man > unicode(7)

unicode(7)                        Miscellaneous Information Manual                        unicode(7)

NAME
       unicode - universal character set

DESCRIPTION
       The international standard ISO/IEC 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 in‐
       cludes 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, Chero‐
       kee, Mongolian, Ogham, Myanmar, Sinhala, Thaana, Yi, and others.  For scripts  not  yet  cov‐
       ered,  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 sci‐
       entific symbols, including those provided by TeX, Postscript, APL, MS-DOS, MS-Windows, Macin‐
       tosh, OCR fonts, as well as many word processing and publishing systems, and more  are  being
       added.

       The  UCS standard (ISO/IEC 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/IEC 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/IEC 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/IEC 10646-2 cover only more exotic characters for spe‐
       cial scientific, dictionary printing, publishing industry, higher-level protocol and enthusi‐
       ast 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 charac‐
       ters 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/IEC 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 mod‐
       ify.  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 mathemat‐
       ical typesetting and users of the International Phonetic Alphabet.

   Implementation levels
       As  not  all  systems  are expected to support advanced mechanisms like combining characters,
       ISO/IEC 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/IEC 10646-1:2000.  Unicode
       3.1 added the supplemental planes of ISO/IEC 10646-2.  The Unicode standard and technical re‐
       ports published by the Unicode Consortium provide much additional information on  the  seman‐
       tics  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 sig‐
       naled 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  suit‐
       able 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 func‐
       tions such as wctomb(3) and mbsrtowcs(3) can be used to transform the internal wchar_t  char‐
       acters  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 exten‐
       sions 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/uni‐
       code.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:  Ar‐
          chitecture and Basic Multilingual Plane.  International Standard ISO/IEC 10646-1, Interna‐
          tional Organization for Standardization, Geneva, 2000.

          This is the official specification of UCS.  Available from ]8;;http://www.iso.ch/\http://www.iso.ch/]8;;\.

       •  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 func‐
          tions 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.
          ]8;;http://www.unicode.org/reports/\http://www.unicode.org/reports/]8;;\

       •  Markus Kuhn: UTF-8 and Unicode FAQ for UNIX/Linux.
          ]8;;http://www.cl.cam.ac.uk/~mgk25/unicode.html\http://www.cl.cam.ac.uk/~mgk25/unicode.html]8;;\

       •  Bruno Haible: Unicode HOWTO.
          ]8;;http://www.tldp.org/HOWTO/Unicode-HOWTO.html\http://www.tldp.org/HOWTO/Unicode-HOWTO.html]8;;\

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

Linux man-pages 6.7                          2024-01-28                                   unicode(7)