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

[Yves Parès]

Thanks for your explanation Albert, it makes things clearer.

So StablePtrs are just useful so that C code can:
1) call back into Haskell (through a foreign exported function like
doSomethingWithTheObjectIGaveYou
:: StablePtr MyObjectType -> Stuff -> IO ())
2) store them to return them later to Haskell when prompted (through a
foreign imported function like getObject :: Stuff -> IO (StablePtr
MyObjectType))
That's it?

But then,
In use case 1), how can a Haskell function modify the data addressed?

If StablePtrs cannot have their pointed value modified (either C or
Haskell-side), that mostly limits their interest, doesn't it?


2012/2/12 Albert Y. C. Lai <trebla at vex.net>

> 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<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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