* Guile dynamic FFI, C function expecting pointer
@ 2020-11-22 22:50 Tim Meehan
2020-11-23 0:16 ` Matt Wette
0 siblings, 1 reply; 2+ messages in thread
From: Tim Meehan @ 2020-11-22 22:50 UTC (permalink / raw)
To: guile-user
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;
}
|#
;;----------------------------------------------------------------------------;;
;; Test loading and using the library.
(use-modules (system foreign))
(define libstuff (dynamic-link "./libstuff.so"))
(define int-ptr-example1
(pointer->procedure
void
(dynamic-func "int_ptr_example1" libstuff)
(list '*)))
;; Following:
;;
https://nalaginrut.com/archives/2015/03/27/do-some-quick-and-dirty-with-guile-ffi
(let ([a %null-pointer])
(int-ptr-example1 a)
(display a)
(newline))
;;----------------------------------------------------------------------------;;
;; Sadly, when it runs, I get a segmentation fault at the call to
;; int-ptr-example1 :(
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: Guile dynamic FFI, C function expecting pointer
2020-11-22 22:50 Guile dynamic FFI, C function expecting pointer Tim Meehan
@ 2020-11-23 0:16 ` Matt Wette
0 siblings, 0 replies; 2+ messages in thread
From: Matt Wette @ 2020-11-23 0:16 UTC (permalink / raw)
To: guile-user
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.
^ permalink raw reply [flat|nested] 2+ messages in thread
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