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

       unicode - universal character set

       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).

       *  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,
          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.

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

       *  Bruno Haible: Unicode HOWTO.
          ⟨http://www.tldp.org/HOWTO/Unicode-HOWTO.htmlSEE ALSO
       locale(1), setlocale(3), charsets(7), utf-8(7)

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       latest version of this page, can be found at

GNU                                2020-08-13                         UNICODE(7)