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

Yes, but from C side, a StablePtr* is **already a **void**. (See HsFFI.h which typedefs HsStablePtr to void*) So its sufficient for a use as an opaque pointer, no need to cast it. So what is the use of casting it to a Ptr () if this doesn't allow to access the memory space addressed? 2012/2/12 Antoine Latter

On Sun, Feb 12, 2012 at 8:18 AM, Yves Parès wrote:

...

Hello,

According to the documentation ( http://hackage.haskell.org/packages/archive/base/4.5.0.0/doc/html/Foreign-St... ), 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; }

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-- 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.

I think that 'castStablePtrToPtr' exists because many C APIs use 'void*' to mean 'opaque lump of data', and these exist to conform to that sort of API.

Antoine