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       Unicode - universal character set


       The  international  standard  ISO 10646 defines the Universal Character
       Set (UCS).  UCS contains all characters  of  all  other  character  set
       standards.   It  also  guarantees  "round-trip compatibility"; in other
       words, conversion tables can be built such that no information is  lost
       when a string is converted from any other encoding 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 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, Myanmar,
       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
       architecture  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
       Multilingual  Plane  (BMP),  that is plane 0 in group 0.  Part 2 of the
       standard (ISO 10646-2) adds characters to group 0 outside  the  BMP  in
       several  supplementary  planes in 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 the foreseeable future.  The BMP contains
       all  characters  found  in the commonly used other character sets.  The
       supplemental planes  added  by  ISO  10646-2  cover  only  more  exotic
       characters  for  special  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  forms UTF-8 for backward compatibility with ASCII
       processing software and UTF-16 for the backward-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
       classic US-ASCII character set and the characters in the  range  0x0000
       to 0x00ff are identical to those in ISO 8859-1 (Latin-1).

   Combining characters
       Some  code  points  in  UCS have been assigned to combining characters.
       These are similar to the nonspacing 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  UCS,
       however, 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 the Thai
       script or for mathematical typesetting and users 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
       implementation levels of UCS:

       Level 1  Combining  characters  and  Hangul Jamo (a variant encoding of
                the Korean script, where a Hangul syllable glyph is coded as a
                triplet or pair of vowel/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).

       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  ISO  10646-1:2000.   Unicode  3.1 added the supplemental
       planes of ISO 10646-2.  The  Unicode  standard  and  technical  reports
       published by the Unicode Consortium provide much additional information
       on the semantics and recommended usages of  various  characters.   They
       provide  guidelines  and  algorithms  for  editing, sorting, comparing,
       normalizing, 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 all locales), a convention that is signaled by the GNU C library to
       applications  by  defining the constant __STDC_ISO_10646__ as specified
       in the ISO C99 standard.

       UCS/Unicode can be  used  just  like  ASCII  in  input/output  streams,
       terminal  communication,  plaintext  files,  filenames, and environment
       variables in the ASCII compatible UTF-8 multibyte encoding.  To  signal
       the  use  of  UTF-8  as  the  character encoding to all applications, a
       suitable locale has to be selected  via  environment  variables  (e.g.,

       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 and  back  and  wcwidth(3)  tells,  how  many
       positions (0–2) the cursor is advanced by the output of a character.

   Private area
       In  the Basic Multilingual Plane, 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 the range 0xf000 to
       0xf8ff where extensions are coordinated among  all  Linux  users.   The
       registry  of the characters assigned to the Linux zone is maintained by
       LANANA and the registry  itself  is  Documentation/unicode.txt  in  the
       Linux kernel sources.

       *  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  from

       *  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,
          Prentice 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 C90  standard,  which
          adds a large number of new C library functions for handling wide and
          multibyte character encodings, but it does not yet  cover  ISO  C99,
          which improved wide and multibyte character support even further.

       *  Unicode Technical Reports.

       *  Markus Kuhn: UTF-8 and Unicode FAQ for UNIX/Linux.

       *  Bruno Haible: Unicode HOWTO.


       locale(1), setlocale(3), charsets(7), utf-8(7)


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