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A finalizer object helps Lisp code clean up after objects that are no longer needed. A finalizer holds a Lisp function object. When a finalizer object becomes unreachable after a garbage collection pass, Emacs calls the finalizer's associated function object. When deciding whether a finalizer is reachable, Emacs does not count references from finalizer objects themselves, allowing you to use finalizers without having to worry about accidentally capturing references to finalized objects themselves.
Errors in finalizers are printed to *Messages*. Emacs runs
a given finalizer object's associated function exactly once, even
if that function fails.
Make a finalizer that will run function. function will be called after garbage collection when the returned finalizer object becomes unreachable. If the finalizer object is reachable only through references from finalizer objects, it does not count as reachable for the purpose of deciding whether to run function. function will be run once per finalizer object.