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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 guar‐
       antees "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  en‐
       coding 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, Myan‐
       mar, 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 in‐
       dustry, 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 back‐
       ward-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 al‐
       lows 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 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/con‐
                sonant 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,  compar‐
       ing, 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  com‐
       patible 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)

GNU                                                                            2021-03-22                                                                     UNICODE(7)