Except when calling a varargs function, when you need to cast to the correct pointer type

But 0 isn't always null right?

Literal zero in a pointer context is interpreted by the compiler to mean "the null pointer." The compiler then compiles the null pointer into the code.

But then how do you get a pointer to zero (not null) in those systems?

First, that's inherently non-portable. That aside...

On most systems, the null pointer happens to be a pointer to address zero. For other systems, e.g. 8086 16-bit mode, you can create a intptr_t (which is the same size as a pointer) and set it to zero. Then cast it to a pointer. Such casting is always done bitwise, so it will work.

The compiler knows to turn the following into the null pointer

  void *p = (void *) 0;

but the compiler will NOT turn the following into a null pointer

  void *p = (void *) (uintptr_t) 0;

because the literal 0 is now in an integer context, not a pointer context.

BTW 0 is a valid literal value of a pointer type (so you do not need a cast).

Thank you!