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.\" 1995-11-26  Markus Kuhn <mskuhn@cip.informatik.uni-erlangen.de>
.\"      First version written
.\" 2001-05-11  Markus Kuhn <mgk25@cl.cam.ac.uk>
.\"      Update
.\"
.TH UNICODE 7 2017-09-15 "GNU" "Linux Programmer's Manual"
.SH NAME
unicode \- universal character set
.SH DESCRIPTION
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.
.PP
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.
.PP
The UCS standard (ISO 10646) describes a
31-bit character set architecture
consisting of 128 24-bit
.IR groups ,
each divided into 256 16-bit
.I planes
made up of 256 8-bit
.I rows
with 256
.I 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
.IR "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
.I "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.
.PP
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.
.PP
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).
.SS Combining characters
Some code points in UCS
have been assigned to
.IR "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.
.PP
Combining characters are essential for instance for encoding the Thai
script or for mathematical typesetting and users of the International
Phonetic Alphabet.
.SS Implementation levels
As not all systems are expected to support advanced mechanisms like
combining characters, ISO 10646-1 specifies the following three
.I implementation levels
of UCS:
.TP 0.9i
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.
.TP
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).
.TP
Level 3
All UCS characters are supported.
.PP
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.
.SS Unicode under Linux
Under GNU/Linux, the C type
.I 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
.B __STDC_ISO_10646__
as specified in the ISO C99 standard.
.PP
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
.I locale
has to be selected via environment variables (e.g.,
"LANG=en_GB.UTF-8").
.PP
The
.B nl_langinfo(CODESET)
function returns the name of the selected encoding.
Library functions such as
.BR wctomb (3)
and
.BR mbsrtowcs (3)
can be used to transform the internal
.I wchar_t
characters and strings into the system character encoding and back
and
.BR wcwidth (3)
tells, how many positions (0\(en2) the cursor is advanced by the
output of a character.
.PP
.SS 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
.I Documentation/admin\-guide/unicode.rst
in the Linux kernel sources
.\" commit 9d85025b0418163fae079c9ba8f8445212de8568
(or
.I Documentation/unicode.txt
before Linux 4.10).
.PP
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).
.SS Literature
.IP * 3
Information technology \(em Universal Multiple-Octet Coded Character
Set (UCS) \(em Part 1: Architecture and Basic Multilingual Plane.
International Standard ISO/IEC 10646-1, International Organization
for Standardization, Geneva, 2000.
.IP
This is the official specification of UCS .
Available from
.UR http://www.iso.ch/
.UE .
.IP *
The Unicode Standard, Version 3.0.
The Unicode Consortium, Addison-Wesley,
Reading, MA, 2000, ISBN 0-201-61633-5.
.IP *
S.\& Harbison, G.\& Steele. C: A Reference Manual. Fourth edition,
Prentice Hall, Englewood Cliffs, 1995, ISBN 0-13-326224-3.
.IP
A good reference book about the C programming language.
The fourth