Socket Options

[Glynn Clements]

Peter Simons wrote:

> the Network module provides the data type SocketOption. I am
> particularly interested in setting the RecvTimeOut and
> SendTimeOut values, but I wonder how to set them. The
> function
> 
>   setSocketOption :: Socket -> SocketOption -> Int -> IO ()
> 
> allows me only 'Int' parameters, but the kernel expects a
> struct timeval here -- or more accurately, a pointer to one.
> Do I really have to engage in FFI pointer wizardry here, or
> is there a simpler way to set these values?

You can't set the send/receive timeouts using that function; it always
passes a C int (with the optlen parameter set to sizeof(int)). You
would have to re-write it with a more flexible interface, e.g.:

> import Foreign
> import Foreign.C
> import Network.Socket hiding (setSocketOption)
> 
> foreign import CALLCONV unsafe "setsockopt"
>   c_setsockopt :: CInt -> CInt -> CInt -> Ptr () -> CInt -> IO CInt
> 
> setSocketOption :: (Storable a) => Socket 
> 		-> SocketOption -- Option Name
> 		-> a		-- Option Value
> 		-> IO ()
> setSocketOption (MkSocket s _ _ _ _) so v = do
>    with v $ \ptr_v -> do
>    throwErrnoIfMinus1_ "setSocketOption" $
>        c_setsockopt s (socketOptLevel so) (packSocketOption so) (castPtr ptr_v)
> 	  (fromIntegral (sizeOf v))
>    return ()

Note: neither socketOptLevel nor packSocketOption are exported from
Network.Socket, so you would need to copy those (or just pass a pair
of integers instead of the SocketOption).

> Am I even supposed to set them, or is there a better way to
> specify general I/O timeouts than on the socket level?

Non-blocking I/O and select/poll; although I don't know how well that
is supported.

-- 
Glynn Clements <glynn.clements at virgin.net>