ForeignPtr's - why can't they be passed directly to foreign functions?

[kahl at cas.mcmaster.ca]

Brian Hulley <brianh at metamilk.com> wrote:
 
 > My other question is what happens if I want to have a function that takes 
 > more than one ForeignPtr as argument ie
 > 
 > foreign import ccall duma_test :: Ptr (Window a) -> Ptr (Window a) -> IO ()
 > 
 > test :: ForeignPtr (Window a) -> ForeignPtr (Window a) -> IO ()
 > test p q = withForeignPtr p (\p' -> withForeignPtr q $ duma_test p')
 > 
 > Is this the only way to achieve this? It seems a bit long-winded and 
 > possibly a bit inefficient...

I use:

\begin{code}
{-# INLINE with2ForeignPtrs #-}
{-# INLINE with3ForeignPtrs #-}
with2ForeignPtrs :: ForeignPtr a -> ForeignPtr b -> (Ptr a -> Ptr b -> IO c) -> IO c
with2ForeignPtrs f1 f2 m = withForeignPtr f1 (withForeignPtr f2 . m)
\end{code}

\begin{code}
with3ForeignPtrs :: ForeignPtr a -> ForeignPtr b -> ForeignPtr c ->
  (Ptr a -> Ptr b -> Ptr c -> IO d) -> IO d
with3ForeignPtrs f1 f2 f3 m = withForeignPtr f1 (with2ForeignPtrs f2 f3 . m)
\end{code}


 > 
 > foreign import ccall duma_init :: IO ()
                        ^^^^

Any relation with duma.sourceforge.net ?
``D.U.M.A. - Detect Unintended Memory Access''

We just used that for hunting down memory leaks in a C library
we produced an FFI binding to...



Wolfram