Re: won't work on MacOS X (JVM 1.6.0_05)

Sebastian Staudt <>
Thu, 31 Jul 2008 10:46:40 -0700 (PDT)
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 goes
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(2=


                dgc.register(s, SelectionKey.OP_READ);

                while (true) {

                    Set<SelectionKey> keys = s.sele=


                    for (Iterator<SelectionKey> i ==


i.hasNext();) {
                        SelectionKey key =


                        if (key.isReadable()) {

 channel =;

                            if (channel ==

= dgc) {


r buffer =


ress 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 =;
        dc.connect(new InetSocketAddress(SOME_IP, SOME_PORT);

        ByteBuffer bb = ByteBuffer.wrap("test".getBytes());

        Selector selector =;
        dc.register(selector, SelectionKey.OP_READ);
        if( == 0)
            throw new TimeoutException();

        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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