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



NAME
       Unicode - the Universal Character Set

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, i.e., 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  Syl-
       labics,  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  charac-
       ter.   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 charac-
       ters 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  char-
       acters),  whereas UCS-4 is the representation of each character by a 4-byte word.  In addition, there exist two
       encoding forms UTF-8 for backwards compatibility with ASCII processing software and UTF-16  for  the  backwards
       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 char-
       acters 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  non-spacing
       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  diaere-
       sis")  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 vovel/consonant codes) are not supported.

       Level 2  In  addition to level 1, combining characters are now allowed for some languages where they are essen-
                tial (e.g., Thai, Lao, Hebrew, Arabic, Devanagari, Malayalam, etc.).

       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  addi-
       tional  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 applica-
       tions 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, file-
       names, 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.

       Under Linux, in general only the BMP at implementation level 1 should be used at the moment.  Up to two combin-
       ing  characters  per  base  character for certain scripts (in particular Thai) are also supported by some UTF-8
       terminal emulators and ISO 10646 fonts (level 2), but in general precomposed  characters  should  be  preferred
       where available (Unicode calls this Normalization Form C).

   Private Area
       In the BMP, 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 currently maintained by H. Peter Anvin <Peter.AnvinATlinux.org>.

   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 as a PDF file on CD-ROM 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  multi-byte
         character  encodings, but it does not yet cover ISO C99, which improved wide and multi-byte character support
         even further.

       * Unicode Technical Reports.
         http://www.unicode.org/unicode/reports/

       * Markus Kuhn: UTF-8 and Unicode FAQ for Unix/Linux.
         http://www.cl.cam.ac.uk/~mgk25/unicode.html

         Provides subscription information for the linux-utf8 mailing list, which is the best place to look for advice
         on using Unicode under Linux.

       * Bruno Haible: Unicode HOWTO.
         ftp://ftp.ilog.fr/pub/Users/haible/utf8/Unicode-HOWTO.html

BUGS
       When this man page was last revised, the GNU C Library support for UTF-8 locales was mature and XFree86 support
       was in an advanced state, but work on making applications (most notably editors)  suitable  for  use  in  UTF-8
       locales  was still fully in progress.  Current general UCS support under Linux usually provides for CJK double-
       width characters and sometimes even simple overstriking combining characters, but usually does not include sup-
       port  for  scripts  with  right-to-left writing direction or ligature substitution requirements such as Hebrew,
       Arabic, or the Indic scripts.  These scripts are currently only supported in  certain  GUI  applications  (HTML
       viewers, word processors) with sophisticated text rendering engines.

SEE ALSO
       setlocale(3), charsets(7), utf-8(7)

COLOPHON
       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



GNU                               2001-05-11                        UNICODE(7)