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