Re: [ooc-compiler] Handling C "in" parameters

[Stewart G. <sgr...@ii...>]

Hi August,

> In my program I want to use some functionality from the SDL (Simple
> Directmedia Layer) C library. The API contains the function:
>
> int SDL_PollEvent(SDL_Event *event)
>
> where `SDL_Event' is a union type. In the interface file I have the
> definitions:
>
> EventPtr* = POINTER TO Event;
> Event* = RECORD [UNION] ... END;
>
> PROCEDURE ["SDL_PollEvent"] PollEvent*(event: EventPtr): C.int;
>
> If the client code contains:
>
> event: EventPtr
> ...
> NEW(event)
>
> I get an error about `_td_Sdl__EventPtr' being undeclared. How do I
> use/port this function?

You can't use NEW to allocate a non-Oberon record type, not foreign "C" 
types. I think the compiler should have given you a more intelligent 
error message (ie. at the NEW() statement), but the problem is 
occurring later because the record has no type descriptor. If you 
really need to allocate the record on the heap use RT0.NewBlock, but I 
think it is OK to just allocate the event as a local variable.

I suggest that you do something like this:

PROCEDURE ["SDL_PollEvent"] PollEvent*(VAR event: Event): C.int;

Then you can use it like:

PROCEDURE HandleEvents;
VAR
   e : SDL.Event;
BEGIN
   ...
   WHILE SDL.PollEvent(e) # 0 DO
     CASE e.type OF
       SDL.KEYDOWN:

Another option is wrapping the event in an Oberon-2 RECORD. This might 
be useful if (for example) you want to queue the events for later 
processing.

TYPE
   Event = RECORD
     event : SDL.Event;
   END:
   PtrEvent = POINTER TO Event;

PROCEDURE HandleEvents;
VAR
   se : SDL.Event;
   e : PtrEvent;
BEGIN
   ...
   WHILE SDL.PollEvent(se) # 0 DO
     NEW(e);
     e.event := se;
     Enqueue(e);
   END;

Hope this helps.

Cheers,
   Stewart