Re: portable finalizer

Marco Gidde <marco.gidde@xxxxxxxxxx> writes:

Tamas Papp <tkpapp@xxxxxxxxx> writes:

I am wrapping a void * cffi pointer and some extra information in a
CLOS object. When the object is garbage-collected, I would like to
have the memory location freed. I have found out how to do
finalization in SBCL, but I want to make my code portable.

I have seen finalize implementations with #+ directives floating
around, could somebody post a reasonably comprehensive version? Or is
there a library to do this?

there is a small library out there called trivial-garbage with
finalization code for several CL implementations.

Thanks Marco,

I also found that there is a cffi:finalize that does the thing I want.

Since this thread has drifted towards resources, I have a conceptual
question. Suppose I have a resource (eg a Cairo context in
cl-cairo2), is a pointer to a C structure.

(defclass foo () ((pointer :initform nil)))

The user has to be able to close the resource explicitly (eg in
cl-cairo2, this indicates that he has finished drawing and the file
can be closed), so there is a destroy method:

(defmethod destroy ((object foo))
(with-slots (pointer) object
(when pointer
(call_c_api_to_destroy pointer)
(setf pointer nil))))

But suppose that I also want to register a finalizer, and have the
object destroyed when it is garbage collected. The documentation of
cffi:finalize says

For portability reasons, function should not attempt to look at
object by closing over it because, in some lisps, object will
already have been garbage-collected and is therefore not accessible
when function is invoked.

So how is the finalizer supposed to look at pointer to determine if
the resource has already been freed explicitly with destroy?

Thanks,

Tamas
.

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