如何计算结构的偏移量

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

为什么不使用stddef.h中定义的标准offsetof（）宏？

Answer 2

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.

Answer 3

int should never be used to save a memory address, use uintptr_t . Or just use offsetof instead of your own macro.

0 will never be defined as something other than 0, substracting (char*)0 is just wrong. What book was it that suggested this? Of course the addresses you deal with are just virtual addresses but your program will never see the real addresses.

The statement might make sense on some very obscure system where NULL is not 0, and you replace 0 with NULL in the macro. Of course, if this system exists it is easy to understand why it's not known and probably a big fail.