The conversion functions mentioned so far in this chapter all had in
common that they operate on character sets that are not directly
specified by the functions. The multibyte encoding used is specified by
the currently selected locale for the
LC_CTYPE category. The
wide character set is fixed by the implementation (in the case of the GNU C Library
it is always UCS-4 encoded ISO 10646.
This has of course several problems when it comes to general character conversion:
LC_CTYPEcategory, one has to change the
LC_CTYPE locale introduces major problems for the rest
of the programs since several more functions (e.g., the character
classification functions, see Classification of Characters) use the
LC_CTYPEselection is global and shared by all threads.
wchar_trepresentation, there is at least a two-step process necessary to convert a text using the functions above. One would have to select the source character set as the multibyte encoding, convert the text into a
wchar_ttext, select the destination character set as the multibyte encoding, and convert the wide character text to the multibyte (= destination) character set.
Even if this is possible (which is not guaranteed) it is a very tiring work. Plus it suffers from the other two raised points even more due to the steady changing of the locale.
The XPG2 standard defines a completely new set of functions, which has none of these limitations. They are not at all coupled to the selected locales, and they have no constraints on the character sets selected for source and destination. Only the set of available conversions limits them. The standard does not specify that any conversion at all must be available. Such availability is a measure of the quality of the implementation.
In the following text first the interface to
iconv and then the
conversion function, will be described. Comparisons with other
implementations will show what obstacles stand in the way of portable
applications. Finally, the implementation is described in so far as might
interest the advanced user who wants to extend conversion capabilities.