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SBCL provides additional garbage collection functionality not specified by ANSI. Weak pointers allow references to objects to be maintained without keeping them from being garbage collected, and “finalization” hooks are available to cause code to be executed when an object has been garbage collected. Additionally users can specify their own cleanup actions to be executed with garbage collection.
Arrange for the designated
functionto be called when there are no more references toobject, including references infunctionitself.If
dont-saveis true, the finalizer will be cancelled whensave-lisp-and-dieis called: this is useful for finalizers deallocating system memory, which might otherwise be called with addresses from the old image.In a multithreaded environment
functionmay be called in any thread. In both single and multithreaded environmentsfunctionmay be called in any dynamic scope: consequences are unspecified iffunctionis not fully re-entrant.Errors from
functionare handled and cause awarningto be signalled in whichever thread thefunctionwas called in.Examples:
;;; good (assumes RELEASE-HANDLE is re-entrant) (let* ((handle (get-handle)) (object (make-object handle))) (finalize object (lambda () (release-handle handle))) object);;; bad, finalizer refers to object being finalized, causing ;;; it to be retained indefinitely (let* ((handle (get-handle)) (object (make-object handle))) (finalize object (lambda () (release-handle (object-handle object)))));;; bad, not re-entrant (defvar *rec* nil)(defun oops () (when *rec* (error "recursive OOPS")) (let ((*rec* t)) (gc))) ; or just cons enough to cause one(progn (finalize "oops" #'oops) (oops)) ; causes GC and re-entry to #'oops due to the finalizer ; -> ERROR, caught, WARNING signalled