I'm trying to use the standard library's qsort to sort an array of wide characters:
wchar_t a = L'a';
wchar_t a1 = L'ä';
wchar_t b = L'z';
wchar_t chararray[] = {b, a, a1};
length = wcslen(chararray);
qsort(chararray, length, sizeof(wchar_t), wcscoll);
Now I think the functions involved have these prototypes:
int wcscoll(const wchar_t *ws1, const wchar_t *ws2);
void qsort(void *base, size_t num, size_t size, int (*comp_func)(const void *, const void *))
The results are completely as expected, but why am I getting the compiler warning "passing argument 4 of ‘qsort’ from incompatible pointer type"? And how can I cast wcscoll to fit the prototype?
The warning goes away if I define and pass in a separate comparison function:
int widecharcomp(const void *arg1, const void *arg2)
{
return wcscoll(arg1, arg2);
}
... but this one looks like it should have error handling for when the arguments are not of type wchar_t *.
There are two problems: you've mixed up wchar_t and wchar_t*, and you've tried to pass off a wchar_t* as a void*.
First, you've told qsort to sort an array of wchar_t. But wcscoll doesn't compare wchar_t, it compares wide character strings which have the type wchar_t*. The fact that your comparison appears to have worked is due to your test data which just happens to work well under both interpretations.
If you wanted to sort characters, you need to call an appropriate function (I don't know the wide character API well enough to tell you which one). If you wanted to sort strings, you need to allocate an array of strings (of type wchar_t *).
Furthermore, even if you had an array of wchar_t*, you could not portably pass wcscoll as an argument to qsort. The issue is that there is no guarantee that wchar_t* and void* have the same representation. Some machines have word pointers that have a different representation from byte pointers; on such a machine, qsort would pass byte pointers to elements of the array to wcscoll, and this wouldn't work because wcscoll expects byte pointers. The solution is to write a trivial wrapper function that performs the conversion if necessary. A trivial wrapper is often necessary with qsort.