On 21/08/06, Udo Stenzel
Neil Mitchell wrote:
I'm trying to write out a binary file, in particular I want the following functions:
hPutInt :: Handle -> Int -> IO ()
hGetInt :: Handle -> IO Int
For the purposes of these functions, Int = 32 bits, and its got to roundtrip - Put then Get must be the same.
How would I do this? I see Ptr, Storable and other things, but nothing which seems directly useable for me.
hPutInt h = hPutStr h . map chr . map (0xff .&.) . take 4 . iterate (`shiftR` 8)
hGetInt h = replicateM 4 (hGetChar h) >>= return . foldr (\i d -> i `shiftL` 8 .|. ord d) 0
This of course assumes that a Char is read/written as a single low-order byte without any conversion. But you'd have to assume a lot more if you started messing with pointers. (Strange, somehow I get the feeling, the above is way too easy to be the answer you wanted.)
Udo.
What's wrong with the following i.e. what assumptions is it making (w.r.t. pointers) that I've missed? Is endian-ness an issue here? Alistair hPutInt :: Handle -> Int32 -> IO () hGetInt :: Handle -> IO Int32 int32 :: Int32 int32 = 0 hPutInt h i = do alloca $ \p -> do poke p i hPutBuf h p (sizeOf i) hGetInt h = do alloca $ \p -> do bytes <- hGetBuf h p (sizeOf int32) when (bytes < sizeOf int32) (error "too few bytes read") peek p