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"; in other
words, 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, Sin-
hala, 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 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 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 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).
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
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 vowel/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).
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 C99 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 multibyte 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.,
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 posi-
tions (0-2) the cursor is advanced by the output of a character.
In the Basic Multilingual Plane, the range 0xe000 to 0xf8ff will never
be assigned to any characters by the standard and is reserved for pri-
vate usage. For the Linux community, this private area has been subdi-
vided further into the range 0xe000 to 0xefff which can be used indi-
vidually 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 maintained by
LANANA and the registry itself is Documentation/unicode.txt in the
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 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.
* Markus Kuhn: UTF-8 and Unicode FAQ for UNIX/Linux.
* Bruno Haible: Unicode HOWTO.
locale(1), setlocale(3), charsets(7), utf-8(7)
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