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

NAME
       Unicode - the 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 compatibility, i.e., conver-
       sion 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	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,  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-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 architec-
       ture  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 Multilin-
       gual  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  charac-
       ters  found in the commonly used	other character	sets. The supplemental
       planes added by ISO 10646-2 cover only more exotic characters for  spe-
       cial 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 backwards compatibility with ASCII
       processing software and UTF-16 for the backwards	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 clas-
       sic 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	non-spacing 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, how-
       ever,  the  combining  character	mechanism allows us to add accents and
       other diacritical marks to any character. The combining characters  al-
       ways  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	imple-
       mentation 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, Malayalam, etc.).

       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  pub-
       lished by the Unicode Consortium	provide	much additional	information on
       the semantics and recommended usages of various characters.  They  pro-
       vide guidelines and algorithms for editing, sorting, comparing, normal-
       izing, 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 C 99 standard.

       UCS/Unicode can be used just like ASCII in input/output streams,	termi-
       nal  communication,  plaintext  files, filenames, and environment vari-
       ables 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 en-
       coding. 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 posi-
       tions (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  charac-
       ter for certain scripts (in particular 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 as-
       signed to the Linux zone	is currently maintained	by H. Peter Anvin <Pe-
       ter.Anvin@linux.org>.

LITERATURE
       * Information  technology  --  Universal	Multiple-Octet Coded Character
	 Set (UCS) -- Part 1: Architecture and Basic Multilingual Plane.   In-
	 ternational  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,	 Pren-
	 tice 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	C 90  standard,	 which
	 adds  a large number of new C library functions for handling wide and
	 multi-byte character encodings, but it	does not yet cover ISO	C  99,
	 which improved	wide and multi-byte 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  sup-
       port  under  Linux usually provides for CJK double-width	characters and
       sometimes even simple overstriking combining  characters,  but  usually
       does  not include support for scripts with right-to-left	writing	direc-
       tion or ligature	substitution requirements such as Hebrew,  Arabic,  or
       the  Indic  scripts. These scripts are currently	only supported in cer-
       tain GUI	applications (HTML viewers, word  processors)  with  sophisti-
       cated text rendering engines.

AUTHOR
       Markus Kuhn <mgk25@cl.cam.ac.uk>

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

GNU				  2001-05-11			    UNICODE(7)

NAME | DESCRIPTION | COMBINING CHARACTERS | IMPLEMENTATION LEVELS | UNICODE UNDER LINUX | PRIVATE AREA | LITERATURE | BUGS | AUTHOR | SEE ALSO

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