On 12-02-12 09:18 AM, Yves Parès wrote:
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.
The doc multiply warns again and again that StablePtr, as well as whatever Ptr you get from castStablePtrToPtr, are opague (meaningless) to the C side. This is sanctioned by Haskell 2010, and GHC certainly exploits it to the fullest. The following example shows what kind of "pointer" values the C side receives for real (I deliberately do not free anything to show you more possible values): #include <stdio.h> void expose(void *p, void *q) { printf("%p %p\n", p, q); } import Foreign.StablePtr import Foreign.Ptr main = do printout (0 :: Int) printout (let x = not x in x) printout ([] :: [Integer]) printout :: a -> IO () printout thunk = do p <- newStablePtr thunk expose p (castStablePtrToPtr p) -- I deliberately do not free foreign import ccall expose :: StablePtr a -> Ptr b -> IO () Typically the output is like 0xf 0xf 0x10 0x10 0x11 0x11 Looks more like keys of a lookup table than pointers. I do not know what good is castStablePtrToPtr for, given that StablePtr is already translated to C side void*, so that no intermediate Ptr step is necessary. Perhaps there is a story from a historical perspective.