[Haskell-cafe] How can I pass IOUArrays to FFI functions?

Stefan O'Rear stefanor at cox.net
Mon Aug 20 14:49:50 EDT 2007

On Mon, Aug 20, 2007 at 05:27:04AM -0700, Ryan Ingram wrote:
> I have a C function of type
>    void f ( HsWord32* p0, HsWord32* p1, HsWord32 size );
> along with the FFI declaration:
>    foreign import ccall unsafe f :: Ptr Word32 -> Ptr Word32 -> Word32 -> IO
> ()
> In my Haskell code I have an unboxed IO array of Word32; IOUArray Int
> Word32.
> I want to pass the pointer to this array to f().  How can I get the pointer
> out of the array?  Or, is there a better way to declare f() to do this?

Short answer: You can't.

Longer: GHC uses a copying/compacting garbage collector, so most objects
don't have stable addresses.  In particular MutableByteArray# is movable
by default.  So, you need an array structure built on pinned memory.

If you want MArray operations, that leaves you with
Data.Array.Storable, as already mentioned.

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