Determining offset of a struct member via casting NULL to a struct pointer-CodePudding

Consider the following code:

#include <inttypes.h>

struct test {
    int a;
    long b;
};

int main(void){
    intptr_t ptr = (intptr_t) &(((struct test *)NULL)->b);
    printf("%"PRIiPTR"\n", ptr); //prints 8
    //...
}

I've been using this sort of construction for pretty long time to determine offset of struct member, but now questioned if this construction a well defined in terms of the Standard standpoint.

The thing that's not clear to me is why is it legitimate to perform indirection on a NULL pointer with later taking address?

CodePudding user response：

This is not valid code, as it dereferences a NULL pointer and attempts to obtain an lvalue for a non-existent object.

There is a language construct that gives the offset of a struct member, namely the offsetof macro.

Although it is possible that such a macro could expand to an expression similar to what you have, that macro is considered part of the implementation, and such code can do things that "user" code is not allowed to do