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     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, dictio-
          nary printing, publishing industry, higher-level protocol
          and enthusiast needs.

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          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 soft-
          ware 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 com-
          bining 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 combi-
          nation 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 mecha-
          nisms 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 syl-
                   lable 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, Devana-
                   gari, 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 imple-
          mentation level 3, as described in ISO 10646-1:2000.

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          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 inte-
          ger 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 char-
          acter 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 (0en2) 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 reg-
          istry of the characters assigned to the Linux zone is main-
          tained 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

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             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 Consor-
             tium, 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 fur-
             ther.

          *  Unicode Technical Reports.

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

          *  Bruno Haible: Unicode HOWTO.

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

     COLOPHON
          This page is part of release 5.10 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 https://www.kernel.org/doc/man-pages/.

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