[Z]Introduction to Extended Characters

http://www.gnu.org/software/libc/manual/html_node/Extended-Char-Intro.html#Extended-Char-Intro

Introduction to Extended Characters

A variety of solutions is available to overcome the differences between character sets with a 1:1 relation between bytes and characters and character sets with ratios of 2:1 or 4:1. The remainder of this section gives a few examples to help understand the design decisions made while developing the functionality of the C library.

A distinction we have to make right away is between internal and external representation. Internal representation means the representation used by a program while keeping the text in memory. External representations are used when text is stored or transmitted through some communication channel. Examples of external representations include files waiting in a directory to be read and parsed.

Traditionally there has been no difference between the two representations. It was equally comfortable and useful to use the same single-byte representation internally and externally. This comfort level decreases with more and larger character sets.

One of the problems to overcome with the internal representation is handling text that is externally encoded using different character sets. Assume a program that reads two texts and compares them using some metric. The comparison can be usefully done only if the texts are internally kept in a common format.

For such a common format (= character set) eight bits are certainly no longer enough. So the smallest entity will have to grow: wide characters will now be used. Instead of one byte per character, two or four will be used instead. (Three are not good to address in memory and more than four bytes seem not to be necessary).

As shown in some other part of this manual, a completely new family has been created of functions that can handle wide character texts in memory. The most commonly used character sets for such internal wide character representations are Unicode and ISO 10646 (also known as UCS for Universal Character Set). Unicode was originally planned as a 16-bit character set; whereas, ISO 10646 was designed to be a 31-bit large code space. The two standards are practically identical. They have the same character repertoire and code table, but Unicode specifies added semantics. At the moment, only characters in the first 0x10000 code positions (the so-called Basic Multilingual Plane, BMP) have been assigned, but the assignment of more specialized characters outside this 16-bit space is already in progress. A number of encodings have been defined for Unicode and ISO 10646 characters: UCS-2 is a 16-bit word that can only represent characters from the BMP, UCS-4 is a 32-bit word than can represent any Unicode and ISO 10646 character, UTF-8 is an ASCII compatible encoding where ASCII characters are represented by ASCII bytes and non-ASCII characters by sequences of 2-6 non-ASCII bytes, and finally UTF-16 is an extension of UCS-2 in which pairs of certain UCS-2 words can be used to encode non-BMP characters up to 0x10ffff.

To represent wide characters the char type is not suitable. For this reason the ISO C standard introduces a new type that is designed to keep one character of a wide character string. To maintain the similarity there is also a type corresponding to int for those functions that take a single wide character.

— Data type:  wchar_t

This data type is used as the base type for wide character strings. In other words, arrays of objects of this type are the equivalent of char[] for multibyte character strings. The type is defined in stddef.h.

The ISO C90 standard, where wchar_t was introduced, does not say anything specific about the representation. It only requires that this type is capable of storing all elements of the basic character set. Therefore it would be legitimate to define wchar_t as char, which might make sense for embedded systems.

But in the GNU C Library wchar_t is always 32 bits wide and, therefore, capable of representing all UCS-4 values and, therefore, covering all of ISO 10646. Some Unix systems definewchar_t as a 16-bit type and thereby follow Unicode very strictly. This definition is perfectly fine with the standard, but it also means that to represent all characters from Unicode and ISO 10646 one has to use UTF-16 surrogate characters, which is in fact a multi-wide-character encoding. But resorting to multi-wide-character encoding contradicts the purpose of the wchar_t type.

— Data type:  wint_t

wint_t is a data type used for parameters and variables that contain a single wide character. As the name suggests this type is the equivalent of int when using the normal charstrings. The types wchar_t and wint_t often have the same representation if their size is 32 bits wide but if wchar_t is defined as char the type wint_t must be defined as int due to the parameter promotion.

This type is defined in wchar.h and was introduced in Amendment 1 to ISO C90.

As there are for the char data type macros are available for specifying the minimum and maximum value representable in an object of type wchar_t.

— Macro: wint_t  WCHAR_MIN

The macro WCHAR_MIN evaluates to the minimum value representable by an object of type wint_t.

This macro was introduced in Amendment 1 to ISO C90.

— Macro: wint_t  WCHAR_MAX

The macro WCHAR_MAX evaluates to the maximum value representable by an object of type wint_t.

This macro was introduced in Amendment 1 to ISO C90.

Another special wide character value is the equivalent to EOF.

— Macro: wint_t  WEOF

The macro WEOF evaluates to a constant expression of type wint_t whose value is different from any member of the extended character set.

WEOF need not be the same value as EOF and unlike EOF it also need not be negative. In other words, sloppy code like

          {
            int c;
            ...
            while ((c = getc (fp)) < 0)
              ...
          }

has to be rewritten to use WEOF explicitly when wide characters are used:

          {
            wint_t c;
            ...
            while ((c = wgetc (fp)) != WEOF)
              ...
          }

This macro was introduced in Amendment 1 to ISO C90 and is defined in wchar.h.

These internal representations present problems when it comes to storing and transmittal. Because each single wide character consists of more than one byte, they are effected by byte-ordering. Thus, machines with different endianesses would see different values when accessing the same data. This byte ordering concern also applies for communication protocols that are all byte-based and therefore require that the sender has to decide about splitting the wide character in bytes. A last (but not least important) point is that wide characters often require more storage space than a customized byte-oriented character set.

For all the above reasons, an external encoding that is different from the internal encoding is often used if the latter is UCS-2 or UCS-4. The external encoding is byte-based and can be chosen appropriately for the environment and for the texts to be handled. A variety of different character sets can be used for this external encoding (information that will not be exhaustively presented here–instead, a description of the major groups will suffice). All of the ASCII-based character sets fulfill one requirement: they are "filesystem safe." This means that the character '/'is used in the encoding only to represent itself. Things are a bit different for character sets like EBCDIC (Extended Binary Coded Decimal Interchange Code, a character set family used by IBM), but if the operation system does not understand EBCDIC directly the parameters-to-system calls have to be converted first anyhow.

• The simplest character sets are single-byte character sets. There can be only up to 256 characters (for 8 bit character sets), which is not sufficient to cover all languages but might be sufficient to handle a specific text. Handling of a 8 bit character sets is simple. This is not true for other kinds presented later, and therefore, the application one uses might require the use of 8 bit character sets.

• The ISO 2022 standard defines a mechanism for extended character sets where one character can be represented by more than one byte. This is achieved by associating a state with the text. Characters that can be used to change the state can be embedded in the text. Each byte in the text might have a different interpretation in each state. The state might even influence whether a given byte stands for a character on its own or whether it has to be combined with some more bytes.

  In most uses of ISO 2022 the defined character sets do not allow state changes that cover more than the next character. This has the big advantage that whenever one can identify the beginning of the byte sequence of a character one can interpret a text correctly. Examples of character sets using this policy are the various EUC character sets (used by Sun's operations systems, EUC-JP, EUC-KR, EUC-TW, and EUC-CN) or Shift_JIS (SJIS, a Japanese encoding).

  But there are also character sets using a state that is valid for more than one character and has to be changed by another byte sequence. Examples for this are ISO-2022-JP, ISO-2022-KR, and ISO-2022-CN.

• Early attempts to fix 8 bit character sets for other languages using the Roman alphabet lead to character sets like ISO 6937. Here bytes representing characters like the acute accent do not produce output themselves: one has to combine them with other characters to get the desired result. For example, the byte sequence 0xc2 0x61 (non-spacing acute accent, followed by lower-case `a') to get the “small a with acute” character. To get the acute accent character on its own, one has to write 0xc2 0x20 (the non-spacing acute followed by a space).

  Character sets like ISO 6937 are used in some embedded systems such as teletex.

• Instead of converting the Unicode or ISO 10646 text used internally, it is often also sufficient to simply use an encoding different than UCS-2/UCS-4. The Unicode and ISO 10646standards even specify such an encoding: UTF-8. This encoding is able to represent all of ISO 10646 31 bits in a byte string of length one to six.

  There were a few other attempts to encode ISO 10646 such as UTF-7, but UTF-8 is today the only encoding that should be used. In fact, with any luck UTF-8 will soon be the only external encoding that has to be supported. It proves to be universally usable and its only disadvantage is that it favors Roman languages by making the byte string representation of other scripts (Cyrillic, Greek, Asian scripts) longer than necessary if using a specific character set for these scripts. Methods like the Unicode compression scheme can alleviate these problems.

The question remaining is: how to select the character set or encoding to use. The answer: you cannot decide about it yourself, it is decided by the developers of the system or the majority of the users. Since the goal is interoperability one has to use whatever the other people one works with use. If there are no constraints, the selection is based on the requirements the expected circle of users will have. In other words, if a project is expected to be used in only, say, Russia it is fine to use KOI8-R or a similar character set. But if at the same time people from, say, Greece are participating one should use a character set that allows all people to collaborate.

The most widely useful solution seems to be: go with the most general character set, namely ISO 10646. Use UTF-8 as the external encoding and problems about users not being able to use their own language adequately are a thing of the past.

One final comment about the choice of the wide character representation is necessary at this point. We have said above that the natural choice is using Unicode or ISO 10646. This is not required, but at least encouraged, by the ISO C standard. The standard defines at least a macro __STDC_ISO_10646__ that is only defined on systems where the wchar_t type encodes ISO 10646characters. If this symbol is not defined one should avoid making assumptions about the wide character representation. If the programmer uses only the functions provided by the C library to handle wide character strings there should be no compatibility problems with other systems.