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unicode(7) Miscellaneous Information Manual unicode(7)
unicode - universal character set
The international standard ISO/IEC 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/IEC 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/IEC 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/IEC 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/IEC 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/IEC 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/IEC 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/IEC 10646-1:2000.
Unicode 3.1 added the supplemental planes of ISO/IEC 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/admin-guide/unicode.rst in the Linux kernel sources
(or Documentation/unicode.txt before Linux 4.10).
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,
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/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)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
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⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
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⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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Linux man-pages 6.10 2025-01-22 unicode(7)
Pages that refer to this page: locale(5), charsets(7), locale(7), utf-8(7)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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