Casting function pointer arguments without a helper function

April 29, 2022

void qsort(void *base, size_t nitems, size_t size, int (*compar)(const void *, const void*))

Is there a way to pass, let’s say strcmp to qsort without making a helper function?

I was trying to do:

qsort(..., (int (*) (const void*, const void*) (strcmp)));

>Solution :

Your attempt at the cast simply has a misplaced right (closing) parenthesis. The one at the end should be after the type of the cast. So, you can change:

(int (*) (const void*, const void*) (strcmp))
//                                          ^ wrong

(int (*) (const void*, const void*)) (strcmp)
//                                 ^ right

Alternatively, although hiding pointer types in typedef aliases is severely frowned-upon, function pointer types are an exception to that guideline. So, it is easier/clearer to define the required type for the qsort comparator first:

typedef int (*QfnCast) (const void*, const void*);

Then, you can cast to that type:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef int (*QfnCast) (const void*, const void*);

int main(void)
{
    char list[5][8] = {
        "Fred",
        "Bob",
        "Anna",
        "Gareth",
        "Joe"
    };

    qsort(list, 5, 8, (QfnCast)(strcmp));
    for (int i = 0; i < 5; ++i) printf("%s\n", list[i]);
    return 0;
}

As others have mentioned, you don’t actually need that cast; but, without it, some compilers will emit a warning. MSVC doesn’t give a warning but clang-cl gives:

warning : incompatible function pointer types passing ‘int (const char
*, const char )’ to parameter of type ‘_CoreCrtNonSecureSearchSortCompareFunction’ (aka ‘int ()(const void
*, const void *)’)