Null pointers, one month later

"In fact the C language doesn't know the identifier "NULL", it just knows that its value is zero because the preproceessor defines that."

Wrong:
The fact that the C compiler, preprocessor excluded, does not know about the symbol NULL is irrelevant. NULL is defined as the null pointer constant, and the C language, even preprocessor excluded, does know about the null pointer constant. And even if the null pointer constant can be literally written (in a strictly conforming program) as (void*)0, that does not mean that the representation of the null pointer constant must be zero.