17 Language support library [support]

17.2 Common definitions [support.types]

17.2.3 Null pointers [support.types.nullptr]

The type nullptr_­t is a synonym for the type of a nullptr expression, and it has the characteristics described in [basic.fundamental] and [conv.ptr].
Although nullptr's address cannot be taken, the address of another nullptr_­t object that is an lvalue can be taken.
— end note
The macro NULL is an implementation-defined null pointer constant.182
See also: ISO C 7.19
Possible definitions include 0 and 0L, but not (void*)0.