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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 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, Sin-
     hala, 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 architec-
     ture  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 Multilin-
     gual  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  charac-
     ters found in the commonly used other character sets.  The supplemental
     planes added by ISO 10646-2 cover only more exotic characters for  spe-
     cial 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 clas-
     sic 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, how-
     ever,  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  imple-
     mentation 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 pub-
     lished by the Unicode Consortium provide much additional information on
     the semantics and recommended usages of various characters.  They  pro-
     vide guidelines and algorithms for editing, sorting, comparing, normal-
     izing, 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, termi-
     nal  communication,  plaintext  files, filenames, and environment vari-
     ables 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 posi-
     tions (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  pri-
     vate usage.  For the Linux community, this private area has been subdi-
     vided further into the range 0xe000 to 0xefff which can be  used  indi-
     vidually  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  (Supplemen-
     tary  Private  Use Area-A, range 0xf0000 to 0xffffd) and plane 16 (Sup-
     plementary 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

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, Pren-

tice 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.