Question

I’m qsort()ing some memory pointers in a list to allow a bsearch of them later in a function. My question is, do I need to type cast those values to something other than const void * to do a legal comparison in C? I could just do the conversion and let the compiler tell me, but I have a feeling that this might be compiler dependent.

Answer 1

Comparisons are fine, arithmetic and dereferencing are not (because you don’t know the size of the underlying data). So yes, you can compare void pointers quite well ^(a).

However, keep in mind that you don’t usually compare the pointers passed to a qsort comparison function, unless you want to sort on their addresses. But, since they’ll be sorted in that order already (being an array), there’s not much of a use case for that 🙂

You generally cast the void pointers to a specific pointer and then compare what they point to. Something like:

int compfn (const void *p1, const void *p2) {
    const char *str1 = *((const char **)(p1));
    const char *str2 = *((const char **)(p2));
    return strcmp (str1, str2);
}

You don’t have to create temporaries like str1 and str2 (even though any decent compiler would optimise them out anyway). Other than a slight readability issue, there’s nothing wrong with:

int compfn (const void *p1, const void *p2) {
    return strcmp (*((const char **)(p1)), *((const char **)(p2)));
}

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^(a) Subject to the normal rules that the pointers must both point to elements of the same array or one beyond that array – anything else is undefined. I mention that for completeness but, if you’re using qsort, you’ll be working on an array anyway.