[Cffi-devel] Fw: What is the proper way to do GC in CFFI

[Joeish W]

Stephan,  I was needing the output of the code to be a pointer not a struct..should I just use with-foreign-object, or is there a way to output as a pointer in the code you provided.  Also I'm getting a " The variable X is defined but never used."..this will be my template for future binding like this so..I'd really like to get this right according to your vision, unless I find it's necessary I,d like to stay away from with macros. 

On Friday, April 11, 2014 8:52 AM, Joeish W <joeish80829 at yahoo.com> wrote:
 
Thanks for getting back to me Stephan, I  like your idea, its what I was advised to do, I think, but do you agree with Martins comment that it could be error prone?
>
>On Friday, April 11, 2014 3:53 AM, Martin Simmons <martin at lispworks.com> wrote:
> 
>I think it is better to avoid garbage collection of foreign objects because it
>>is too easy to introduce use-after-free errors if you pass the foreign object
>>to foreign code.  Also, finalizers interact poorly with generational GC.
>>
>>The simplest way to solve the original problem is to implement a macro that
>>allocates and frees the object, e.g. with-temporary-mat
>>
>>(defmacro with-temporary-mat (mat &body body)
>>  (let ((m (gensym "mat")))
>>    `(let ((,m (mat)))
>>       (unwind-protect
>>           (let ((,mat ,m))
>>             , at body)
>>         (del-mat ,m)))))
>>
>>__Martin
>>
>>
>>
>>>>>>> On Fri, 11 Apr 2014 08:59:20 +0200, Stephan Frank said:
>>> 
>>> Hi Joeish,
>>> 
>>> have a look at trivial-garbage (
>>> http://common-lisp.net/project/trivial-garbage/). This project wraps the
>>> finalizers of different Lisp implementations with a common interface.
>>> 
>>> So in your case it is best to wrap the pointer in a Lisp struct and attach
>>> that the finalizer like this (not tested, so syntax errors may apply). You
>>> can remove the type annotation of the struct slot if you don't want any
>>> SBCL-specific code.
>>> 
>>> (defstruct (cvmatrix (:constructor %make-cvmatrix))
>>>   (sap (mat) :type sb-sys:system-area-pointer :read-only t))
>>> 
>>> (defun make-cvmatrix ()
>>>   (let* ((matrix (%make-cvmatrix))
>>>          (sap (cvmatrix-sap matrix)))
>>>     (tg:finalize matrix (lambda (x) (del-mat sap)))
>>>     matrix))
>>> 
>>> 
>>> Now you create matrices with make-cvmatrix which returns you a cvmatrix
>>> struct. Whenever this returned struct is garbage collected the finalizer is
>>> called.
>>> If you prefer classes to structs, of course a similar approach with
>>> make-sintance and after methods can be easily implemented.
>>> 
>>> Regs,
>>> Stephan
>>> 
>>> 
>>> 
>>> 2014-04-10 5:41 GMT+02:00 Joeish W <joeish80829 at yahoo.com>:
>>> 
>>> > I have Lisp functions that wrap C wrappers for C++ functions that contain
>>> > a new operator. The C wrappers have to stay the same I can't change those
>>> > but I was advised to update my method of GC.
>>> >
>>> > Here are the C wrappers:
>>> >
>>> > Mat* cv_create_Mat() {
>>> >     return new Mat();
>>> > }
>>> >
>>> > void cv_delete_Mat(void* ptr) {
>>> >     delete (~Mat*)ptr;
>>> > }
>>> >
>>> > Here are the Lisp wrappers for them:
>>> >
>>> > (defcfun ("cv_create_Mat" mat) (:pointer mat)
>>> >   "MAT constructor")
>>> >
>>> > (defcfun ("cv_delete_Mat" del-mat) :void
>>> >   (ptr :pointer))
>>> >
>>> > In a loop, as below, I would normally after calling a MAT function, delete
>>> > it when I'm done with it manually with DEL-MAT, but I heard there was a
>>> > better way by actually entering in the DEL-MAT function into Lisps natural
>>> > GC methods so it would know when it comes across a function with a new it
>>> > should call delete to get rid of it when it know i'm done with it.. The
>>> > thing is I could use help to get started.  Hopefully a
 concrete example of
>>> > how to do this so I don't have to call DEL-MAT manually
>>> >
>>> > (dotimes (n 1000)
>>> > (setf m (mat))
>>> > (del-mat m))
>>> >
>>> > _______________________________________________
>>> > Cffi-devel mailing list
>>> > Cffi-devel at common-lisp.net
>>> > http://common-lisp.net/cgi-bin/mailman/listinfo/cffi-devel
>>> >
>>> >
>>> 
>>
>>
>>
>
>
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