Unicode — the Universal Character Set
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., 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 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 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 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 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.
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, 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 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.
As not all systems are expected to support advanced mechanisms like combining characters, ISO 10646-1 specifies the following three implementation levels of UCS:
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.
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.).
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 editing, sorting, comparing, normalizing, converting and displaying Unicode strings.
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 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).
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>.
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 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 C99, 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-utf8mailing 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
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 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
viewers, word processors) with sophisticated text rendering
engines.
This page is part of release 2.79 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/.
|
Copyright (C) Markus Kuhn, 1995, 2001 This is free documentation; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. The GNU General Public License's references to "object code" and "executables" are to be interpreted as the output of any document formatting or typesetting system, including intermediate and printed output. This manual is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this manual; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111, USA. 1995-11-26 Markus Kuhn <mskuhncip.informatik.uni-erlangen.de> First version written 2001-05-11 Markus Kuhn <mgk25cl.cam.ac.uk> Update |