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NAME | DESCRIPTION | SEE ALSO | COLOPHON |
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"; 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, 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).
Combining characters
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.
Implementation levels
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 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
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.
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,
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.
Private Use Areas (PUA)
In the Basic Multilingual Plane, 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
maintained by LANANA and the registry itself is
Documentation/unicode.txt in the Linux kernel sources.
Two other planes are reserved for private usage, plane 15
(Supplementary Private Use Area-A, range 0xf0000 to 0xffffd) and
plane 16 (Supplementary Private Use Area-B, range 0x100000 to
0x10fffd).
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.
This is the official specification of UCS . Available from
⟨http://www.iso.ch/⟩.
* The Unicode Standard, Version 3.0. The Unicode Consortium, Addi‐
son-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 stan‐
dard, 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/reports/⟩
* Markus Kuhn: UTF-8 and Unicode FAQ for UNIX/Linux.
⟨http://www.cl.cam.ac.uk/~mgk25/unicode.html⟩
* Bruno Haible: Unicode HOWTO.
⟨http://www.tldp.org/HOWTO/Unicode-HOWTO.html⟩
locale(1), setlocale(3), charsets(7), utf-8(7)
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GNU 2016-03-15 UNICODE(7)
Pages that refer to this page: groff(1), locale(5), charsets(7), locale(7), utf-8(7)