问题
I test the std::condition_variable using the below code:
class CondWait{
public:
std::condition_variable cv;
std::mutex mu;
int i=0;
public:
void mainTask(){
std::unique_lock<std::mutex> lk(mu);
cv.wait(lk);
i++;
std::cout<<"main task, "<<i<<std::endl;
}
void notifyTask(){
std::unique_lock<std::mutex> lk(mu);
i = 0;
std::cout<<"notify task, "<<i<<std::endl;
cv.notify_one();
std::cout<<"notify task, sleep 5 sec"<<std::endl;
std::this_thread::sleep_for(std::chrono::seconds(5));
}
};
int main()
{
CondWait condwait;
std::thread t1(&CondWait::mainTask,&condwait);
std::thread t2(&CondWait::notifyTask,&condwait);
t1.join();
t2.join();
return 0;
}
Sometimes, the output is below and the program is blocked:
notify task, 0
notify task, sleep 5 sec
Sometimes, the program will run well,i.e. after 5 seconds of sleep, it will output main task, 1, the complete output is:
notify task, 0
notify task, sleep 5 sec
main task, 1
In my opinion, int the notifyTask thread, the mutex is still used after notify_one, so the wait in the mainTask cannot lock the mutex. But I don't know what will happen next, why the example will have an ambiguity performance. Could you please provide some advice? Thanks a lot!
回答1:
Since you start both threads concurrently perhaps a situation occurs when cv.notify_one() is called before cv.wait() in mainTask and since no threads are waiting yet, the function does nothing. After that, wait() is executed and hangs waiting for a notification
来源：https://stackoverflow.com/questions/62636456/what-happen-between-after-notify-all-and-before-wait-get-the-lock