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., conversion tables can be built
such that no information is lost when a string is con
verted from any other encoding to UCS and back.
UCS contains the characters required to represent practi
cally 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 Syl
labics, 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 pro
vided by TeX, Postscript, APL, MS-DOS, MS-Windows, Macin
tosh, OCR fonts, as well as many word processing and pub
lishing 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 Multi
lingual 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 supplemental planes added by ISO 10646-2 cover
only more exotic characters for special scientific, dic
tionary printing, publishing industry, higher-level proto
col 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 charac
ters), whereas UCS-4 is the representation of each
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 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 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 alter
natively as the combination of a normal "Latin capital
letter A" followed by a "combining diaeresis": 0x0041
0x0308.
Combining characters are essential for instance for encod
ing 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 mecha
nisms 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, Malayalam, etc.).
Level 3 All UCS characters are supported.
The Unicode 3.0 Standard published by the Unicode Consor
tium 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
ters. 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 C 99 standard.
UCS/Unicode can be used just like ASCII in input/output
streams, terminal communication, plaintext files, file
names, 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 par
ticular 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 exten
sions 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.
a PDF file on CD-ROM from http://www.iso.ch/.
* The Unicode Standard, Version 3.0. The Unicode Consor
tium, 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 char
acter 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 support
under Linux usually provides for CJK double-width charac
ters 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 proces
sors) with sophisticated text rendering engines.
AUTHOR
Markus Kuhn <mgk25@cl.cam.ac.uk>
SEE ALSO
utf-8(7), charsets(7), setlocale(3)
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