Casting function pointers

Question

I am writing a function that receives a pointer to a comparison function and an array of MyStructs and is supposed to sort the array according to the comparison function:

void myStructSort(
                  struct MyStruct *arr,
                  int size,
                  int (*comp)(const struct MyStruct *, const struct MyStruct *)) {
  qsort(arr, size, sizeof(struct MyStruct), comp);
}

Unfortunately this doesn't compile because qsort expects the comparator to receive void * arguments and not const struct MyStruct *. I thought of several bad solutions and was wondering what the correct solution is.

Option 1

Cast comp to int (*)(const void *, const void*). This compiles but is undefined behavior (see this SO question).

Option 2

Create a global variable int (*global_comp)(const struct MyStruct *, const struct MyStruct *) and set global_comp=comp inside myStructSort. Then create a function:

int delegatingComp(const void *a, const void *b) {
  return globalComp((const struct MyStruct *)a, (const struct MyStruct *)b);
}

And in myStructSort call qsort(arr, size, sizeof(struct MyStruct), delegatingComp). The problem with this is the icky global variable.

Option 3

Reimplement qsort. This is functionally safe but very bad practice.

Is there a magical perfect fourth option?

Edit

I can't change the API of myStructSort and I am compiling my code using gcc c99 -Wall -Wextra -Wvla.

Answer 1

Option 2 breaks thread-safety, so I wouldn't choose that one.

Option 3 is just plain wrong as you point out. There is no reason to re-implement quicksort and potentially make a mistake.

Option 1 is UB but it will work on any sane compiler. If you choose this option be sure to add a comment.

I would also consider:

Option 4. Redesign the interface of myStructSort to take int (*)(const void *, const void*) or scrap it entirely and call qsort directly. Basically send it back to the architecht, because he made a poor design choice.