1UNICODE(7)                 Linux Programmer's Manual                UNICODE(7)


6       unicode - universal character set


9       The  international  standard  ISO 10646 defines the Universal Character
10       Set (UCS).  UCS contains all characters  of  all  other  character  set
11       standards.   It  also  guarantees  "round-trip compatibility"; in other
12       words, conversion tables can be built such that no information is  lost
13       when a string is converted from any other encoding to UCS and back.
15       UCS contains the characters required to represent practically all known
16       languages.  This includes not only the Latin, Greek, Cyrillic,  Hebrew,
17       Arabic,  Armenian, and Georgian scripts, but also Chinese, Japanese and
18       Korean Han ideographs as well as scripts such  as  Hiragana,  Katakana,
19       Hangul,  Devanagari, Bengali, Gurmukhi, Gujarati, Oriya, Tamil, Telugu,
20       Kannada,  Malayalam,  Thai,  Lao,  Khmer,  Bopomofo,  Tibetan,   Runic,
21       Ethiopic, Canadian Syllabics, Cherokee, Mongolian, Ogham, Myanmar, Sin‐
22       hala, Thaana, Yi, and others.  For scripts not yet covered, research on
23       how  to  best encode them for computer usage is still going on and they
24       will be added eventually.   This  might  eventually  include  not  only
25       Hieroglyphs and various historic Indo-European languages, but even some
26       selected artistic scripts such as Tengwar,  Cirth,  and  Klingon.   UCS
27       also  covers  a large number of graphical, typographical, mathematical,
28       and scientific symbols, including those provided  by  TeX,  Postscript,
29       APL,  MS-DOS,  MS-Windows,  Macintosh,  OCR fonts, as well as many word
30       processing and publishing systems, and more are being added.
32       The UCS standard (ISO 10646) describes a 31-bit character set architec‐
33       ture  consisting  of  128  24-bit  groups, each divided into 256 16-bit
34       planes made up of 256 8-bit rows with 256  column  positions,  one  for
35       each character.  Part 1 of the standard (ISO 10646-1) defines the first
36       65534 code positions (0x0000 to 0xfffd), which form the Basic Multilin‐
37       gual  Plane  (BMP), that is plane 0 in group 0.  Part 2 of the standard
38       (ISO 10646-2) adds characters to group 0 outside  the  BMP  in  several
39       supplementary  planes  in  the range 0x10000 to 0x10ffff.  There are no
40       plans to add characters beyond 0x10ffff to the standard,  therefore  of
41       the  entire  code  space, only a small fraction of group 0 will ever be
42       actually used in the foreseeable future.  The BMP contains all  charac‐
43       ters found in the commonly used other character sets.  The supplemental
44       planes added by ISO 10646-2 cover only more exotic characters for  spe‐
45       cial scientific, dictionary printing, publishing industry, higher-level
46       protocol and enthusiast needs.
48       The representation of each UCS character as a 2-byte word  is  referred
49       to  as  the  UCS-2 form (only for BMP characters), whereas UCS-4 is the
50       representation of each character by a 4-byte word.  In addition,  there
51       exist  two  encoding  forms UTF-8 for backward compatibility with ASCII
52       processing software and UTF-16 for the backward-compatible handling  of
53       non-BMP characters up to 0x10ffff by UCS-2 software.
55       The UCS characters 0x0000 to 0x007f are identical to those of the clas‐
56       sic US-ASCII character set and the characters in the  range  0x0000  to
57       0x00ff are identical to those in ISO 8859-1 (Latin-1).
59   Combining characters
60       Some  code  points  in  UCS have been assigned to combining characters.
61       These are similar to the nonspacing accent keys  on  a  typewriter.   A
62       combining character just adds an accent to the previous character.  The
63       most important accented characters have codes of their own in UCS, how‐
64       ever,  the  combining  character mechanism allows us to add accents and
65       other diacritical marks to any  character.   The  combining  characters
66       always follow the character which they modify.  For example, the German
67       character Umlaut-A ("Latin capital letter A with diaeresis") can either
68       be  represented by the precomposed UCS code 0x00c4, or alternatively as
69       the combination of a normal "Latin capital  letter  A"  followed  by  a
70       "combining diaeresis": 0x0041 0x0308.
72       Combining  characters  are essential for instance for encoding the Thai
73       script or for mathematical typesetting and users of  the  International
74       Phonetic Alphabet.
76   Implementation levels
77       As  not  all  systems  are expected to support advanced mechanisms like
78       combining characters, ISO 10646-1 specifies the following three  imple‐
79       mentation levels of UCS:
81       Level 1  Combining  characters  and  Hangul Jamo (a variant encoding of
82                the Korean script, where a Hangul syllable glyph is coded as a
83                triplet or pair of vowel/consonant codes) are not supported.
85       Level 2  In  addition  to level 1, combining characters are now allowed
86                for some languages where they are essential (e.g., Thai,  Lao,
87                Hebrew, Arabic, Devanagari, Malayalam).
89       Level 3  All UCS characters are supported.
91       The  Unicode  3.0 Standard published by the Unicode Consortium contains
92       exactly the UCS Basic Multilingual Plane at implementation level 3,  as
93       described  in  ISO  10646-1:2000.   Unicode  3.1 added the supplemental
94       planes of ISO 10646-2.  The Unicode standard and technical reports pub‐
95       lished by the Unicode Consortium provide much additional information on
96       the semantics and recommended usages of various characters.  They  pro‐
97       vide guidelines and algorithms for editing, sorting, comparing, normal‐
98       izing, converting, and displaying Unicode strings.
100   Unicode under Linux
101       Under GNU/Linux, the C type wchar_t is a signed  32-bit  integer  type.
102       Its  values  are always interpreted by the C library as UCS code values
103       (in all locales), a convention that is signaled by the GNU C library to
104       applications  by  defining the constant __STDC_ISO_10646__ as specified
105       in the ISO C99 standard.
107       UCS/Unicode can be used just like ASCII in input/output streams, termi‐
108       nal  communication,  plaintext  files, filenames, and environment vari‐
109       ables in the ASCII compatible UTF-8 multibyte encoding.  To signal  the
110       use  of UTF-8 as the character encoding to all applications, a suitable
111       locale  has  to  be   selected   via   environment   variables   (e.g.,
112       "LANG=en_GB.UTF-8").
114       The  nl_langinfo(CODESET)  function  returns  the  name of the selected
115       encoding.  Library functions such as wctomb(3) and mbsrtowcs(3) can  be
116       used  to transform the internal wchar_t characters and strings into the
117       system character encoding and back and wcwidth(3) tells, how many posi‐
118       tions (0–2) the cursor is advanced by the output of a character.
120   Private Use Areas (PUA)
121       In  the Basic Multilingual Plane, the range 0xe000 to 0xf8ff will never
122       be assigned to any characters by the standard and is reserved for  pri‐
123       vate usage.  For the Linux community, this private area has been subdi‐
124       vided further into the range 0xe000 to 0xefff which can be  used  indi‐
125       vidually  by  any  end-user  and  the Linux zone in the range 0xf000 to
126       0xf8ff where extensions are coordinated among  all  Linux  users.   The
127       registry  of the characters assigned to the Linux zone is maintained by
128       LANANA and the registry itself is Documentation/admin-guide/unicode.rst
129       in  the Linux kernel sources (or Documentation/unicode.txt before Linux
130       4.10).
132       Two other planes are reserved for private usage, plane  15  (Supplemen‐
133       tary  Private  Use Area-A, range 0xf0000 to 0xffffd) and plane 16 (Sup‐
134       plementary Private Use Area-B, range 0x100000 to 0x10fffd).
136   Literature
137       *  Information technology — Universal  Multiple-Octet  Coded  Character
138          Set  (UCS)  —  Part  1:  Architecture  and Basic Multilingual Plane.
139          International Standard ISO/IEC 10646-1,  International  Organization
140          for Standardization, Geneva, 2000.
142          This  is  the  official  specification  of  UCS  .   Available  from
145       *  The Unicode Standard, Version 3.0.  The Unicode Consortium, Addison-
146          Wesley, Reading, MA, 2000, ISBN 0-201-61633-5.
148       *  S. Harbison, G. Steele. C: A Reference Manual. Fourth edition, Pren‐
149          tice Hall, Englewood Cliffs, 1995, ISBN 0-13-326224-3.
151          A good reference book about the C programming language.  The  fourth
152          edition  covers  the 1994 Amendment 1 to the ISO C90 standard, which
153          adds a large number of new C library functions for handling wide and
154          multibyte  character  encodings,  but it does not yet cover ISO C99,
155          which improved wide and multibyte character support even further.
157       *  Unicode Technical Reports.
160       *  Markus Kuhn: UTF-8 and Unicode FAQ for UNIX/Linux.
163       *  Bruno Haible: Unicode HOWTO.


167       locale(1), setlocale(3), charsets(7), utf-8(7)


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177GNU                               2017-09-15                        UNICODE(7)