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



NAME
       Unicode - the Universal Character Set

DESCRIPTION
       The  international  standard  ISO 10646 defines the Universal Character
       Set (UCS).  UCS contains all characters  of  all  other  character  set(7,n,1 builtins)
       standards.  It  also guarantees round-trip compatibility, i.e., conver-
       sion tables can be built such that no information is lost when a string(3,n)
       is converted from any other encoding(3,n) to UCS and back.

       UCS contains the characters required to represent practically all known
       languages. This includes not only the Latin, Greek,  Cyrillic,  Hebrew,
       Arabic, Armenian, and Georgian scripts, but also also Chinese, Japanese
       and Korean  Han  ideographs  as  well  as  scripts  such  as  Hiragana,
       Katakana,  Hangul,  Devanagari,  Bengali,  Gurmukhi,  Gujarati,  Oriya,
       Tamil, Telugu, Kannada, Malayalam, Thai, Lao, Khmer, Bopomofo, Tibetan,
       Runic,  Ethiopic, Canadian Syllabics, Cherokee, Mongolian, Ogham, Myan-
       mar, Sinhala, Thaana, Yi, and others.  For  scripts  not  yet  covered,
       research  on  how to best encode them for computer usage is still going
       on and they will be added eventually. This might eventually include not
       only Hieroglyphs and various historic Indo-European languages, but even
       some selected artistic scripts such as Tengwar, Cirth, and Klingon. UCS
       also  covers  a  large number of graphical, typographical, mathematical
       and scientific symbols, including those provided  by  TeX,  Postscript,
       APL,  MS-DOS,  MS-Windows,  Macintosh,  OCR fonts, as well as many word
       processing and publishing systems, and more are being added.

       The UCS standard (ISO 10646) describes a 31-bit character set(7,n,1 builtins) architec-
       ture  consisting  of  128  24-bit  groups, each divided into 256 16-bit
       planes made up of 256 8-bit rows with 256  column  positions,  one  for
       each  character. Part 1 of the standard (ISO 10646-1) defines the first
       65534 code positions (0x0000 to 0xfffd), which form the Basic Multilin-
       gual  Plane  (BMP),  that is plane 0 in(1,8) group 0. Part 2 of the standard
       (ISO 10646-2) adds characters to group 0 outside  the  BMP  in(1,8)  several
       supplementary  planes  in(1,8)  the  range 0x10000 to 0x10ffff. There are no
       plans to add characters beyond 0x10ffff to the standard,  therefore  of
       the  entire  code  space, only a small fraction of group 0 will ever be
       actually used in(1,8) the foreseeable future. The BMP contains  all  charac-
       ters  found in(1,8) the commonly used other character sets. The supplemental
       planes added by ISO 10646-2 cover only more exotic characters for  spe-
       cial scientific, dictionary printing, publishing industry, higher-level
       protocol and enthusiast needs.

       The representation of each UCS character as a 2-byte word  is  referred
       to  as  the  UCS-2 form (only for BMP characters), whereas UCS-4 is the
       representation of each character by a 4-byte word.  In addition,  there
       exist  two  encoding(3,n) forms UTF-8 for backwards compatibility with ASCII
       processing software and UTF-16 for the backwards compatible handling of
       non-BMP characters up to 0x10ffff by UCS-2 software.

       The UCS characters 0x0000 to 0x007f are identical to those of the clas-
       sic US-ASCII character set(7,n,1 builtins) and the characters in(1,8) the  range  0x0000  to
       0x00ff are identical to those in(1,8) ISO 8859-1 Latin-1.

COMBINING CHARACTERS
       Some  code  points  in(1,8)  UCS have been assigned to combining characters.
       These are similar to the non-spacing accent keys  on  a  typewriter.  A
       combining  character just adds an accent to the previous character. The
       most important accented characters have codes of their own in(1,8) UCS, how-
       ever,  the  combining  character mechanism allows us to add accents and
       other diacritical marks to  any  character.  The  combining  characters
       always  follow the character which they modify. For example, the German
       character Umlaut-A ("Latin capital letter A with diaeresis") can either
       be  represented by the precomposed UCS code 0x00c4, or alternatively as
       the combination of a normal "Latin capital  letter  A"  followed  by  a
       "combining diaeresis": 0x0041 0x0308.

       Combining  characters  are essential for instance for encoding(3,n) the Thai
       script or for mathematical typesetting and users(1,5) of  the  International
       Phonetic Alphabet.

IMPLEMENTATION LEVELS
       As  not  all  systems  are expected to support advanced mechanisms like
       combining characters, ISO 10646-1 specifies the following three  imple-
       mentation levels of UCS:

       Level 1  Combining  characters  and  Hangul Jamo (a variant encoding(3,n) of
                the Korean script, where a Hangul syllable glyph is coded as a
                triplet or pair of vovel/consonant codes) are not supported.

       Level 2  In  addition  to level 1, combining characters are now allowed
                for some languages where they are essential (e.g., Thai,  Lao,
                Hebrew, Arabic, Devanagari, Malayalam, etc.).

       Level 3  All UCS characters are supported.

       The  Unicode  3.0 Standard published by the Unicode Consortium contains
       exactly the UCS Basic Multilingual Plane at implementation level 3,  as
       described  in(1,8)  ISO  10646-1:2000.   Unicode  3.1 added the supplemental
       planes of ISO 10646-2. The Unicode standard and technical reports  pub-
       lished by the Unicode Consortium provide much additional information on
       the semantics and recommended usages of various characters.  They  pro-
       vide guidelines and algorithms for editing, sorting, comparing, normal-
       izing, converting and displaying Unicode strings.

UNICODE UNDER LINUX
       Under GNU/Linux, the C type wchar_t is a signed  32-bit  integer  type.
       Its  values  are always interpreted by the C library as UCS code values
       (in(1,8) all locales), a convention that is signaled by the GNU C library to
       applications  by  defining the constant __STDC_ISO_10646__ as specified
       in(1,8) the ISO C 99 standard.

       UCS/Unicode can be used just like ASCII in(1,8) input/output streams, termi-
       nal  communication,  plaintext  files, filenames, and environment vari-
       ables in(1,8) the ASCII compatible UTF-8 multi-byte encoding. To signal(2,7)  the
       use  of UTF-8 as the character encoding(3,n) to all applications, a suitable
       locale(3,5,7)  has  to  be   selected   via   environment   variables   (e.g.,
       "LANG=en_GB.UTF-8").

       The  nl_langinfo(CODESET)  function  returns  the  name of the selected
       encoding. Library functions such as wctomb(3) and mbsrtowcs(3)  can  be
       used  to transform the internal wchar_t characters and strings into the
       system character encoding(3,n) and back and wcwidth(3) tells, how many posi-
       tions (0-2) the cursor is advanced by the output of a character.

       Under  Linux,  in(1,8) general only the BMP at implementation level 1 should
       be used at the moment. Up to two combining characters per base  charac-
       ter for certain scripts (in(1,8) particular Thai) are also supported by some
       UTF-8 terminal emulators and ISO 10646 fonts (level 2), but in(1,8)  general
       precomposed  characters  should  be  preferred where available (Unicode
       calls this Normalization Form C).

PRIVATE AREA
       In the BMP, the range 0xe000 to 0xf8ff will never be  assigned  to  any
       characters  by  the standard and is reserved for private usage. For the
       Linux community, this private area has been subdivided further into the
       range  0xe000  to 0xefff which can be used individually by any end-user
       and the Linux zone in(1,8) the range 0xf000 to 0xf8ff where  extensions  are
       coordinated  among  all  Linux  users.  The  registry of the characters
       assigned to the Linux zone is currently maintained by  H.  Peter  Anvin
       <Peter.Anvin@linux.org>.

LITERATURE
       * Information  technology  --  Universal Multiple-Octet Coded Character
         Set (UCS) -- Part  1:  Architecture  and  Basic  Multilingual  Plane.
         International  Standard  ISO/IEC  10646-1, International Organization
         for Standardization, Geneva, 2000.

         This is the official specification of UCS.  Available as a  PDF  file(1,n)
         on CD-ROM from http://www.iso.ch/.

       * The  Unicode Standard, Version 3.0.  The Unicode Consortium, Addison-
         Wesley, Reading, MA, 2000, ISBN 0-201-61633-5.

       * S. Harbison, G. Steele. C: A Reference Manual. Fourth edition,  Pren-
         tice Hall, Englewood Cliffs, 1995, ISBN 0-13-326224-3.

         A  good  reference  book about the C programming language. The fourth
         edition covers the 1994 Amendment 1 to the ISO C 90  standard,  which
         adds  a large number of new C library functions for handling wide and
         multi-byte character encodings, but it does not yet cover ISO  C  99,
         which improved wide and multi-byte character support even further.

       * Unicode Technical Reports.
         http://www.unicode.org/unicode/reports/

       * Markus Kuhn: UTF-8 and Unicode FAQ for Unix/Linux.
         http://www.cl.cam.ac.uk/~mgk25/unicode.html

         Provides  subscription  information  for the linux-utf8 mailing list,
         which is the best place to look(1,8,3 Search::Dict) for advice  on  using  Unicode  under
         Linux.

       * Bruno Haible: Unicode HOWTO.
         ftp://ftp.ilog.fr/pub/Users/haible/utf8/Unicode-HOWTO.html

BUGS
       When  this  man(1,5,7)  page  was  last revised, the GNU C Library support for
       UTF-8 locales was mature and XFree86 support was in(1,8) an advanced  state,
       but work on making applications (most notably editors) suitable for use
       in(1,8) UTF-8 locales was still fully in(1,8) progress. Current general UCS  sup-
       port  under  Linux usually provides for CJK double-width characters and
       sometimes even simple overstriking combining  characters,  but  usually
       does  not include support for scripts with right-to-left writing direc-
       tion or ligature substitution requirements such as Hebrew,  Arabic,  or
       the  Indic  scripts. These scripts are currently only supported in(1,8) cer-
       tain GUI applications (HTML viewers, word  processors)  with  sophisti-
       cated text rendering engines.

AUTHOR
       Markus Kuhn <mgk25@cl.cam.ac.uk>

SEE ALSO
       setlocale(3), charsets(7), utf-8(7)



GNU                               2001-05-11                        UNICODE(7)

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