Hi all, GHC defines ForeignPtr as: data ForeignPtr a = ForeignPtr Addr# ForeignPtrContents -- we cache the Addr# in the ForeignPtr object, but attach -- the finalizer to the IORef (or the MutableByteArray# in -- the case of a MallocPtr). The aim of the representation -- is to make withForeignPtr efficient; in fact, withForeignPtr -- should be just as efficient as unpacking a Ptr, and multiple -- withForeignPtrs can share an unpacked ForeignPtr. Note -- that touchForeignPtr only has to touch the ForeignPtrContents -- object, because that ensures that whatever the finalizer is -- attached to is kept alive. Is it ok to modify the Addr# field? Or do the libraries assume that it always points to the start of the block described by ForeignPtrContents? Changing the Addr# field would be quite useful for pointing to the middle of a memory block. For instance, Storable vectors are currently defined as: data Vector a = Vector {-# UNPACK #-} !(Ptr a) {-# UNPACK #-} !Int {-# UNPACK #-} !(ForeignPtr a) If I could use the Addr# field of the ForeignPtr, I could get rid of the Ptr. Alternatively, I could define my own version of ForeignPtr if only ForeignPtrContents was exported (abstractedly would be enough). Roman