Try something like this:

module Main where

import Network.Socket

main = withSocketsDo $ do
    -- Make a UDP socket
    s <- socket AF_INET Datagram defaultProtocol

    -- We want to listen on all interfaces (0.0.0.0)
    bindAddr <- inet_addr "0.0.0.0"

    -- Bind to 0.0.0.0:30000
    bindSocket s (SockAddrInet 30000 bindAddr)

    -- Read a message of max length 1000 from some one
    (msg,len,from) <- recvFrom s 1000

    putStrLn $ "Got the following message from " ++ (show from)
    putStrLn msg

Does this help? As Stephan said, you missed the bind step.

/jve


On Sun, Jan 25, 2009 at 11:22 AM, Andrew Coppin <andrewcoppin@btinternet.com> wrote:
I'm trying to write a simple program that involves UDP. I was hoping something like this would work:

module Main where

import Network.Socket

main = withSocketsDo main2

main2 = do
 s <- socket AF_INET Datagram defaultProtocol
 putStrLn "Waiting..."
 x <- recv s 100
 putStrLn x

Unfortunately, that doesn't work at all. It immediately throws an exception ("unknown error"). But then, the whole module seems to be completely undocumented. I managed to find a tiny amount of info online about the underlying C API, but I still don't get how the Haskell interface is supposed to be used. Any hints?

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