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UNICODE(7) Linux Programmer's Manual UNICODE(7)
NAME
unicode - universal character set
DESCRIPTION
The international standard ISO 10646 defines the Universal Character Set (UCS). UCS con-
tains 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 Syllab-
ics, 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 char-
acters for special scientific, dictionary printing, publishing industry, higher-level pro-
tocol 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 charac-
ter 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 dia-
critical 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 alterna-
tively 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 mathe-
matical 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, Malaya-
lam).
Level 3 All UCS characters are supported.
The Unicode 3.0 Standard published by the Unicode Consortium contains exactly the UCS Ba-
sic Multilingual Plane at implementation level 3, as described in ISO 10646-1:2000. Uni-
code 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 se-
mantics and recommended usages of various characters. They provide guidelines and algo-
rithms 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 al-
ways 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 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 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/ad-
min-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, In-
ternational 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>
SEE ALSO
locale(1), setlocale(3), charsets(7), utf-8(7)
COLOPHON
This page is part of release 5.10 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this page, can be
found at https://www.kernel.org/doc/man-pages/.
GNU 2020-08-13 UNICODE(7)
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