I'd agree if GP hadn't also used C++-specific optional parameters. Given the minefield that is vendor specific C++ish extensions to C, it seems worth a mention, even if I'm unaware of a compiler vendor that allows default parameters in your C code.
It was just an illustration in C++ (the C version would just document it, obviously). There is no way to do this in a vendor specific way without some sort of name mangling that will hopelessly break the ABI...
void foo(void* p=0); // has exactly the same ABI as
void bar(void* p);
bar(); // error: missing parameter
foo(); // shorthand for foo(0);
// void(*)(void*) is a different type than void(*)(void*=0)
// but they can be implicitly converted the same as
// eg void* and char*
