Sample Haskell FFI read

read.hs

foreign import ccall unsafe "read" c_read :: CInt -> Ptr () -> CSize -> IO CSsize

read :: Int -- ^ file descriptor
     -> Int64 -- ^ Bytes to allocate
     -> Word64 -- ^ Read upto this many bytes
     -> IO (ByteString, Int64)
read fd bytes len = do
  (ptr :: Ptr ()) <- mallocBytes (fromIntegral bytes)
  size <- c_read (fromIntegral fd) ptr (fromIntegral len)
  bstring <- packCString (castPtr ptr)
  free ptr
  return (bstring, (fromIntegral size))