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(libc.info)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 for GNU systems `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 define
`wchar_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 `char' strings.
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, thereforet 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 10646 standards
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 10646 characters. 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.
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