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 Use Areas (PUA)
   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.

   Two   other   planes   are   reserved   for  private  usage,  plane  15
   (Supplementary Private Use Area-A, range 0xf0000 to 0xffffd) and  plane
   16 (Supplementary Private Use Area-B, range 0x100000 to 0x10fffd).

   *  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)


   This  page  is  part of release 4.09 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


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