man-pages/man7/charsets.7

.\" Copyright (c) 1996 Eric S. Raymond <esr@thyrsus.com>
.\"                and Andries Brouwer <aeb@cwi.nl>
.\"
.\" This is free documentation; you can redistribute it and/or
.\" modify it under the terms of the GNU General Public License as
.\" published by the Free Software Foundation; either version 2 of
.\" the License, or (at your option) any later version.
.\"
.\" This is combined from many sources, including notes by aeb and
.\" research by esr.  Portions derive from a writeup by Roman Czyborra.
.\"
.\" Last changed by David Starner <dstarner98@aasaa.ofe.org>.
.TH CHARSETS 7 2001-05-07 "Linux" "Linux Programmer's Manual"
.SH NAME
charsets \- programmer's view of character sets and internationalization
.SH 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.
.LP
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.
.LP
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.)
.SH 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.
.LP
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.)
.LP
As Linux was written for hardware designed in the US, it natively
supports ASCII.
.SH 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.
.LP
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.
.LP
Console support for the other 8859 character sets is available under
Linux through user-mode utilities (such as
.BR setfont (8))
.\" // some distributions still have the deprecated consolechars
that modify keyboard bindings and the EGA graphics
table and employ the "user mapping" font table in the console
driver.
.LP
Here are brief descriptions of each set:
.TP
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.
.TP
8859-2 (Latin-2)
Latin-2 supports most Latin-written Slavic and Central European
languages: Croatian, Czech, German, Hungarian, Polish, Rumanian,
Slovak, and Slovene.
.TP
8859-3 (Latin-3)
Latin-3 is popular with authors of Esperanto, Galician, and Maltese.
(Turkish is now written with 8859-9 instead.)
.TP
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).
.TP
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.
.TP
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.
.TP
8859-7
Supports Modern Greek.
.TP
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.
.TP
8859-9 (Latin-5)
This is a variant of Latin-1 that replaces Icelandic letters with
Turkish ones.
.TP
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.
.TP
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.
.TP
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.
.TP
8859-13 (Latin-7)
Supports the Baltic Rim languages; in particular, it includes Latvian
characters not found in Latin-4.
.TP
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.
.TP
8859-15 (Latin-9)
This adds the Euro sign and French and Finnish letters that were missing in
Latin-1.
.TP
8859-16 (Latin-10)
This set covers many of the languages covered by 8859-2, and supports
Romanian more completely then that set does.
.SH 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.
.LP
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.
.\" Thanks to Tomohiro KUBOTA for the following sections about
.\" national standards.
.SH 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.
.SH 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.
.SH 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.
.SH 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.
.SH 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.
.SH 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>.
.LP
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.
.LP
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.
.LP
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.)
.LP
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.
.LP
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.
.LP
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.
.LP
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.
.LP
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.
.SH "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
.BR xterm (1).
It is popular in Japan and Korea.
.LP
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.
.LP
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 LS\fIn\fP makes character set G\fIn\fP the current one
for codes with high bit zero.
The function LS\fIn\fPR makes character set G\fIn\fP the current one
for codes with high bit one.
The function SS\fIn\fP makes character set G\fIn\fP (\fIn\fP=2 or 3)
the current one for the next character only (regardless of the value
of its high order bit).
.LP
A 94-character set is designated as G\fIn\fP 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.
.LP
A 96-character set is designated as G\fIn\fP 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.
.LP
A multibyte character set is designated as G\fIn\fP 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.
.LP
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.
.SH "SEE ALSO"
.BR console (4),
.BR console_codes (4),
.BR console_ioctl (4),
.BR ascii (7),
.BR iso_8859-1 (7),
.BR unicode (7),
.BR utf-8 (7)