Calling a function when a something is garbage collected.

Hello,


My question is in object, when the garbage collector releases memory
from a variable for example does GCL allow to add a "hook" function to
do some other things. Namely I also need to release memory in another
place not known to the garbage collector. Here is a piece of code for
other CL implementations:

; TODO: add Clozure CL 'terminate'
(defun make_jlref (val type)
     (let* ((ret (make-instance 'jlref :val val :type type))
            (hash (write-to-string (sxhash ret) :base 16)))
        #+:lispworks (flag-special-free-action hash)
        #+:cmu (ext:finalize ret (lambda () (free_jlref hash)))
        #+:sbcl (sb-ext:finalize ret (lambda () (free_jlref hash)))
        #+:allegro (excl:schedule-finalization ret (lambda ()
(free_jlref hash)))
        (setf (jlrefId ret) hash)
        ret))

And another question, where can I find documentation for calling C
from GCL Common Lisp?

Regards,
- Greg