void
swap(void *a, void *b)
{
     void *tmp;
     tmp = a;
     a = b;
     b = tmp;
}

struct heap_s
{
     heap left, right;
     void* data;
};
 
struct event_s {
     etime       time;
     int     e_num;
     void        (*event_fn)(event,  pqueue);
     void        *details;
     };

void swap(void** a, void** b)
{
    void* temp = *a;
     *a = *b;
      *b = temp;
}

int main()
{
    int a = 1;
    int b = 2;
    swap(&a, &b); // <<<<<<< wrong

    int* c = &a;
    int* d = &b;
    swap(&c, &d); // <<< OK
// when the swap returns, pointer c will point to b and pointer d will point to a.
// The value of a and b are still the same, only the two pointers
// were swapped