegian, Portuguese, Spanish, and Swedish. The lack of the ligatures Dutch ij, French oe and old-style ,,German`` quotation marks is considered tolerable.

8859-2 (Latin-2)

Latin-2 supports most Latin-written Slavic and Central European languages: Croatian, Czech, German, Hungarian, Polish, Rumanian, Slovak, and Slovene.

8859-3 (Latin-3)

Latin-3 is popular with authors of Esperanto, Galician, and Maltese. (Turkish is now written with 8859-9 instead.)

8859-4 (Latin-4)

Latin-4 introduced letters for Estonian, Latvian, and Lithuanian. It is essentially obsolete; see 8859-10 (Latin-6) and 8859-13 (Latin-7).

8859-5

Cyrillic letters supporting Bulgarian, Byelorussian, Macedonian, Russian, Serbian and Ukrainian. Ukrainians read the letter `ghe' with downstroke as `heh' and would need a ghe with upstroke to write a correct ghe. See the discussion of KOI8-R below.

8859-6

Supports Arabic. The 8859-6 glyph table is a fixed font of separate letter forms, but a proper display engine should combine these using the proper initial, medial, and final forms.

8859-7

Supports Modern Greek.

8859-8

Supports modern Hebrew without niqud (punctuation signs). Niqud and full-fledged Biblical Hebrew are outside the scope of this character set; under Linux, UTF-8 is the preferred encoding for these.

8859-9 (Latin-5)

This is a variant of Latin-1 that replaces Icelandic letters with Turkish ones.

8859-10 (Latin-6)

Latin 6 adds the last Inuit (Greenlandic) and Sami (Lappish) letters that were missing in Latin 4 to cover the entire Nordic area. RFC 1345 listed a preliminary and different `latin6'. Skolt Sami still needs a few more accents than these.

8859-11

This only exists as a rejected draft standard. The draft standard was identical to TIS-620, which is used under Linux for Thai.

8859-12

This set does not exist. While Vietnamese has been suggested for this space, it does not fit within the 96 (non-combining) characters ISO 8859 offers. UTF-8 is the preferred character set for Vietnamese use under Linux.

8859-13 (Latin-7)

Supports the Baltic Rim languages; in particular, it includes Latvian characters not found in Latin-4.

8859-14 (Latin-8)

This is the Celtic character set, covering Gaelic and Welsh. This charset also contains the dotted characters needed for Old Irish.

8859-15 (Latin-9)

This adds the Euro sign and French and Finnish letters that were missing in Latin-1.

8859-16 (Latin-10)

This set covers many of the languages covered by 8859-2, and supports Romanian more completely then that set does.

KOI8-R

KOI8-R is a non-ISO character set popular in Russia. The lower half is US ASCII; the upper is a Cyrillic character set somewhat better designed than ISO 8859-5. KOI8-U is a common character set, based off KOI8-R, that has better support for Ukrainian. Neither of these sets are ISO-2022 compatible, unlike the ISO-8859 series.

Console support for KOI8-R is available under Linux through user-mode utilities that modify keyboard bindings and the EGA graphics table, and employ the "user mapping" font table in the console driver.

JIS X 0208

JIS X 0208 is a Japanese national standard character set. Though there are some more Japanese national standard character sets (like JIS X 0201, JIS X 0212, and JIS X 0213), this is the most important one. Characters are mapped into a 94x94 two-byte matrix, whose each byte is in the range 0x21-0x7e. Note that JIS X 0208 is a character set, not an encoding. This means that JIS X 0208 itself is not used for expressing text data. JIS X 0208 is used as a component to construct encodings such as EUC-JP, Shift_JIS, and ISO-2022-JP. EUC-JP is the most important encoding for Linux and includes US ASCII and JIS X 0208. In EUC-JP, JIS X 0208 characters are expressed in two bytes, each of which is the JIS X 0208 code plus 0x80.

KS X 1001

KS X 1001 is a Korean national standard character set. Just as JIS X 0208, characters are mapped into a 94x94 two-byte matrix. KS X 1001 is used like JIS X 0208, as a component to construct encodings such as EUC-KR, Johab, and ISO-2022-KR. EUC-KR is the most important encoding for Linux and includes US ASCII and KS X 1001. KS C 5601 is an older name for KS X 1001.

GB 2312

GB 2312 is a mainland Chinese national standard character set used to express simplified Chinese. Just like JIS X 0208, characters are mapped into a 94x94 two-byte matrix used to construct EUC-CN. EUC-CN is the most important encoding for Linux and includes US ASCII and GB 2312. Note that EUC-CN is often called as GB, GB 2312, or CN-GB.

Big5

Big5 is a popular character set in Taiwan to express traditional Chinese. (Big5 is both a character set and an encoding.) It is a superset of US ASCII. Non-ASCII characters are expressed in two bytes. Bytes 0xa1-0xfe are used as leading bytes for two-byte characters. Big5 and its extension is widely used in Taiwan and Hong Kong. It is not ISO 2022-compliant.

TIS 620

TIS 620 is a Thai national standard character set and a superset of US ASCII. Like ISO 8859 series, Thai characters are mapped into 0xa1-0xfe. TIS 620 is the only commonly used character set under Linux besides UTF-8 to have combining characters.

UNICODE

Unicode (ISO 10646) is a standard which aims to unambiguously represent every character in every human language. Unicode's structure permits 20.1 bits to encode every character. Since most computers don't include 20.1-bit integers, Unicode is usually encoded as 32-bit integers internally and either a series of 16-bit integers (UTF-16) (needing two 16-bit integers only when encoding certain rare characters) or a series of 8-bit bytes (UTF-8). Information on Unicode is available at <http://www.unicode.com>.

Linux represents Unicode using the 8-bit Unicode Transformation Format (UTF-8). UTF-8 is a variable length encoding of Unicode. It uses 1 byte to code 7 bits, 2 bytes for 11 bits, 3 bytes for 16 bits, 4 bytes for 21 bits, 5 bytes for 26 bits, 6 bytes for 31 bits.

Let 0,1,x stand for a zero, one, or arbitrary bit. A byte 0xxxxxxx stands for the Unicode 00000000 0xxxxxxx which codes the same symbol as the ASCII 0xxxxxxx. Thus, ASCII goes unchanged into UTF-8, and people using only ASCII do not notice any change: not in code, and not in file size.

A byte 110xxxxx is the start of a 2-byte code, and 110xxxxx 10yyyyyy is assembled into 00000xxx xxyyyyyy. A byte 1110xxxx is the start of a 3-byte code, and 1110xxxx 10yyyyyy 10zzzzzz is assembled into xxxxyyyy yyzzzzzz. (When UTF-8 is used to code the 31-bit ISO 10646 then this progression continues up to 6-byte codes.)

For most people who use ISO-8859 character sets, this means that the characters outside of ASCII are now coded with two bytes. This tends to expand ordinary text files by only one or two percent. For Russian or Greek users, this expands ordinary text files by 100%, since text in those languages is mostly outside of ASCII. For Japanese users this means that the 16-bit codes now in common use will take three bytes. While there are algorithmic conversions from some character sets (esp. ISO-8859-1) to Unicode, general conversion requires carrying around conversion tables, which can be quite large for 16-bit codes.

Note that UTF-8 is self-synchronizing: 10xxxxxx is a tail, any other byte is the head of a code. Note that the only way ASCII bytes occur in a UTF-8 stream, is as themselves. In particular, there are no embedded NULs or '/'s that form part of some larger code.

Since ASCII, and, in particular, NUL and '/', are unchanged, the kernel does not notice that UTF-8 is being used. It does not care at all what the bytes it is handling stand for.

Rendering of Unicode data streams is typically handled through `subfont' tables which map a subset of Unicode to glyphs. Internally the kernel uses Unicode to describe the subfont loaded in video RAM. This means that in UTF-8 mode one can use a character set with 512 different symbols. This is not enough for Japanese, Chinese and Korean, but it is enough for most other purposes.

At the current time, the console driver does not handle combining characters. So Thai, Sioux and any other script needing combining characters can't be handled on the console.

ISO 2022 AND ISO 4873

The ISO 2022 and 4873 standards describe a font-control model based on VT100 practice. This model is (partially) supported by the Linux kernel and by xterm(1). It is popular in Japan and Korea.

There are 4 graphic character sets, called G0, G1, G2 and G3, and one of them is the current character set for codes with high bit zero (initially G0), and one of them is the current character set for codes with high bit one (initially G1). Each graphic character set has 94 or 96 characters, and is essentially a 7-bit character set. It uses codes either 040-0177 (041-0176) or 0240-0377 (0241-0376). G0 always has size 94 and uses codes 041-0176.

Switching between character sets is done using the shift functions ^N (SO or LS1), ^O (SI or LS0), ESC n (LS2), ESC o (LS3), ESC N (SS2), ESC O (SS3), ESC ~ (LS1R), ESC } (LS2R), ESC | (LS3R). The function LSn makes character set Gn the current one for codes with high bit zero. The function LSnR makes character set Gn the current one for codes with high bit one. The function SSn makes character set Gn (n=2 or 3) the current one for the next character only (regardless of the value of its high order bit).

A 94-character set is designated as Gn character set by an escape sequence ESC ( xx (for G0), ESC ) xx (for G1), ESC * xx (for G2), ESC + xx (for G3), where xx is a symbol or a pair of symbols found in the ISO 2375 International Register of Coded Character Sets. For example, ESC ( @ selects the ISO 646 character set as G0, ESC ( A selects the UK standard character set (with pound instead of number sign), ESC ( B selects ASCII (with dollar instead of currency sign), ESC ( M selects a character set for African languages, ESC ( ! A selects the Cuban character set, etc. etc.

A 96-character set is designated as Gn character set by an escape sequence ESC - xx (for G1), ESC . xx (for G2) or ESC / xx (for G3). For example, ESC - G selects the Hebrew alphabet as G1.

A multibyte character set is designated as Gn character set by an escape sequence ESC $ xx or ESC $ ( xx (for G0), ESC $ ) xx (for G1), ESC $ * xx (for G2), ESC $ + xx (for G3). For example, ESC $ ( C selects the Korean character set for G0. The Japanese character set selected by ESC $ B has a more recent version selected by ESC & @ ESC $ B.

ISO 4873 stipulates a narrower use of character sets, where G0 is fixed (always ASCII), so that G1, G2 and G3 can only be invoked for codes with the high order bit set. In particular, ^N and ^O are not used anymore, ESC ( xx can be used only with xx=B, and ESC ) xx, ESC * xx, ESC + xx are equivalent to ESC - xx, ESC . xx, ESC / xx, respectively.

SEE ALSO

console(4), console_ioctl(4), console_codes(4), ascii(7), iso_8859-1(7), unicode(7), utf-8(7)