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.pa...@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@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe