NAME

       unicode - universal character set

DESCRIPTION

       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., "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 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/admin-guide/unicode.rst in the Linux kernel sources  (or  Documentation/unicode.txt  before
       Linux 4.10).

       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).

   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 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, 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.
          ⟨http://www.unicode.org/reports/⟩

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

       *  Bruno Haible: Unicode HOWTO.
          ⟨http://www.tldp.org/HOWTO/Unicode-HOWTO.html⟩

SEE ALSO

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

COLOPHON

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