[gambit-list] Re: more than one thread, no return to C (was: SQLite 2.4.7)

Marc Feeley feeley at IRO.UMontreal.CA
Wed Jan 26 21:03:33 EST 2005

> Given the general case, that a Scheme procedure calls a C function
> which in turn calls another Scheme procedure: does it matter whether
> the inner (latter) Scheme procedure is a callback procedure (i.e.
> given by the caller of the outer Scheme procedure) or not? I don't
> think so. Is this then the general rule for avoiding the problem:
> don't nest Scheme procedure calls indirectly via C?

Whether it is a callback or not is immaterial.  The important thing is
that you have a call from Scheme to C in which is nested a call from C
to Scheme.  In such a situation the "continuation" contains C frames.
There is a risk that these C frames will be popped by an (unrelated)
call to a continuation that contains another C frame (that was pushed
on the C stack earlier).  This might happen in a multithreaded program
if more than one thread does these Scheme -> C -> Scheme calls and the
scheduler interleaves the execution of the threads (because of
preemption, I/O blocking the threads, etc).  When there are only
Scheme to C calls the issue does not arise.


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