<div class="gmail_quote">On Fri, Oct 26, 2012 at 10:26 PM, Matthew Flatt <<a href="mailto:mflatt@cs.utah.edu" target="_blank">mflatt@cs.utah.edu</a>> wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> 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. </blockquote><div> Ok, so as Jon pointed out, the last part may be a problem then. And _union is not the right thing to use here apparently. Now I think that Jon's solution is actually quite good. The XNextEvent* procedure does all that is needed (add the correct pointer-tag) for the user, who can then use the corresponding "sub-struct" on the event. I'm just wondering if it would not be possible to simply add all relevant tags to the any XEvent, e.g.: (define (make-dummy-XEvent) (let ((s (malloc _int (* 24 (ctype-sizeof _long))))) (memset s 0 24 _long) (cpointer-push-tag! s XEvent-tag) (cpointer-push-tag! s XAnyEvent-tag) (cpointer-push-tag! s XKeypressedEvent-tag) .... s) without caring about the actual kind of event received, and let the user use the event kind it wants. But it's probably not so important now. Thanks for your answers, I understand it better now. Laurent </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> <div><div class="h5"> 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 <<a href="mailto:laurent.orseau@gmail.com">laurent.orseau@gmail.com</a>> 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 > > > > </div></div>> ____________________ > Racket Users list: > <a href="http://lists.racket-lang.org/users" target="_blank">http://lists.racket-lang.org/users</a> </blockquote></div>