Re: Protecting C code from continuations

[Neil Jerram]

Steve Tell wrote:

> On Sun, 4 Jul 2004, Neil Jerram wrote:
>
>
> > Hi all,
> >
> > I'm working with some C code that is not continuation-safe; [...]
> >
> > Any comments/ideas?
>
>
> Might (call-with-dynamic-root thunk) be what you want?

Thanks for reminding me - I had forgotten about call-with-dynamic-root 
(cwdr).  However, I'm not sure that it gives me what I need, because I 
believe that a continuation captured within one cwdr cannot then be 
invoked within a different cwdr.

To explain further: in my program, the main loop is in C; some events 
from the main loop are handled purely within C code, while others are 
dispatched into Scheme.  The way I want to use continuations is such 
that a Scheme coder can write

(begin
  (do-thing-1)
  (do-thing-2)
  (do-thing-3))

without having to worry about the fact that (do-thing-2) will usually 
have to escape and then let the C main loop run for a few iterations 
until the information needed to complete (do-thing-2) comes back.

So, the continuation to "finish (do-thing-2) and then continue" needs to 
be captured within one call from C into Scheme, but invoked within some 
later call from C into Scheme (not necessarily the next such call).

Obviously (I think!) I can implement this using CPS 
(continuation-passing style), and then define convenience macros to try 
to hide the CPS, but it would be much nicer to use real continuations if 
that is possible.

Regards,
     Neil