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 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 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:

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 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 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 multibyte 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/unicode.txt in the Linux kernel sources.

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