[Haskell-cafe] Stable pointers: use of cast to/from Ptr

[Albert Y. C. Lai]

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-StablePtr.html),
> 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.