Question

I got a struct and an initializing function like following:

struct someStruct {
    int* b, r;  // r is a pointer to an integer in b
};

someStruct* init_struct(size_t size) {
    someStruct* p = (someStruct*) malloc(sizeof(someStruct));
    p->b = (int*) malloc(size);
    p->b[0] = 16;
    p->r = &p->b;  // Here's the error, why?
    return p;
}

Answer 1

In your code the r field is an integer, not a pointer to an integer ! Also, &(p->b) is a pointer to a pointer to an integer (int**). You need to define your struct like this if you want r to be pointing a integer in the b array :

struct someStruct {
    int *b, *r;
}

And assign the r pointer by dereferencing an element of b (int), not the pointer itself (int*):

p->r = &(p->b[0]);

Or don’t dereference b at all if you want to point to b[0] :

p->r = p->b;