CHARSETS(4) Linux Programmer's Manual CHARSETS(4)
charsets - programmer's view of character sets and inter-
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 dif-
ferent character-set standards and how they fit together
on Linux. Standards discussed include ASCII, ISO 8859,
KOI8-R, Unicode, ISO 2022 and ISO 4873.
ASCII (American Standard Code For Information) is the
original 7-bit character set, originally designed for
American English. It is currently described by the ECMA-6
An ASCII variant replacing the American
crosshatch/octothorpe/hash pound symbol with the British
pound-sterling symbol is used in Great Britain; when
needed, the American and British variants may be distin-
guished as "US ASCII" and "UK ASCII".
As Linux was written for hardware designed in the US, it
natively supports US ASCII.
ISO 8859 is a series of 10 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
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:
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
Latin-2 supports most Latin-written Slavic and Cen-
tral European languages: Croatian, Czech, German,
Hungarian, Polish, Rumanian, Slovak, and Slovene.
Latin-3 is popular with authors of Esperanto, Gali-
cian, Maltese, and Turkish.
Latin-4 introduced letters for Estonian, Latvian,
and Lithuanian. It is essentially obsolete; see
8859-5 Cyrillic letters supporting Bulgarian, Byelorus-
sian, 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 ini-
tial, medial, and final forms.
8859-7 Supports Modern Greek.
8859-8 Supports Hebrew.
This is a variant of Latin-1 that replaces rarely-
used Icelandic letters with Turkish ones.
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.
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.
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.
Unicode (ISO 10646) is a standard which aims to unambigu-
ously represent every known glyph in every human language.
Unicode's native encoding is 32-bit (older versions used
16 bits). Information on Unicode is available at
Linux represents Unicode using the 8-bit Unicode Transfer
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 ISO-8859-1 users this means that the characters with
high bit set now are coded with two bytes. This tends to
expand ordinary text files by one or two percent. There
are no conversion problems, however, since the Unicode
value of ISO-8859-1 symbols equals their ISO-8859-1 value
(extended by eight leading zero bits). For Japanese users
this means that the 16-bit codes now in common use will
take three bytes, and extensive mapping tables are
required. Many Japanese therefore prefer ISO 2022.
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 han-
dling 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 sym-
bols. This is not enough for Japanese, Chinese and
Korean, but it is enough for most other purposes.
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 pop-
ular 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 (ini-
tially G1). Each graphic character set has 94 or 96 char-
acters, 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 Reg-
ister 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 num-
ber 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, respec-
console(4), console_ioctl(4), console_codes(4)
Linux November 5th, 1996 1