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UNICODE(7) Linux Programmer's Manual UNICODE(7)
Unicode - 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 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 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 nonspacing 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:
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 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 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., "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 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.
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
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.
setlocale(3), charsets(7), utf-8(7)
This page is part of release 3.32 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/.
GNU 2001-05-11 UNICODE(7)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface