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 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 C 99 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
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
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 C 99, which improved
wide and multi-byte character support even further.
Unicode Technical Reports.
Markus Kuhn: UTF-8 and Unicode FAQ for Unix/Linux.
Provides subscription information for the linux-utf8 mailing list, which
is the best place to look for advice on using Unicode under Linux.
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.