What if the condition_variable::wait_for delay parameter changes during the wait process?

Question

I prefer using condition_variable::wait_for as a timer over chrono based timer since I can override the wait condition by specifying a predicate. So I wrote a test program to check what if the delay duration changes once the wait process has already started. The condition_variable::wait_for seems to ignore the change and instead comes out of the wait process entirely. Why is this? Can I ever change it in the middle of a wait process?

enum STATE {START,STOP,NONE};

STATE  state ;
condition_variable cv;
mutex mu;
chrono::milliseconds delay;

void cv_wait_thread()
{
    {
        unique_lock<mutex> lock(mu);
        cv.wait(lock, [](){ return state == START; });
    }

    chrono::time_point<chrono::high_resolution_clock> start_time = chrono::high_resolution_clock::now();

    {
        unique_lock<mutex> lock(mu);
        cv.wait_for(lock, delay);       
    }

    auto diff = chrono::duration_cast<chrono::milliseconds>(chrono::high_resolution_clock::now() - start_time);


    cout << "Conditional_wait:  "<< diff.count() << " milliseconds" << endl;

}


void main()
{
    thread thread1([](){ cv_wait_thread(); });

    state = NONE;   

    this_thread::sleep_for(chrono::seconds(1));

    delay = chrono::milliseconds(3000);

    state = START;
    cv.notify_all(); // ask thread to sleep for 3 sec

    this_thread::sleep_for(chrono::milliseconds(2000)); // let cv_wait thread start the wait process for at least 2 sec already

    delay = chrono::milliseconds(5000); // ask thread to correct and sleep for 5 sec instead

    cv.notify_all();

    thread1.join(); // thread prints 2000 milli
}

Sam Varshavchik · Accepted Answer · 2017-01-25 17:02:33Z

4

As you can see in the specification of the wait_for() method, the delay interval is specified as a const parameter:

template< class Rep, class Period >
std::cv_status wait_for( std::unique_lock<std::mutex>& lock,
                         const std::chrono::duration<Rep, Period>& rel_time);

Furthermore, there's nothing in the specification of wait_for() that indicates that any changes to the parameter will be observable by the thread that's executing the wait_for().

In fact, the only sequencing here comes as an indirect result of releasing and re-acquiring the underlying mutex. Note, however, that this will not occur until after the specified timeout expires. As such, any changes made to the delay parameter by another thread will not be sequenced until that time, and will not be observable by the thread that's executing the wait_for() call.

Note, that if the thread in question was doing something other than locking the mutex and executing wait_for(), unless the main execution thread explicitly does something that sequences its change to delay with respect to the other thread, the other thread is not guaranteed to observe the new value of delay ever.

answered Jan 25, 2017 at 17:02

Sam Varshavchik

119k6 gold badges109 silver badges164 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

ark1974 Over a year ago

Thanks for the comprehensive reply. But still that don't answer why the wait process breaks after the delay parameter value had changed.Why the original if not not the second value was not taken at all?

Sam Varshavchik Over a year ago

Because that's what your code told it to do. wait_for() returns when the condition variable has been notify()-ed, or the timeout expires, whichever comes first. Your code notify()-ed the condition variable, so wait_for() returned. Whether or not the delay value was changed is completely irrelevant.

Collectives™ on Stack Overflow

What if the condition_variable::wait_for delay parameter changes during the wait process?

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related