Bulat Ziganshin wrote:
my program uses datastructure that contains plain Ptr, this Ptr points to the memory area allocated by 'malloc':
createRawMemBuf size = do buf <- mallocBytes (fromIntegral size) bufRef <- newURef buf ... return (Mem bufRef ...)
i need to free this memory buffer on GC if there are no more references to Mem structure. how i can accomplish this? i can't allocate this buffer in GHC heap because later i can use 'realloc' on it.
i think that i should use ForeignPtr what points to nothing and performs 'readURef bufRef >>= free' in it's finalizer?
something like this:
createRawMemBuf size = do buf <- mallocBytes (fromIntegral size) bufRef <- newURef buf ... fin <- mkFinalizer (\_ -> readURef bufRef >>= free) fptr <- newForeignPtr fin nullPtr return (Mem bufRef ... fptr)
I hope you surround each use of the actual Ptr with 'withForeignPtr'? If so, I imagine this is safe. I would rather package this up as a library, maybe MutForeignPtr, with the same operations as ForeignPtr.
type Finalizer a = Ptr a -> IO () foreign import ccall "wrapper" mkFinalizer :: Finalizer a -> IO (FinalizerPtr a)
.... well, i implemented this and it seems to work - at least memory freed at performGC. another question is how to free 'fin' - i should apply 'freeHaskellFunPtr' to it, but i think i can't do it in finalizer itself
You can call this from inside the finalizer. There was a discussion about this recently on one of the GHC lists, IIRC. I don't think the FFI spec explicitly allows it.
The second question is how to make buffer reallocation reliable. Currently i use the following code:
reallocBuffer (Mem bufRef ...) newsize = do buf <- readURef bufRef writeURef bufRef nullPtr newbuf <- reallocBytes buf newsize writeURef bufRef newbuf
First 'writeURef' is used to prevent repetitive memory deallocation by finalizer i this routine will be interrupted just after 'reallocBytes' operation. will it be enough to use 'block' instead? i.e.:
reallocBuffer (Mem bufRef ...) newsize = do buf <- readURef bufRef block $ do newbuf <- reallocBytes buf newsize writeURef bufRef newbuf
You're missing a 'withForeignPtr'. Something like this, I think: reallocBuffer (Mem bufRef ... fp) newsize = withForeignPtr fp $ do buf <- readURef bufRef newbuf <- reallocBytes buf newsize writeURef bufRef newbuf Cheers, Simon