TFA assumes pre-C89 C, I think:

> The caller could see the declaration int f();, the actual call could have n>0 arguments, and the actual function could have m≤n parameters.

Certainly if `f()` were `int f(void);` then that wouldn't be the case. But even for `int f();` C17 6.5.2.2p6 says that "If the number of arguments does not equal the number of parameters, the behavior is undefined." Near as I can tell that was made UB in C89. So TFA is a) right about K&R C, b) just wrong for pretty much all post-K&R C. C23 makes `int f();` be the same as `int f(void);`.

That calling a non-variadic function with more / fewer arguments than expected by its definition is UB is enough to make TCO possible for that function's body.

The point about K&R C is well taken though: to turn a tail call into a jump, the caller needs to know how much to pop off the stack.

For variadic if you `va_start()`, `va_arg()` as needed, then `va_end()` with no `va_copy()` left alive then you can still tail-call out correctly, otherwise you can't.

For non-variadic functions post K&R C TCO should always be possible and not UB, provided you're not triggering UB to begin with by using the incorrect number of arguments.