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

[Bulat Ziganshin]

Hello Haskell-Cafe,

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)

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


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

-- 
Best regards,
 Bulat                          mailto:Bulat.Ziganshin at gmail.com