Why can this C code run correctly? [duplicate]

Question

The C code likes this:

 #include <stdio.h>  #include <unistd.h>   #define DIM(a) (sizeof(a)/sizeof(a[0]))   struct obj  {      int a[1];  };  int main()  {      struct obj *p = NULL;      printf("%d\n",DIM(p->a));      return 0;  } 

This object pointer p is NULL, so, i think this p->a is illegal. But i have tested this code in Ubuntu14.04, it can execute correctly. So, I want to know why...

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Note: the original code had int a[0] above but I've changed that to int a[1] since everyone seems to be hung up on that rather than the actual question, which is:

Is the expression sizeof(p->a) valid when p is equal to NULL?

Answer 1

Because sizeof is a compile time construction, it does not depend on evaluating the input. sizeof(p->a) gets evaluated based on the declared type of the member p::a solely, and becomes a constant in the executable. So the fact that p points to null makes no difference.

The runtime value of p plays absolutely no role in the expression sizeof(p->a).

In C and C++, sizeof is an operator and not a function. It can be applied to either a type-id or an expression. Except in the case that of an expression and the expression is a variable-length array (new in C99) (as pointed out by paxdiablo), the expression is an unevaluated operand and the result is the same as if you had taken sizeof against the type of that expression instead. (C.f. C11 references due to paxdiablo below, C++14 working draft 5.3.3.1)