Author: James CE Johnson

ACE Tutorial 017
Using the ACE_Barrier synch object

First, lets take a look at the main() routine and how it will use the Barrier wrapper class. A simple ACE_Task derivative is used so that we can perform work in multiple threads. These threads will use the barrier to synch in a couple of places.

Obviously this isn't a very realistic example but you should be able to get the idea of how to use a Barrier without getting hung up in application-level details.

    // page02.html,v 1.8 1999/04/03 21:56:32 jcej Exp

    #include "Barrier_i.h"
    #include "ace/Task.h"

    /* We'll use a simple Task<> derivative to test our new Barrier
       object.
    */
    class Test : public ACE_Task<ACE_NULL_SYNCH>
    {
    public:

            // Construct the object with a desired thread count
        Test(int _threads);

            // Open/begin the test.  As usual, we have to match the
            // ACE_Task signature.
        int open(void * _unused = 0);

            // Change the threads_ value for the next invocation of open()
        void threads(int _threads);

            // Get the current threads_ value.
        int threads(void);

            // Perform the test
        int svc(void);

    protected:
            // How many threads the barrier will test.
        int threads_;

            // The Barrier object we'll use in our tests below
        Barrier barrier_;
    };

    /* Construct the object & initialize the threads value for open() to
       use.
    */
    Test::Test(int _threads)
            : threads_(_threads)
    {
    }

    /* As usual, our open() will create one or more threads where we'll do
       the interesting work.
    */
    int Test::open(void * _unused)
    {
        ACE_UNUSED_ARG(_unused);

            // One thing about the barrier:  You have to tell it how many
            // threads it will be synching.  The threads() mutator on my
            // Barrier class lets you do that and hides the implementation
            // details at the same time.
        barrier_.threads(threads_);

            // Activate the tasks as usual...
        return this->activate(THR_NEW_LWP, threads_);
    }

    void Test::threads(int _threads)
    {
        threads_ = _threads;
    }

    int Test::threads(void)
    {
        return threads_;
    }

    /* svc() will execute in each thread & do a few things with the
       Barrier we have.
     */
    int Test::svc(void)
    {
        ACE_DEBUG ((LM_INFO, "(%P|%t|%T)\tTest::svc() Entry\n"));

            // Initialize the random number generator.  We'll use this to
            // create sleep() times in each thread.  This will help us see
            // if the barrier synch is working.
        ACE_Time_Value now(ACE_OS::gettimeofday());
        ACE_RANDR_TYPE seed = now.usec();
        ACE_OS::srand(seed);
        int delay;

            // After saying hello above, sleep for a random amount of time
            // from 1 to 6 seconds.  That will cause the next message
            // "Entering wait()" to be staggered on the output as each
            // thread's sleep() returns.
        delay = ACE_OS::rand_r(seed)%5;
        ACE_OS::sleep(abs(delay)+1);

            // When executing the app you should see these messages
            // staggered in an at-most 6 second window.  That is, you
            // won't likely see them all at once.
        ACE_DEBUG ((LM_INFO, "(%P|%t|%T)\tTest::svc() Entering wait()\n"));

            // All of the threads will now wait at this point.  As each
            // thread finishes the sleep() above it will join the waiters.
        if( barrier_.wait() == -1 )
        {
            ACE_DEBUG ((LM_INFO, "(%P|%t|%T)\tbarrier_.wait() failed!\n"));
            return 0;
        }

            // When all threads have reached wait() they will give us this
            // message.  If you execute this, you should see all of the
            // "Everybody together" messages at about the same time.
        ACE_DEBUG ((LM_INFO, "(%P|%t|%T)\tTest::svc() Everybody together?\n"));

            // Now we do the sleep() cycle again...
        delay = ACE_OS::rand_r(seed)%5;
        ACE_OS::sleep(abs(delay)+1);

            // As before, these will trickle in over a few seconds.
        ACE_DEBUG ((LM_INFO, "(%P|%t|%T)\tTest::svc() Entering done()\n"));

            // This time we call done() instead of wait().  done()
            // actually invokes wait() but before returning here, it will
            // clean up a few resources.  The goal is to prevent carrying
            // around objects you don't need.
        if( barrier_.done() == -1 )
        {
            ACE_DEBUG ((LM_INFO, "(%P|%t|%T)\tbarrier_.done() failed!\n"));
            return 0;
        }

            // Since done() invokes wait() internally, we'll see this
            // message from each thread simultaneously
        ACE_DEBUG ((LM_INFO, "(%P|%t|%T)\tTest::svc() Is everyone still here?\n"));

            // A final sleep()
        delay = ACE_OS::rand_r(seed)%5;
        ACE_OS::sleep(abs(delay)+1);

            // These should be randomly spaced like all of the other
            // post-sleep messages.
        ACE_DEBUG ((LM_INFO, "(%P|%t|%T)\tTest::svc() Chaos and anarchy for all!\n"));

        return(0);
    }

    /* Our test application...
     */
    int main(int, char**)
    {
            // Create the test object with 10 threads
        Test test(10);

            // and open it to test the barrier.
        test.open();
            // Now wait for them all to exit.
        test.wait();

            // Re-open the Test object with just 5 threads
        test.threads(5);
        test.open();
            // and wait for them to complete also.
        test.wait();

        return(0);
    }

[Tutorial Index] [Continue This Tutorial]