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 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...

-R. Kyle Murphy -- Curiosity was framed, Ignorance killed the cat.

On Wed, Dec 19, 2012 at 4:49 PM, Matthew Hayden 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_8...

...

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

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