unicode(7) - man - phpman

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 compati‐
       bility"; 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, Ma‐
       layalam, Thai, Lao, Khmer, Bopomofo, Tibetan, Runic, Ethiopic, Canadian Syllabics,  Cherokee,
       Mongolian, Ogham, Myanmar, Sinhala, Thaana, Yi, and others.  For scripts not yet covered, re‐
       search 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-Eu‐
       ropean languages, but even some selected artistic scripts such as Tengwar, Cirth,  and  Klin‐
       gon.   UCS  also  covers a large number of graphical, typographical, mathematical, and scien‐
       tific 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 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  out‐
       side  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  supplemen‐
       tal  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 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 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
       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 edit‐
       ing, 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 ⟨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 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.
          ⟨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)

COLOPHON
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GNU                                          2020-08-13                                   UNICODE(7)