Re: [Caml-list] RFH: can't figure out why my QT5 widget bindings segfault

[Goswin von Brederlow <goswin-v-b@web.de>]

On Thu, Mar 24, 2016 at 11:25:59AM +0100, Goswin von Brederlow wrote:
> On Wed, Mar 23, 2016 at 06:18:12PM +0100, François Bobot wrote:
> > On 23/03/2016 16:18, Anatoly Zaretsky wrote:
> > >On Wed, Mar 23, 2016 at 12:50 PM, Goswin von Brederlow <goswin-v-b@web.de
> > ><mailto:goswin-v-b@web.de>> wrote:
> > >
> > >    I'm stuck with a bug in the Tetrix example for my QT5 bindings:
> > >
> > >    https://github.com/mrvn/ocaml-qt5
> > >
> > >    The segfault happens when you click start and the first piece is moved
> > >    one tile down in caml_mrvn_QT5_OPainter_fillRect. The arguments to the
> > >    call all look ok but something must corrupt the painter. The segfault
> > >    goes away when I force a Gc.full_major before creating a new OPainter
> > >    in TetrixBoard:148.

Turns out the problem has nothing directly to do with the ocaml
bindings. The problem is that Qt only allows one active painter per
paint device at a time and has explicit begin()/end() functions to
start and top painting where begin() does error checking. The QPainter
constructor also allows passing the paint device directly, which
automatically calls begin(device) but reports no errors. The
destructor then also automatically calls end().

Problem in ocaml now is that the destructor no longer runs when the
painter goes out of scope but only when the Gc gets around to it. And
for some reason creating a second painter for a device kind of works
(in disregard of what the API says) as long as you don't destroy the
first one half way through, which the Gc does.

I've converted to explicit painter#begin_ and painter#end_ calls and
added some checks in the bindings to enforce them and now everything
works. Now off to adding KeyEvent parsing and one can actually play.

MfG
	Goswin