CODEWITHSUNDEEP
c++multithreadingpthreads
abrahab
abrahab

Reputation: 2496

mutex cond wait class error or bug

I have problem with this simple wrapper of mutex / cond wait class. I use it to protect queue of connected clients and to notify threads in the wait state that there are new clients connected, but application crashed with Segmentation Failed at 

client_socket_fd = clients_for_threads_q.front();

when its is under stress test only (many clients connected )

Wait at the thread:

    MUTEXCONDSIGNAL_for_workers.lock();
    if ( clients_for_threads_q.empty() )
    {
        MUTEXCONDSIGNAL_for_workers.wait_signal();
    }

    client_socket_fd = clients_for_threads_q.front(); // CRASHED !!!!
    clients_for_threads_q.pop();
    MUTEXCONDSIGNAL_for_workers.unlock();

of course clients_for_threads_q ptotected on writing 

    MUTEXCONDSIGNAL_for_workers.lock();
    clients_for_threads_q.push (client_socket_fd);
    MUTEXCONDSIGNAL_for_workers.send_signal();
    MUTEXCONDSIGNAL_for_workers.unlock();

Upvotes: 2

Views: 177

Answers (1)

Mat
Mat

Reputation: 206841

pthread_cond_wait can have spurious wake ups.

When using condition variables there is always a Boolean predicate involving shared variables associated with each condition wait that is true if the thread should proceed. Spurious wakeups from the pthread_cond_wait() or pthread_cond_timedwait() functions may occur. Since the return from pthread_cond_wait() or pthread_cond_timedwait() does not imply anything about the value of this predicate, the predicate should be re-evaluated upon such return.

You need to change your waiter if condition to a loop:

while ( clients_for_threads_q.empty() )
{
    MUTEXCONDSIGNAL_for_workers.wait_signal();
}

Upvotes: 4

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