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Richard Frith-Macdonald <richard@brainstorm.co.uk> wrote: > ... > 1. Guile passes a SCM representing a procedure to some C library code > which stores it somewhere. > 2. Control returns from the C code to Guile and other things happen. > 3. From the C library, I want to call the Guile procedure whose SCM I > recorded earlier. > How can I be sure that the procedure has not been garbage collected > between stage 1 and stage 2? Any kind of Guile structure which is visible from the top level (except weak vectors) will protect the procedure from being garbage collected. This includes the C stack, which is probably where the object is stored between 1 and 2 above. You may think, how can the C-stack be used like that?, but that's a "trick" performed by the gc, which makes it "conservative". When it's scanning the C-stack it have to guess what things may be scheme objects. The principle is, better to mark than not, which means that there is a possibility for garbage to be retained, but not the opposite. Best regards Roland Orre