How to access hs_free_fun_ptr from a Visual Studio DLL

Simon Marlow simonmarhaskell at
Mon Mar 27 05:49:10 EST 2006

Sven Panne wrote:
> Am Samstag, 25. März 2006 20:00 schrieb Brian Hulley:
>>I've found a workaround to the problem below: instead of trying to use
>>hs_free_fun_ptr, I instead pass a FunPtr to the Haskell function
>>freeHaskellFunPtr into my DLL, and use this to free everything, finally
>>using it to free itself (!) which I assume should be safe. [...]
> It has been quite some time since I've worked on GHC's Adjustor.c and Hugs' 
> FFI, but IIRC it is in general *not* safe to do this. On some platforms code 
> is being generated dynamically for these kind of callbacks, which has already 
> been freed by the time the callback returns. This might appear to work, 
> depending on your processor architecture and dynamic memory management 
> behaviour, but it's better not to rely on this. Perhaps the FFI spec should 
> be clearer here.

I'm pretty sure this *does* work with GHC, at least on some platforms 
(definitely x86 and x86_64, I'm not sure about the others).  We're 
careful not to require any dynamically-allocated code to run when the 
callback returns.

I agree the FFI spec should be clearer here.


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