condition-variable

I'll present some code first since explaining is easier that way. Assume that mutexes are correctly used with the condition variables to keep it simple: // Thread 1 while(1) { conditionVariable.wait(); // Do some work } // Thread 2 while(1) { // Do some work conditionVariable.notify_one(); } // Thread 3 while(1) { // Do some work conditionVariable.notify_one(); } What I would like to achieve is that thread 1 is guaranteed to be waiting on the condition variable when thread 2 or Thread 3 notifies. As the code stands, there is a large gap between notify_one() and wait() in the form of some other

For simplicity, let's assume that we have only one conditional variable to match a single condition that is reflected by a boolean. 1) Why does std::condition_variable::wait(...) locks the mutex again after a "notify" has been sent to un-sleep it? 2) Seeing the behaviour in "1)", does that mean that when you do std::condition_variable::notify_all it only makes it so that all of the waiting threads are unblocked/woken up... but in order instead of all at once? If so, what can be done to do it all at once? 3) If I only care about threads sleeping until a condition is met and not care a single

Consider the following simplistic example of condition variables: bool pause = true; boost::mutex::scoped_lock lock(m_mutex); while (!pause) cv.wait(lock); and boost::mutex::scoped_lock lock(m_mutex); pause = false; cv.notify_one(); Do we essentially need the scoped_lock or any other lock here, if we are running the code on a processor which supports updates on byte-granularity. This essentially means that the assignment of bools are atomic which is typically the case with x86 processors. Has it got something to do with syncing of the variable in the case when the two threads are running on

I've looked into the VC++ implementation of std::condition_variable(lock,pred) , basically, it looks like this: template<class _Predicate> void wait(unique_lock<mutex>& _Lck, _Predicate _Pred) { // wait for signal and test predicate while (!_Pred()) wait(_Lck); } Basically , the naked wait calls _Cnd_waitX which calls _Cnd_wait which calls do_wait which calls cond->_get_cv()->wait(cs); (all of these are in the file cond.c). cond->_get_cv() returns Concurrency::details::stl_condition_variable_interface . If we go to the file primitives.h , we see that under windows 7 and above, we have the

As you know, condition variables should be called in cycle to avoid spurious wake-ups. Like this: while (not condition) condvar.wait(); If another thread wants to wake up waiting thread, it must set condition flag to true. E.g.: condition = true; condvar.notify_one(); I wonder, is it possible for condition variable to be blocked by this scenario: 1)Waiting thread checks condition flag, and finds it is equal to FALSE, so, it's going to enter condvar.wait() routine. 2)But just before this (but after condition flag checking) waiting thread is preempted by kernel (e.g. because of time slot

I have thread 1 executing the following code: unique_lock<mutex> ul(m); while(condition == true) cv.wait(ul); And thread 2 executing this code: condition = false; cv.notify_one(); Unfortunately I'm hitting a timing issue: T1: condition checks true T2: condition set to false T2: cv.notify_one() T1: cv.wait() Thread 1 misses the notification completely and remains blocked on wait(). I tried using the version of wait() which takes a predicate but with essentially the same result. That is, the body of the predicate performs the check, but before it returns, the condition's value is changed and the

As it is said that Mutex are needed to protect the Condition Variables. Is the reference here to the actual condition variable declared as pthread_cond_t OR A normal shared variable count whose values decide the signaling and wait. ? is the reference here to the actual condition variable declared as pthread_cond_t or a normal shared variable count whose values decide the signaling and wait? The reference is to both. The mutex makes it so that the shared variable ( count in your question) can be checked and if the value of that variable doesn't meet the desired condition the wait that is

I have a situation where a notify() 'can' be called before a wait(). I am trying to make a simulator to schedule its next event when I 'notify' him by sending him messages. So I have devised a wait->notify->scedule chain void Broker::pause() { boost::unique_lock<boost::mutex> lock(m_pause_mutex); { std::cout << "pausing the simulation" << std::endl; m_cond_cnn.wait(lock); std::cout << "Simulation UNpaused" << std::endl; // the following line causes the current function to be called at // a later time, and a notify() can happen before the current function // is called again Simulator::Schedule