I think I haven't run into this because `r` and `foo` are usually
combined into one value that's more Rackety. Maybe `allocator` should
have a `get-result` optional argument similar to `deallocator`.
Meanwhile:
;; Not exported:
(define-fooapi make_foo
(_fun (foo : (_ptr o _foo))
-> (r : _int)
-> (values r foo)))
;; Not exported:
(define made-foo ((allocator destroy-foo) (lambda (foo) foo)))
;; Exported:
(define (make-foo)
(start-atomic)
(let-values ([(r foo) (make_foo)])
(made-foo foo)
(end-atomic)
(values r foo)))
With Dmitry's solution as written, you don't get atomicity of the
foreign function producing a result and registering it, if that
matters. But you could use `#:wrap` to add a `call-as-atomic` wrapper.
(That doesn't let you call the destructor more eagerly, though.)
At Wed, 24 Jan 2018 22:53:41 +0300, Dmitry Pavlov wrote:
> Konrad,
>
> I would create a wrapper like this:
>
> (define-fooapi make-foo
> (_fun (foo : (_ptr o _foo)
> -> (r : _int)
> -> (if r (begin (register-finalizer-and-custodian-shutdown foo
> destroy-foo) foo)
> (error "can not make foo")))
>
>
> Regards,
>
> Dmitry
>
> On 01/24/2018 10:47 PM, Konrad Hinsen wrote:
> > Hi everyone,
> >
> > I am working on an FFI which overall progresses well - the Racket FFI
> library is really a very nice tool to have.
> >
> > The one problem I couldn't solve yet is handling resource allocation. My
> > use
> case is very similar to what allocator/deallocator are meant for, except that
> the allocated object is not the return value of any function, and therefore I
> cannot use allocator.
> >
> > I have an API like
> >
> > int make_foo(foo** new_foo);
> > int destroy_foo(foo* a_foo);
> >
> > so my FFI definitions look like
> >
> > (define _foo (_cpointer 'foo))
> >
> > (define-fooapi make-foo
> > (_fun (foo : (_ptr o _foo)
> > -> (r : _int)
> > -> (values r foo))
> >
> > (define-fooapi destroy-foo
> > (_fun _foo -> _void))
> >
> > Given that make-foo returns two values, I cannot use #:wrap to make it an
> allocator.
> >
> > Since the C API I am wrapping is pretty standard, I suspect others have had
> this problem before. Any suggestions?
> >
> > Thanks in advance,
> > Konrad.
> >
>
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