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



NAME
       Unicode - universal character set

DESCRIPTION
       The  international standard ISO 10646 defines the Universal Character Set (UCS).  UCS con‐
       tains 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 Syllab‐
       ics,  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 char‐
       acters for special scientific, dictionary printing, publishing industry, higher-level pro‐
       tocol 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 charac‐
       ter 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  dia‐
       critical  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 alterna‐
       tively 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 mathe‐
       matical 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  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,  Malay‐
                alam).

       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 tech‐
       nical 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  Uni‐
       code 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 multi‐
       byte 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 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/uni‐
       code.txt in the Linux kernel sources.

   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
       This  page  is  part of release 3.74 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 http://www.kernel.org/doc/man-pages/.



GNU                                         2014-06-13                                 UNICODE(7)


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