I have an extention for mzscheme in C++. This is how my code looks like: struct Widget {     Scheme_Object* callback;         Widget() : callback(scheme_null) { } }; Scheme_Object* callback(int argc, Scheme_Object** argv) {     Widget* w = new Widget;     w->callback = argv[0];     return scheme_true; } argv[0] is a procedure assigned from scheme code. This is getting garbage collected when i call (collect-garbage) even though a reference is held in w->callback. I want to tell the GC about this reference. scheme_dont_gc_ptr(argv[0]) did not help. thanks,  -- Vijay