Macro with Parameters

Question

In the code which follows, I keep getting an error. How to modify the third line? Why's that keep happening? What's wrong?

#include <stdio.h> 
#include "stdlib.h"
#define ARRAY_IDX(type, array, i) ((type *)(array+i)) // you can only modify this line!

int main(int argc, const char * argv[]) {
    void *ptr = malloc(10*sizeof(int));
#ifdef ARRAY_IDX
    for (int i = 0; i < 10; i++) {
        ARRAY_IDX(int, ptr, i) = i * 2;
    }

    for (int i = 0; i < 10; i++) {
        printf("%d ", ARRAY_IDX(int, ptr, i));
    }
    free(ptr);
#else
    printf("Implement ARRAY_IDX first");
#endif
}

Answer 1

Looking at

ARRAY_IDX(int, ptr, i) = i * 2;

and

printf("%d ", ARRAY_IDX(int, ptr, i));

shows that the expression

ARRAY_IDX(int, whatever, whatever)

should expand into an expression of type int (and an lvalue, so that we can assign to it).

Starting off with a void * you first need to change (cast) it to a pointer that allows indexing, and since you want to index the elements of that array (not its individual bytes, which would be a violation of aliasing) you need to make it an int * first:

(int *)(ptr)

Now you have a pointer to an integer (array, hopefully). Increment it:

(int *)(ptr) + (idx)

Finally, you need an lvalue int expression. Dereference the pointer to get that:

(*((int *)(ptr) + (idx)))

Converting that to a preprocessor macro is something that should be doable, so I leave it up to you.

---

Note that whoever is giving you that code is - IMHO - not a teacher you should trust. This won't teach you much about correct C. It might teach you something about the preprocessor. But don't write such code. Just don't. Use correct types if possible. Check for failure of malloc.

Answer 2

There is nothing wrong with adding an int to a void pointer. For many years, compiler designers assumed that this was standard behavior, and it was implemented as such. It's every bit as standard as anonymous structs and unions, which compilers have had for almost 20 years and were only recently added in C11. Practically all compilers will compile this just fine without any warnings or errors, and without having to use any special compiler flags.

Your problem is, as I have pointed out, that you are assigning a value to a pointer. You need to dereference it after the cast.

#define ARRAY_IDX(type, array, i) ((type *)array)[i]