Accessing structure elements using pointers

Question

I got surprised when the following program did not crash.

typedef struct _x {
  int a;
  char b;
  int c;
} x;

main() {
  x *ptr = 0;
  char *d = &ptr->b;
}

As per my understanding the -> operator has higher precedence over & operator. So I expected the program to crash at the below statement when we try to dereference the NULL pointer tr.

char *d = &ptr->b; 

But the statement &ptr->b evaluates to a valid address. Could somebody please explain where I'm wrong?

Answer 1

Your expectations were unfounded. C programs don't necessarily "crash" when you dereference null pointers. C programs exhibit so called undefined behavior when you attempt to do something like that. Undefined behavior can manifest itself in many different ways. It can result in a crash. Or it can produce something that even resembles a "working" program. The latter is what apparently happened in your case.

But in any case, your program's behavior is undefined. And no, it does not produce a "valid address" as you seem to mistakingly believe. A numerical address that corresponds to a location in memory where no object exists is not valid (with the exception of null pointer value, of course).

Answer 2

The reason that your code doesn't crash is that you didn't actually dereference the pointer. Notice that the expression

&ptr->b

doesn't actually try loading the contents of ptr or ptr->b. Instead, it just stores the address of where this is located in memory. What you'll end up getting is a pointer to where the b field of the object pointed at by ptr should be. This will be a few bytes past address 0, so dereferencing the pointer you just created will cause a segfault.