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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 an 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; glibc 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 exists only 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 (noncombining) 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.org⟩.
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 (especially 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 ('\0') 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, and so on.
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 be invoked only 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_codes(4), console_ioctl(4), ascii(7), iso_8859-1(7), unicode(7),
utf-8(7)
COLOPHON
This page is part of release 3.54 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at
http://www.kernel.org/doc/man-pages/.