paul@theV.net wrote:
So am I right to draw a picture like this? [...]
Basically, yes. Or from the Haskell implementation's view: "I don't have a clue what a Ptr really is, but it fits in C's void*."
[ 1. & 2. ]
The requirement for matching allocators and deallocators is not much different from the situation in e.g. C++: You can't "free" an object constructed via "new", you can't simply "delete" an array allocated with "new[]", etc.
3. If the Haskell Ptr is no longer in use, the Haskell GC will collect it, regardless of whether it has a dangling pointer to Foreign Data Block.
It is by no means clear if the pointer is "dangling", even if Haskell knew that a Ptr is really a pointer. Other C pointers or Haskell Ptrs could still reference the data. What the best approach is, depends on your problem: a) The C part does more than a simple malloc/free in its (de-)allocation functions, but the lifetime of the object is restricted. In this case, a "withNewFoo" Haskell action is often a good approach, e.g.: withNewFoo :: (Ptr a -> IO b) -> IO b withNewFoo = bracket c_alloc c_dealloc foreign import ccall unsafe "c_alloc" c_alloc :: IO (Ptr a) foreign import ccall unsafe "c_dealloc" c_dealloc :: Ptr a -> IO () b) Same as a), but the lifetime of the C object should be under control of the Haskell GC. In this case, you can use a ForeignPtr, which is basically a Ptr with attached finalizers. c) If the C part simply needs a buffer for some time, there are quite a few possibilities, depending on the required lifetime and ownership of the buffer: * explicit control via Foreign.Marshal.Alloc.malloc and Foreign.Marshal.Alloc.free * pooled allocation ("all or nothing") via Foreign.Marshal.Pool * Foreign.ForeignPtr.mallocForeignPtr* puts the buffer under control of Haskell's GC * explicitly delimited lifetime via Foreign.Marshal.Alloc.alloca
I only hope the Ptr part of FFI documentation can include some of the malloc/free guidelines, otherwise it is quite confusing for beginners.
The FFI spec is, well, a spec and not intended as a tutorial, but Alastair has written a guide to the FFI: http://www.reid-consulting-uk.ltd.uk/docs/ffi.html Cheers, S.