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 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 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 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
IMPLEMENTATION LEVELS [Toc] [Back]
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 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 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.,
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
* 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, AddisonWesley,
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
* Bruno Haible: Unicode HOWTO.
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.
Markus Kuhn <firstname.lastname@example.org>
utf-8(7), charsets(7), setlocale(3)
GNU 2001-05-11 UNICODE(7)
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