Re: [wxhaskell-users] Strange polygon behaviour

[Jeremy O'D. <jer...@gm...>]

Hi Chris,

On 15/12/06, Chris Mears <ch...@cm...> wrote:
> Chris Mears <ch...@cm...> writes:
>
> > I am using the wxHaskell library built from the Darcs repository at
> > haskell.org, with wxWidgets "gtk-2.6.3" and GHC 6.6.  I am trying to
> > draw a polygon with the "polygon" function, but it isn't doing what I
> > expected.
[...]
> I think I have tracked down the source of the problem.  It works
> correctly on my 32-bit computer, but misbehaves on the 64-bit one.
> Summary: On my 64-bit machine, Haskell Int is 64-bit, C int is 32-bit.

This makes perfect sense. I think you may well be the first person to
have tried wxHaskell on a 64 bit machine, so perhaps no surprise that
you are coming up with these issues.

> The polygon function takes a list of Point, where a Point is a pair
> (basically) of Ints.  On 64-bit GHC, an Int is 64-bits long, so each
> component of a Point is 64-bits.  The Point components are marshalled
> into an array and passed to C code (wxc/src/ewxw/eljdc.c).  The C code
> treats the array as an array of (C) int.  However, on the 64-bit machine
> a C int is 32-bits.
>
> So, when I passed the 64-bit points (50, 50), (100, 100), etc. as 32-bit
> points they were interpreted as (50, 50), (0, 0), (100, 100), etc.,
> which explains the strange drawing I saw.  Changing the cast in the C
> code from (int*) to (long*) corrects the problem, but is not a general
> fix.  Sadly, I don't know enough about FFI and marshalling to suggest
> the proper correction.

Unfortunately, I think your basic solution is the way forward, but
probably mapped onto a suitable set of macros to hide the platform
difference. We could probably do this by working through all of the
wxC implementation files and replacing all int with types of known
size and appropriate casts. Very tedious, but not difficult.

I think we, as the development team, need to put this on our TODO
list, and I am happy to do once I have finished cabalization (so it
may take a while). I'll need help to test it though - I have four
machines available to me, but none has a 64 bit CPU :-(

Regards
Jeremy