Re: DatagramSocket "misses" data on MacOS X (JVM 1.6.0_05)

Sebastian Staudt <>
Fri, 1 Aug 2008 13:32:38 -0700 (PDT)
I did more testing and the problem seems to be really hard to
Like said before, it's not a problem with the DatagramChannel, but
with the DatagramSocket.

Even without using a channel and without using select, the socket
still loses data (aka blocks forever).
Wireshark shows that the data has arrived, but my program is unable to
read it from the socket.

I'm pretty at a loss now, tried every possible code change and JVM
1.5.0 - nothing helps. :(

On 31 Jul., 19:46, Sebastian Staudt <> wrote:

On 31 Jul., 18:54, Owen Jacobson <> wrote:

On Jul 31, 12:01 pm, Sebastian Staudt <> wrote:


I have a problem with a call.
I'm using a DatagramChannel for server queries (i.e. client sends
request, server sends response).
After sending a request, I wait for the response with select(). The
DatagramChannel is registered with OP_READ.
Everything works fine on Linux and Windows, but on MacOS X select()
always returns 0. select() always times out or blocks infinetly
(depending on timeout argument). Wireshark shows that the request goe=


out and the response is received successfully, but the Selector won't

Thanks for your help.

Without seeing the code, it's going to be very difficult to offer a
useful suggestion. A demo program, included below, does not
demonstrate the bug.

import java.nio.ByteBuffer;
import java.nio.channels.DatagramChannel;
import java.nio.channels.SelectableChannel;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.util.Iterator;
import java.util.Set;

public class DatagramSelectDemo {
    public static void main(String[] args) throws IOException {
        Selector s =;
        try {
            DatagramChannel dgc =;
            try {
                dgc.socket().bind(new InetSocketAddress=


                dgc.register(s, SelectionKey.OP_READ);

                while (true) {

                    Set<SelectionKey> keys =


                    for (Iterator<SelectionKey> i =

= keys.iterator();

i.hasNext();) {
                        SelectionKey key = i.=


                        if (key.isReadable()) {

el channel =;

                            if (channel ==

= dgc) {


fer buffer =


ddress source =




     "Received %d bytes from %s\n",


     buffer.position(), source);


            } finally {
        } finally {


Sending messages to this with nc -u hostname 2600 triggers appropriate
"Received 16 bytes from /" messages.


I forgot to mention that this is happening on the client socket. So
your code doesn't exactly match my problem.

Here's a short version of my code:

public class Test
        public void main(String[] argv)
                throws Exception
                DatagramChannel dc = DatagramChannel.op=


                dc.connect(new InetSocketAddress(SOME_IP,=


                ByteBuffer bb = ByteBuffer.wrap("test".=



                Selector selector =;
                dc.register(selector, SelectionKey.OP_REA=


                if( == 0)
                        throw new TimeoutExceptio=



                bb = ByteBuffer.allocate(1500);

                /* MORE CODE HERE */


This code always bails out with a TimeoutException on MacOS X.
An additional note: Commenting out the Selector stuff results in to block forever.
So maybe it's not a Selector problem, but a DatagramSocket problem.

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