charsets(7)



CHARSETS(7)                Linux Programmer's Manual               CHARSETS(7)

NAME
       charsets - character set standards and internationalization

DESCRIPTION
       This manual page gives an overview on different character set standards
       and how they were used on Linux before Unicode became ubiquitous.  Some
       of  this  information  is  still helpful for people working with legacy
       systems and documents.

       Standards discussed include such as ASCII,  GB  2312,  ISO  8859,  JIS,
       KOI8-R, KS, and Unicode.

       The  primary  emphasis  is on character sets that were actually used by
       locale character sets, not the myriad others that  could  be  found  in
       data from other systems.

   ASCII
       ASCII (American Standard Code For Information Interchange) is the orig-
       inal 7-bit character set, originally  designed  for  American  English.
       Also  known as US-ASCII.  It is currently described by the ISO 646:1991
       IRV (International Reference Version) standard.

       Various ASCII variants replacing the dollar sign  with  other  currency
       symbols  and  replacing punctuation with non-English alphabetic charac-
       ters to cover German, French, Spanish, and others in  7  bits  emerged.
       All are deprecated; glibc does not support locales whose character sets
       are not true supersets of ASCII.

       As Unicode, when using UTF-8, is  ASCII-compatible,  plain  ASCII  text
       still renders properly on modern UTF-8 using systems.

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

       Of  these,  the  most important is ISO 8859-1 ("Latin Alphabet No .1" /
       Latin-1).  It was widely adopted and supported  by  different  systems,
       and  is  gradually being replaced with Unicode.  The ISO 8859-1 charac-
       ters are also the first 256 characters of Unicode.

       Console support for the other 8859 character sets  is  available  under
       Linux through user-mode utilities (such as setfont(8)) that modify key-
       board 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  many  West European languages such as Albanian,
              Basque, Danish, English, Faroese,  Galician,  Icelandic,  Irish,
              Italian,  Norwegian, Portuguese, Spanish, and Swedish.  The lack
              of the ligatures Dutch IJ/ij, French oe, and old-style ,,German"
              quotation marks was considered tolerable.

       8859-2 (Latin-2)
              Latin-2  supports  many  Latin-written Central and East European
              languages such as Bosnian, Croatian, Czech,  German,  Hungarian,
              Polish,  Slovak,  and Slovene.  Replacing Romanian / with s/ was
              considered tolerable.

       8859-3 (Latin-3)
              Latin-3 was designed to cover of Esperanto, Maltese,  and  Turk-
              ish, but 8859-9 later superseded it for Turkish.

       8859-4 (Latin-4)
              Latin-4  introduced letters for North European languages such as
              Estonian, Latvian, and Lithuanian, but was superseded by 8859-10
              and 8859-13.

       8859-5 Cyrillic letters supporting Bulgarian, Byelorussian, Macedonian,
              Russian, Serbian, and (almost  completely)  Ukrainian.   It  was
              never widely used, see the discussion of KOI8-R/KOI8-U below.

       8859-6 Was  created for 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 Was created for Modern Greek in 1987, updated in 2003.

       8859-8 Supports Modern Hebrew without niqud (punctuation signs).  Niqud
              and full-fledged Biblical Hebrew were outside the scope of  this
              character set.

       8859-9 (Latin-5)
              This  is  a  variant  of Latin-1 that replaces Icelandic letters
              with Turkish ones.

       8859-10 (Latin-6)
              Latin-6 added the Inuit (Greenlandic) and Sami (Lappish) letters
              that were missing in Latin-4 to cover the entire Nordic area.

       8859-11
              Supports  the  Thai  alphabet  and  is  nearly  identical to the
              TIS-620 standard.

       8859-12
              This set does not exist.

       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 Old Irish, Manx,
              Gaelic, Welsh, Cornish, and Breton.

       8859-15 (Latin-9)
              Latin-9 is similar to the widely used Latin-1 but replaces  some
              less  common  symbols  with the Euro sign and French and Finnish
              letters that were missing in Latin-1.

       8859-16 (Latin-10)
              This set covers many  Southeast  European  languages,  and  most
              importantly supports Romanian more completely than Latin-2.

   KOI8-R / KOI8-U
       KOI8-R  is  a  non-ISO  character set popular in Russia before Unicode.
       The lower half is ASCII; the upper is a Cyrillic character set somewhat
       better  designed  than ISO 8859-5.  KOI8-U, based on KOI8-R, 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.

   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  ASCII  and  GB
       2312.  Note that EUC-CN is often called as GB, GB 2312, or CN-GB.

   Big5
       Big5  was a popular character set in Taiwan to express traditional Chi-
       nese.  (Big5 is both a character set and an encoding.)  It is a  super-
       set  of 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 were widely used in Taiwan and Hong Kong.  It is not ISO
       2022 compliant.

   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 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 ASCII and KS X 1001.  KS C 5601 is an older name
       for KS X 1001.

   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  ker-
       nel  and  by xterm(1).  Several ISO 2022-based character encodings have
       been defined, especially for Japanese.

       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 (ini-
       tially 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  char-
       acter 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.

   TIS-620
       TIS-620 is a Thai national standard character set  and  a  superset  of
       ASCII.  In the same fashion as the ISO 8859 series, Thai characters are
       mapped into 0xa1-0xfe.

   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 inte-
       gers internally and either a series of 16-bit integers (UTF-16)  (need-
       ing  two 16-bit integers only when encoding certain rare characters) or
       a series of 8-bit bytes (UTF-8).

       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 texts in ISO 8859 character sets, this means that the charac-
       ters 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 texts, 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 (espe-
       cially 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  "sub-
       font"  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 the Linux console in UTF-8 mode, one can use a character
       set with 512 different symbols.  This is not enough for Japanese,  Chi-
       nese, and Korean, but it is enough for most other purposes.

SEE ALSO
       iconv(1), ascii(7), iso_8859-1(7), unicode(7), utf-8(7)

COLOPHON
       This  page  is  part of release 4.12 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

Linux                             2016-07-17                       CHARSETS(7)

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