> Because protothreads do not save the stack context across a blocking call, local variables are not preserved when the protothread blocks. This means that local variables should be used with utmost care - if in doubt, do not use local variables inside a protothread!
You could have threads with enough memory for the base levels local variables. It would mean you wouldn't need global variables to keep state, but it would still be stackless as you can't pause execution within a function call.
what does that even mean? Sure you could calculate what stacksize your exact programm would need at maximum, if you unroll everything, but you could not define a fixed "base level" of a stacksize for every single programm out there, could you?
The topmost stack frame. That's what "stackless" generally implies: it doesn't preserve the stack itself, that doesn't mean it preserves nothing.
That's what you see with most async/await systems (as opposed to more integrated "lightweight thread" systems like Erlang or Go), yield points blow the stack (though it may be relinked for debugging purposes), but they don't break local variables.
It is hardly a coroutine at all if no state, other than the instruction pointer, is preserved.
[1] See the lua coroutine papers where the nomenclature is introduced.
That said, I’d hope that someone using a C library like this wouldn’t just start using it without understanding the potentially non-trivial consequences/side-effects of that decision.