[Haskell-cafe] getting data through foreign layer without marshalling

[Bernie Pope]

2009/12/14 Donn Cave <donn at avvanta.com>:
> I'm working with a C++ application library, of the sort where
> you instantiate a subclass of the library object and it dispatches
> your functions on receipt of various events.  With multiple OS
> threads, by the way.
>
> This works all right so far, with some C++ boilerplate and Haskell
> FunPtrs created via the foreign "wrapper" option, but I am not crazy
> about marshalling the application data, to get it to the C++ object
> and back to my application functions.
>
> So, I'm wondering if the way I'm trying to thread application data
> through the C++ layer is reasonably fool-proof - I mean, empirically
> it works in a simple test, but are there going to be storage issues,
> etc.?  Is there a better way to do it?
>
> Briefly,
>  - my callbacks
>        AppData -> CPlusObjectPtr -> P0 ... -> IO Pn
>  - the FunPtr inserted into C++ object
>        FunPtr (CPlusObjectPtr -> P0 ... -> IO Pn)
>        ... i.e, I apply the AppData parameter first
>  - I rewrite the C++ object's FunPtrs every time I update the
>    application data (freeHaskellFunPtr prior values.)
>
> I'm just not sure where AppData lives while it's referenced in a
> FunPtr via partial application, if there might be multiple copies, etc.

I don't fully understand what you want to do, but perhaps you can use
a StablePtr:

   http://www.haskell.org/ghc/docs/latest/html/libraries/base/Foreign-StablePtr.html

Cheers,
Bernie.