On Apache/1.3.37 (Unix) mod_throttle/3.1.2 FrontPage/5.0.2.2635 mod_psoft_traffic/0.2 mod_ssl/2.8.28 OpenSSL/0.9.8b
Under GNU General Public License
2008-12-01 15:32 @38.103.63.55 Crawled by CCBot/1.0 (+http://www.commoncrawl.org/bot.html)
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 guaran-
tees 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 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 even-
tually. 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 consist-
ing 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 sup-
plemental 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
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 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 sim-
ilar 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, however, the combining character mechanism allows
us to add accents and other diacritical marks to any character. The combining char-
acters always follow the character which they modify. For example, the German char-
acter 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 char-
acters, 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 lan-
guages where they are essential (e.g., Thai, Lao, Hebrew, Arabic, Devana-
gari, 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 Uni-
code 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 conven-
tion 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, terminal communi-
cation, plaintext files, filenames, and environment variables in the ASCII compati-
ble 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 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 pri-
vate 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 charac-
ters assigned to the Linux zone is currently maintained by H. Peter Anvin
<Peter.Anvin AT 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, Read-
ing, 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 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 appli-
cations (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 sometimes 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 only supported in certain GUI applications (HTML view-
ers, word processors) with sophisticated text rendering engines.
AUTHOR
Markus Kuhn <mgk25 AT cl.uk>
SEE ALSO
setlocale(3), charsets(7), utf-8(7)
GNU 2001-05-11 UNICODE(7)
Generated by $Id: phpMan.php,v 4.49 2006/02/26 13:18:18 chedong Exp $ Author: Che Dong
On Apache/1.3.37 (Unix) mod_throttle/3.1.2 FrontPage/5.0.2.2635 mod_psoft_traffic/0.2 mod_ssl/2.8.28 OpenSSL/0.9.8b
Under GNU General Public License
2008-12-01 15:32 @38.103.63.55 Crawled by CCBot/1.0 (+http://www.commoncrawl.org/bot.html)