Question

I found a method of computing the offset of struct. as follow:

#define offsetOf(type , f)  ((int)\
 ((char *)&((struct tagName*)0)->f-(char*)(struct tagName*)0)); 

but I often write it like this:

#define offsetOf(type , f)  ((int)(&((struct tagName*)0)->f); 

The book said the subtraction of NULL pointer is to make sure it’s correct when the inner of NULL pointer is non-zero. but i think that the inner of the NULL pointer is not effect the address of the NULL Pointer.

Answer 1

The standard library should supply an offsetof that does exactly what you want, I would strongly recommend that you use it, as many compilers would issue a warning for the kind of code that would be required for a custom variant.

Anyway, the subtract will take your code from the pointer domain to the domain of size_t. I would say that it’s a little bit safer.