[Haskell-beginners] Fwd: FFI bindings to prodecure with an in-out parameter

[Matthew Hayden]

I'm glad my message finally got through!

I've fixed this one now. It was segfaulting because I was missing some
(undocumented) necessary data in one of the other arguments.

Passing it a Ptr (Ptr a) with alloca does the trick, and no need to
allocate memory for the opaque struct.

Thanks,

Matt

On 20 December 2012 03:11, Kyle Murphy <orclev at gmail.com> wrote:

> It's been a while since I've done anything with C, but if I'm reading
> that right it's expecting a pointer pointer as the second argument. In
> other words, it wants the address of a pointer that it will update
> with a pointer to the instantiated instance of the session structure.
> See if this helps:
>
> http://www.haskell.org/haskellwiki/FFICookBook#Passing_pointer-to-pointer-to-thing
>
> -R. Kyle Murphy
> --
> Curiosity was framed, Ignorance killed the cat.
>
>
> On Wed, Dec 19, 2012 at 4:49 PM, Matthew Hayden
> <mrehayden at googlemail.com> wrote:
> > Dear Haskell Beginners,
> >
> > I've gotten stuck developing an FFI binding to libspotify. One of the
> > functions I'm trying to bind to, sp_session_create, uses an in-out
> > parameter. Luckily I have an example of its usage here (in the
> definition of
> > main):
> >
> >
> https://developer.spotify.com/technologies/libspotify/docs/12.1.45/jukebox_8c-example.html
> >
> > It looks to me like they've allocated a pointer to an sp_session and then
> > passed a pointer to that to sp_session_create.
> >
> > My best attempt tries to do the same but comes across a missing Storable
> > instance of the equivalent of the sp_session data structure. Because the
> C
> > definition of this struct is not exposed in the header files I can't work
> > out how to do it myself. The alternative of passing in an empty pointer
> with
> > the assumption that the procedure will allocate memory for us simply
> > segfaults.
> >
> > I've put a source distribution up here (hit "raw" to download):
> >
> https://github.com/mrehayden1/hmdfm/blob/master/dist/hmdfm-0.1.0.0.tar.gz
> > The project can be built automatically with cabal and GHC with hsc2hs
> > installed.
> >
> > Also if you like libspotify can be downloaded and automatically installed
> > (for *nix users I think at least) from here:
> > https://developer.spotify.com/technologies/libspotify/#download
> >
> > Please, can anyone help?
> >
> > Matthew Hayden
> >
> >
> >
> > _______________________________________________
> > Beginners mailing list
> > Beginners at haskell.org
> > http://www.haskell.org/mailman/listinfo/beginners
> >
>
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