Does &((struct name *)NULL -> b) cause undefined behaviour in C11?

Question

Code sample:

struct name
{
    int a, b;
};

int main()
{
    &(((struct name *)NULL)->b);
}

Does this cause undefined behaviour? W

Answer 1

No. Let's take this apart:

&(((struct name *)NULL)->b);

is the same as:

struct name * ptr = NULL;
&(ptr->b);

The first line is obviously valid and well defined.

In the second line, we calculate the address of a field relative to the address 0x0 which is perfectly legal as well. The Amiga, for example, had the pointer to the kernel in the address 0x4. So you could use a method like this to call kernel functions.

In fact, the same approach is used on the C macro offsetof (wikipedia):

#define offsetof(st, m) ((size_t)(&((st *)0)->m))

So the confusion here revolves around the fact that NULL pointers are scary. But from a compiler and standard point of view, the expression is legal in C (C++ is a different beast since you can overload the & operator).