Re: FFI questions

[Jan Synacek]

On Fri, May 15, 2020 at 10:09 PM Taylan Kammer <address@hidden> wrote:
>
> On 15.05.2020 15:47, Jan Synacek wrote:
> > Hello,
> >
> > Currently I have something like this:
> >
> > (define libxcb (dynamic-link "libxcb"))
> >
> > ...
> >
> > (define c-change-window-attributes
> >    (pointer->procedure void
> >       (dynamic-func "xcb_change_window_attributes" libxcb)
> >       (list '* uint32 uint32 '*)))
> >
> > (define (change-window-attributes conn win mask vals)
> >     (c-change-window-attributes conn win mask (bytevector->pointer vals)))
> >
> > The last argument to xcb_change_window_attributes is 'const void *' and I
> > need to pass a u32 vector to it. As it is right now, it segfaults when I
> > try passing #u32(something) to 'change-window-attributes'. Is it possible
> > to make it accept a u32 vector using just Scheme or do I have to work
> > around it on the C level?
>
> I think the reason it segfaults might be that the #u32() object gets
> garbage collected soon after you call bytevector->pointer on it.  See
> that you put it in a variable, and reference that variable at some point
> after xcb_change_windor_attributes should be done with the vector, to
> make sure that this is not the problem.
>
> Otherwise I'm not sure why it should segfault.  A #u32() bytevector
> really should be backed by a contiguous array of uint32_t values, just
> like what you'll get when you define a uint32_t[] in C.

This works, thanks!

(define (change-window-attributes conn win mask vals)
  (define b (list->u32vector vals))
  (c-xcb-change-window-attributes conn win mask (bytevector->pointer b)))

> > My second question is about FFI and structs. Is it possible to access C
> > struct members from Scheme by name? If not, how do I generally approach the
> > problem? I've checked how guile-xcb does it and it seems to be building
> > hash tables with field names as keys. But that basically requires me to
> > "redefine" all the C structs that I would be interested in at the Scheme
> > level.
>
> These might be useful:
>
> https://github.com/TaylanUB/scheme-bytestructures
>
> https://www.nongnu.org/nyacc/ffi-help.html
>
> I maintain bytestructures, am happy to answer questions.  (If I don't
> respond on the mailing list, feel free to mail me directly or open an
> issue on GitHub; mails with address@hidden and address@hidden
> in the To: or Cc: fields land in a folder that I don't always check.)

Very interesting, I'll take a look.

Thank you,
-- 
Jan Synacek
Software Engineer, Red Hat