I would try forking your sending thread, too, rather than using the main thread for sending. Alternatively, use the main thread for receiving and fork your sending thread. – Kyle Marek-Spartz On Sep 23, 2014, 10:14:09 AM, Ben Gunton <ben.gunton@gmail.com> wrote: I am trying to send multicast messages from one thread, and receive them on another. The sending thread reads lines from a file and sends them. However, the receiving thread does not receive all the messages - it seems to miss the last 10 message or so. If I run other programs to listen for the multicast message, they receive them fine. So I think the issue is something to do with receiving, not sending. I keep the program alive with a threadDelay, so it shouldn't be halting prematurely. If I add a small delay after each send, it works fine. And if I make the input file smaller, it works fine. On the provided data file, on my system (64-bit Ubuntu, GHC 7.8.2), the receiver fails to receive lines 125-140. Compiled with no optimizations. Thanks for any help! The relevant code snippets are below, and the sample data file and full program are attached. -- Loop to receive all the packets let receiveMulticast = do (msg, _) <- recvFrom recvSock 32628 putStrLn . BS.unpack $ BS.take 100 msg receiveMulticast _ <- forkIO receiveMulticast -- Send every line from a file as multicast message inputFile <- openFile "data" ReadMode fileLines <- BS.lines <$> BS.hGetContents inputFile let sendMulticast msg = do sendTo sendSock msg addr -- Receiver FAILS to receive last few messages unless this -- thread delay exists... why?! -- threadDelay (1) mapM_ sendMulticast fileLines hClose inputFile threadDelay (1000*1000*1000) -- Delay for 1000 seconds - data, 71 KB - Main.hs, 2.2 KB _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe