double casting in c

Question

I have gone over this code and I have noticed this casting that looks weird

oldidt = (unsigned long long *)(*(unsigned long*)(oldidtr+1));  

To me it looks like the first cast is affecting the +1 so it will move 4 bytes (a pointer), and the second cast is for the result being an unsigned long long *. And the star in the outer shell on the inner cast is an "access this memory" star.

• Is that right?
• Why bother if the original definition was a pointer, and we're casting to another pointer, the +1 will still be a 4 byte jump.
• What would happen if I write instead oldidt = *(oldidt+1);? (Assuming the compiler doesn't complain, and gives out and exec)

The declaration of oldidt was:

static unsigned long long *oldidt;  

I'm calling the casts "first" and "second" according to invocation (left is the second).

Answer 1

It looks like this code is using several unsigned short ints to store pointers in two halves. This assumes that an unsigned short int is half as wide as a pointer. The code is extracting the stored pointer from two adjacent shorts, namely the second and third member of an array, and interpreting it as a pointer to an unsigned long long int.

So, the final cast is necessary to reinterpret an integer as a pointer, while the first (inner) cast serves to read a differently typed value from an existing variable, namely a long from a short (or rather, from two adjacent shorts).