[Haskell-cafe] Re: How to automatically free memory allocated by malloc? and how to reliably realloc such buffer?

[Simon Marlow]

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