[Gtk-sharp-list] Access Violations with Mono.Cairo on Windows

[Michael Hutchinson]

On Tue, Dec 29, 2009 at 9:20 AM, Andy Selvig <ajselvig at gmail.com> wrote:
> Hi-
>
> First of all, this is a Mono.Cairo question and I've noticed there's a lot
> of experience with it on this list, but if there's a better place to
> discuss, please let me know.
>
> I've working on a project using Mono.Cairo. Most of my recent development
> has been on Linux and everything is working well. However, I recently tried
> to run the project on Windows and I keep getting access violations from
> Cairo calls:
>
> "Attempted to read or write protected memory. This is often an indication
> that other memory is corrupt."
>
> The part of the code this seems to be happening in is when I create a Cairo
> image surface and render to it. It looks something like this:
>
>             // remake the image surface, if needed
>             if (surface == null || surface.Width != IntWidth ||
> surface.Height != IntHeight)
>             {
>                 imageData = new byte[IntWidth * IntHeight * 4];
>                 surface = new ImageSurface( ref imageData, Format.ARGB32,
> IntWidth, IntHeight, 4 * IntWidth);
>             }
>
>             // render the control to the surface
>             using (Context cr = new Context(surface))
>             {
>                 cr.Operator = Operator.Source;
>                 cr.Color = new Cairo.Color(1, 1, 1, 0);
>                 cr.Paint();
>
>                 cr.Operator = Operator.Over;
>                 cr.MoveTo(0,0);
>                 RenderCairo(new RenderContext(cr,
> DecoratorService.Get(viewport)));
>
>                 surface.Flush();
>             };
>
> This happens every render cycle and then the image data from the surface
> gets copied to an OpenGL texture and rendered to the screen. So, there's no
> direct Gtk# involved, but I run it inside a Gtk# app on Linux and a WPF app
> on Windows (both using the Tao OpenGL bindings).
>
> The access violations seem to happen randomly, but generally within a couple
> seconds of interacting with the app. It happens on either the cr.Paint()
> call or one of the cr.Operator calls.
>
> One thing that I had to change to get it to compile on Windows is adding a
> ref to the image data argument in the ImageSurface constructor. It compiles
> and runs fine without it on Linux, but not on Windows. I'm not even sure how
> this could be as I'm using the same version of Mono.Cairo (2.0.0).
>
> Anyway, I'm at a bit of a loss as to how to debug this issue. Obviously some
> memory (the image data buffer?) is being moved or freed unintentionally and
> then accessed later, but I just don't see why that would be, especially
> since it works wonderfully on Linux. Any help would be appreciated.

The "ref" version of the ctor is obsolete:
http://anonsvn.mono-project.com/viewvc/trunk/mcs/class/Mono.Cairo/Mono.Cairo/ImageSurface.cs

Because the obsoleted version does nothing, that means that the
ImageSurface's "surface" pointer is zero, so code that dereferences it
will explode.

-- 
Michael Hutchinson
http://mjhutchinson.com