|UNICODE(7)||Linux Programmer's Manual||UNICODE(7)|
NAMEUnicode - universal character set
DESCRIPTIONThe 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.
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 8859-1 Latin-1.
Combining charactersSome 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 levelsAs 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 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 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.
Unicode under LinuxUnder 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.
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 combining 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 areaIn 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.Anvin@linux.org>.
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.
- 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.
- Unicode Technical Reports.
- Markus Kuhn: UTF-8 and Unicode FAQ for UNIX/Linux.
- Bruno Haible: Unicode HOWTO.
BUGSWhen 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 support for scripts with right-to-left writing direction or ligature substitution requirements such as Hebrew, Arabic, or the Indic scripts. These scripts are currently supported only in certain GUI applications (HTML viewers, word processors) with sophisticated text rendering engines.
SEE ALSOsetlocale(3), charsets(7), utf-8(7)
COLOPHONThis page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.