For non-variadic functions, the argument is implicitly converted to the parameter type (if possible). For variadic functions, the compiler doesn't know the par

For non-variadic functions, the argument is implicitly converted to the parameter type (if possible). For variadic functions, the compiler doesn't know the parameter type. So to print a null pointer, you need to do this: printf("NULL = %p\n", (void*)NULL); or printf("null pointer = %p\n", (void*)0); The %p format requires an argument of type void*, so you have to convert to that type if necessary.