[C2hs] Re: [Gtk2hs-users] question on structs with compound members

[Manuel M T Chakravarty]

Axel Simon <A.Simon at kent.ac.uk> wrote,

> On Wed, Oct 22, 2003 at 02:51:15PM +0100, Duncan Coutts wrote:
> > All,
> > 
> > So I'm trying to wrap this structure:
> > 
> > struct GtkSourceTagStyle {
> > 
> > 	/* readonly */
> > 	gboolean is_default;
> > 
> > 	guint mask;
> > 	
> > 	GdkColor foreground;
> > 	GdkColor background;
> > 	
> > 	gboolean italic;
> > 	gboolean bold;
> > 	gboolean underline;
> > 	gboolean strikethrough;
> > 
> > 	/* Reserved for future expansion */
> > 	guint8 reserved[16];	
> > };
> > 
> > My problem is with the GdkColor members foreground & background. They
> > are themselves structures so c2hs cannot make {#get #} mappings for
> > them. Of course Color has already been wrapped (in
> > gtk2hs/gtk/general/Structs.hsc) and is a member of the Storable class so
> > all we need is to get a pointer to the member to be able to apply
> > peek/poke to it. This is not hard if we manually edit the .hs file
> > produced from the .chs binding file:
> 
> I kinda object to use c2hs for marshalling structures. When I wrote the
> marshalling code for graphics contexts for gtk+hs, I used c2hs only to
> find out that it calculates the wrong offsets. 

That was a bug in c2hs, which has been fixed since then.

> Moreover, it is doomed to 
> produce the wrong offsets in some circumstances with bitfields. Windows 
> uses a different layout than Unix, even if you use gcc on both platforms. 
> (gcc behaves differently so that you can use the precompiled Microsoft 
> libraries).

Why do you think that c2hs cannot handle this?  If you look
at the files "c2hs/toplevel/C2HSConfig.hs.in" and
"c2hs/toplevel/c2hs_config.c" in the c2hs source tree, you
will see that c2hs actually goes to quite some trouble to
find out how to properly access bitfields on the particular
OS and architecture it is running on.  If you have got any
example, where this does not work properly, please submit it
as a c2hs bug report.

Cheers,
Manuel