Re: Guile dynamic FFI, C function expecting pointer

[Matt Wette]

On 11/22/20 2:50 PM, Tim Meehan wrote:
> I tried to boil this question down to the most simple thing that
> represented what I needed to understand. I have had luck getting C
> functions that expect arguments "by value," but "by reference" has been
> problematic.
>
> The failure mode is "Segmentation Fault," so I gather that I may not be
> using the right Guile call at all.
>
> The Guile user manual is usually quite excellent, but I seem to be missing
> something important.
>
> Thanks,
>
> ;;----------------------------------------------------------------------------;;
> ;; C source for "libstuff.so":
> ;; file stuff.c, compiled as:
> ;; gcc stuff.c -o libstuff.so -fPIC -shared
> #|
> void int_ptr_example1(int *a) {
>      *a = 5;
> }
> |#
You'll need to make-bytevector a bytevector that holds sizeof(int) bytes.
Then pass (bytevector->pointer <obj>) as the argument.

(let ((obj (make-bytevector (sizeof int))))
  (int-ptr-example (bytevector->pointer obj)))

Now the 5 should be in the bytevector.  You will need to extract it.