Unix/Linux Go Back    


Linux 2.6 - man page for unicode (linux section 7)

Linux & Unix Commands - Search Man Pages
Man Page or Keyword Search:   man
Select Man Page Set:       apropos Keyword Search (sections above)


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 con-
       tains all characters of all other character set standards.  It also guarantees  round-trip
       compatibility,  i.e., 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 Syllab-
       ics,  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 sys-
       tems, 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 char-
       acters for special scientific, dictionary printing, publishing industry, higher-level pro-
       tocol 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 charac-
       ter 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  dia-
       critical  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 alterna-
       tively 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 mathe-
       matical 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  vovel/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,  Malay-
		alam).

       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 tech-
       nical 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 multi-
       byte 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.

       Under Linux, in general only the BMP at implementation level  1	should	be  used  at  the
       moment.	Up to two combining characters per base character for certain scripts (in partic-
       ular Thai) are also supported by some UTF-8 terminal emulators and ISO 10646 fonts  (level
       2),  but  in  general  precomposed characters should be preferred where available (Unicode
       calls this Normalization Form C).

   Private area
       In the BMP, 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 currently maintained by H. Peter Anvin <Peter.Anvin@linux.org>.

   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 as a PDF file on CD-ROM 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/unicode/reports/>

       * Markus Kuhn: UTF-8 and Unicode FAQ for UNIX/Linux.
	 <http://www.cl.cam.ac.uk/~mgk25/unicode.html>

	 Provides  subscription  information  for  the linux-utf8 mailing list, which is the best
	 place to look for advice on using Unicode under Linux.

       * Bruno Haible: Unicode HOWTO.
	 <ftp://ftp.ilog.fr/pub/Users/haible/utf8/Unicode-HOWTO.html>

BUGS
       When this man page was last revised, the GNU C  Library	support  for  UTF-8  locales  was
       mature and XFree86 support was in an advanced state, but work on making applications (most
       notably editors) suitable for use in UTF-8 locales was still fully in  progress.   Current
       general UCS support under Linux usually provides for CJK double-width characters and some-
       times even simple overstriking combining characters, but usually does not include  support
       for  scripts  with  right-to-left  writing direction or ligature substitution requirements
       such as Hebrew, Arabic, or the Indic scripts.  These scripts are currently supported  only
       in  certain  GUI applications (HTML viewers, word processors) with sophisticated text ren-
       dering engines.

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

COLOPHON
       This page is part of release 3.55 of the Linux man-pages project.  A  description  of  the
       project,     and    information	  about    reporting	bugs,	 can	be    found    at
       http://www.kernel.org/doc/man-pages/.

GNU					    2012-08-05				       UNICODE(7)
Unix & Linux Commands & Man Pages : ©2000 - 2018 Unix and Linux Forums


All times are GMT -4. The time now is 03:31 PM.