[plt-scheme] Preventing GC on Scheme_Object

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
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