That seems like an ok solution.
I would be tempted to write
(define-cstruct _XEvent ([type _int]))
(define-cstruct (_XAnyEvent _XEvent) ([serial _ulong]
....))
(define-cstruct (_XKeyEvent _XEvent) ([serial _ulong]
....))
....
Those declarations more directly fit the sub-structuring that is
implemented by the union in "Xlib.h", but it doesn't give you
`XAnyEvent-type', `XKeyEvent-type', etc., and the size of `_XEvent'
isn't the maximum of the variant sizes.
At Fri, 26 Oct 2012 19:00:01 +0200, Laurent wrote:
> Oooohh, I think I understand now: the way it was is already sufficiently
> general.
> The writer of the initial code took a different direction.
> Instead of defining a union type, he changes the tag of the pointer
> depending on what kind of type he needs in the union:
> (cpointer-push-tag! e XAnyEvent-tag)
> (case (XAnyEvent-type e)
> ((KeyPress) (cpointer-push-tag! e XKeyPressedEvent-tag))
> ((KeyRelease) (cpointer-push-tag! e XKeyReleasedEvent-tag))
> ....
>
> It's pretty smart. Is it the right way to go in general, and should _unions
> be avoided?
>
> Laurent
>
> On Fri, Oct 26, 2012 at 6:39 PM, Laurent <laurent.ors...@gmail.com> wrote:
>
> > Hi,
> >
> > I am trying to convert an FFI cstruct definition into a union.
> > More exactly, the previous definition was :
> > (define-cstruct _XEvent
> > ((type EventType)))
> >
> > with EventType being defined elsewhere.
> > But this was an incomplete definition, and I now want to translate the
> > following C union:
> > typedef union _XEvent {
> > int type; /* must not be changed; first element */
> > XAnyEvent xany;
> > XKeyEvent xkey;
> > // ... the rest for later
> > } XEvent;
> >
> > I searched the collects directory, but I see only a single and too simple
> > _union use, so I'm not sure how to use it.
> > I guessed the following:
> > (define _XEvent
> > (_union EventType ; type
> > _XAnyEvent ; xany
> > _XKeyEvent ; xkey
> > ; etc.
> > ))
> > (define (XEvent-type ev) (union-ref ev 0))
> > (define (XEvent-xany ev) (union-ref ev 1))
> > (define (XEvent-xkey ev) (union-ref ev 2))
> >
> > However, now I don't use define-cstruct anymore (it would not work with
> > _union, would it?), so I don't have all the -tag, -pointer, etc. bindings,
> > which I need.
> >
> > What should I do ?
> >
> > Thanks,
> > Laurent
> >
> >
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