[plt-scheme] Re: passing pointer from one FFI call as input into another FFI call

Thanks, changing the _ptr o _st_marker-pointer/null to _st_marker-
pointer/null worked perfectly.

You're correct about the other problems. I was trying to simplify
after I cut and pasted into the email - after a long day. I should
know better. Thanks again.

On Jun 24, 2:34 am, Eli Barzilay <e... at barzilay.org> wrote:
> On Jun 23, scott wrote:
>
> > Thanks for the quick reply. I think I'm close but not quite there
> > yet.The simple example below might help to show where my confusion
> > in accessing the pointer that's being pointed to.
>
> > Below are are two C functions [...]
>
> There are several problems in this code.  I don't have that library,
> so I needed to define the struct myself, there were some typing
> issues, and alloc_marker() did not put the allocated pointer in the
> pointer it received.  The scheme code had some issues too, like
> calling the function `alloc_test_from_scheme'.
>
> But it looks to me like the core of your problem is here:
>
> > ;int display_pointer(struct st_marker *marker);
> > (define display-pointer
> >   (get-ffi-obj "display_pointer" "libvfm"
> >                (_fun [marker : (_ptr i _st_marker-pointer/null)]
> >                      -> [status : _int] -> (values marker status))))
>
> You're passing a newly allocation pointer that points to your pointer,
> but display_pointer() expects to get just your pointer.  One solution
> is to change display_pointer() to expect a pointer to a pointer.
> Another solution is to fix the scheme code to follow the C type, and
> this is what I did below.
>
> ----------------------------------------------------------------------
> #include <stdlib.h>
> #include <stdio.h>
>
> struct st_marker {
>   long timestamp;
>   int  offset;
>   int  value;
>   void *next;
>
> };
>
> int display_pointer(struct st_marker *marker) {
>   fprintf(stderr,"marker: %p, timestamp: %ld\n", marker, marker->timestamp);
>   return 0;
>
> }
>
> int alloc_marker(struct st_marker **marker) {
>     *marker = (struct st_marker *) malloc(sizeof (struct st_marker));
>     (*marker)->timestamp = 1000;
>     fprintf(stderr, "allocated new ");
>     display_pointer(*marker);
>     return 0;}
>
> ----------------------------------------------------------------------
> #lang scheme
> (require scheme/foreign)
> (unsafe!)
>
> (define-cstruct _st_marker
>   ([timestamp _long]
>    [offset _int]
>    [value _int]
>    [next _st_marker/null]))
>
> ;int alloc_test_from_scheme(struct st_marker **marker_list);
> (define alloc-marker
>   (get-ffi-obj "alloc_marker" "./x.so"
>                (_fun [marker : (_ptr o _st_marker-pointer/null)]
>                      -> [status : _int] -> (values marker status))))
>
> ;int display_pointer(struct st_marker *marker);
> (define display-pointer
>   (get-ffi-obj "display_pointer" "./x.so"
>                (_fun [marker : _st_marker-pointer/null]
>                      -> [status : _int] -> (void))))
>
> ;; here's the test
> (define-values (marker status) (alloc-marker))
> (display-pointer marker)
> ----------------------------------------------------------------------
>
> --
>           ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
>                  http://www.barzilay.org/                Maze is Life!
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