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.
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