Actually can't you just use an input specification for the pointer? (_ fun [vf : (_ptr i _OggVorbis_File)])
As long as you can define the OggVorbis_File struct with the ffi. If not your solution is probably fine. On 01/22/2012 03:59 PM, Michael W wrote: > Wow, thanks! > > After a bit of reading, I arrived at this, which seems to work: > > (_fun ... > [vf : _OggVorbis_File-pointer > = (let ([file (malloc _OggVorbis_File 'interior)]) > (set-cpointer-tag! file OggVorbis_File-tag) > file)] > ...) > > 42 minutes ago, Jon Rafkind wrote: >> I'm pretty sure you can use cpointer-push-tag! >> >> http://docs.racket-lang.org/foreign/foreign_tagged-pointers.html?q=cpointer-push&q=in-lines&q=read-bytes&q=command&q=for/fold#(def._((lib._ffi/unsafe..rkt)._cpointer-push-tag!)) >> >> (let ([file (malloc)]) >> (cpointer-push-tag! file _OggVorbis-file-pointer >> file) >> >> On 01/22/2012 02:14 PM, Michael W wrote: >>> Hello! >>> >>> In the FFI library, is there a way to cast one pointer type >>> straight to another? I'm working with libvorbisfile (PLaneT >>> package forthcoming) which expects me to allocate my own >>> OggVorbis_File cstruct and pass that everywhere. >>> >>> I ask because (malloc _OggVorbis_File) returns a _pointer, not an >>> _OggVorbis_File-pointer, and _fun and friends don't accept either >>> type in place of the other. >>> >>> If I say: >>> >>> (cast >>> (malloc _OggVorbis_File) >>> _pointer _OggVorbis_File-pointer) >>> >>> ...That's bad -- because I never keep the original pointer >>> returned by the (malloc) call, the garbage collector reclaims >>> that memory even though the _OggVorbis_File-pointer still points >>> to it. >>> >>> My current workaround is to just use (malloc _OggVorbis_File) >>> everywhere and have _pointer types in my (_fun ...) declarations. >>> Is this the preferred way of doing it? >>> >>> Is there a way to ask (malloc) to return a different pointer type >>> than _pointer? >>> >>> Thanks for your help. >>> ____________________ Racket Users list: http://lists.racket-lang.org/users
