Macro explanation

Hi,

I'm checking the container_of and offsetof macro

#define container_of(ptr, type, member) ({ \
                const typeof( ((type *)0)->member ) *__mptr = (ptr);
                (type *)( (char *)__mptr - offsetof(type,member) );})

#define offsetof(TYPE, MEMBER) \
 ((size_t) &((TYPE *)0)->MEMBER)


I did not understand the first line of both macro. Will it not create
the NULL pointer dereference problem? I know it works and read some
article but it did not explain this part, so can anyone explain why
the NULL pointer error is not coming. Am I missing something?
Any C language specification of such example.