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

[John Meacham]

No, you can do nothing with the pointer on the C side other than pass
it back into haskell. It may not even be a pointer, it may be an index
into an array deep within the RTS for instance. The reason they can be
cast to void *'s is so you can store them in C data structures that
don't know about haskell, which tend to take void *s.

    John

On Sun, Feb 12, 2012 at 6:18 AM, Yves Parès <yves.pares at gmail.com> wrote:
> Hello,
>
> 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.
> 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!
>
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>