Hello, According to the documentation ( http://hackage.haskell.org/packages/archive/base/4.5.0.0/doc/html/Foreign-St...), StablePtrs aims at being opaque on C-side. But they provide functions to be casted to/from regular *void**'s. Does that mean if for instance you have a StablePtr CInt you can cast it to Ptr () and alter it on C-side? void alter(void* data) { int* x = (int*)data; *x = 42; } -------------------------------------------------- -- using 'unsafe' doesn't change anything. foreign import ccall safe "alter" alter :: Ptr () -> IO () main = do sptr <- newStablePtr (0 :: CInt) deRefStablePtr sptr >>= print alter (castStablePtrToPtr sptr) -- SEGFAULTS! deRefStablePtr sptr >>= print freeStablePtr sptr But I tried it, and it doesn't work: I got a segfault when 'alter' is called. Is it normal? Does this mean I can only use my pointer as opaque? (Which I know to be working, as I already got a C function call back into Haskell and pass it the StablePtr via a 'foreign export') But in that case, what is the use of castStablePtrToPtr/castPtrToStablePtr, as you can already pass StablePtrs to and from C code? Thanks!