> it's not in the Scheme standard though (as far as I know), which makes it difficult to rely on, if you are programming to the spec.

If you want to have code work everywhere and thus code defensively, yes. Of course you're not coding defensively when coding for Racket, either. But I guess I may have misunderstood you--Racket simply gives you a guarantee; as do some (other) Scheme implementations, so you get the same benefit with some/many other Scheme implementations as well--but even those "differ from Scheme (the standard)", by giving such a guarantee outside of the standard. It's not "Racket vs. Scheme implementations", it's "the (Racket|Scheme) implementations vs. Scheme (the standard)". Apologies then for my inappropriate rant about false claims.

> It is difficult to use the machine stack and have a fast call/cc. But I think Chez Scheme proves it's possible.

Chez doesn't seem to be using the machine stack either. I've tested with this code and "ulimit -s 1000" on Linux and it runs fine:

    (define (conc a b)
      (if (null? a)
          b
          (cons (car a)
                (conc (cdr a) b))))

    (define (iot n)
      (if (negative? n)
          '()
          (cons n (iot (- n 1)))))

    (display (length (conc (iot 10000000) (iot 10000000))))
    (newline)

Note that it can still be using a stack built from "machine" or C style arrays as building blocks--that's what I meant with "Some Scheme implementations allocate stack frames more efficiently than cons cells". Gambit falls into this category as well. But you can't have a single uninterrupted C style array as the stack (with no copying during GC like Chicken does) and implement call/cc efficiently. So I'd still say that implementing Scheme including call/cc efficiently means moving away from the stack controlled by ulimit -s, and once that is done it's easy/natural for an implementation to also offer unbounded stacks like Racket does.

I think we agree: an implementation that uses a stack to make normal calls fast, needs to move/copy the stack to the heap, when the stack runs out (or when a continuation is captured).

I don't know whether Chez still uses the same strategy, but a major part of Dybvig's thesis (very readable) was dedicated to explain the details of a possible implementation strategy.

http://agl.cs.unm.edu/~williams/cs491/three-imp.pdf