将一个线程与两个正在运行的线程（C ++，Windows）同步

Question

I am a newbie in multithread programming and this is my first post here! So please bear with me.

I have two cameras which I would like to track an object simultaneously (two independent threads) and output its position to the main function or perhaps a third thread. Using the position of the object from each camera, the 3D position of the object should then be calculated. Let's say the first camera outputs x1 and y1 , the second camera outputs x2 and y2 and these should be used to estimate x , y , and z .

I was thinking of defining x1 , y1 , x2 and y2 as global variables so that they are easily accessible for the third thread. But the problem is that the third thread should be synchronized with the first 2 threads. The third thread doesn't change the content of x1 , y1 , x2 and y2 . It only uses these values to obtain x , y and z . But if the value x1 and y1 is updated and x2 and y2 are not yet updated, I want the third thread to pause until the x2 and y2 is updated. Or in other words, I want the third thread to use x1 , y1 , x2 and y2 that are ideally obtained at a time point t , or during a very short period of time.

I appreciate any suggestions on how to approach this problem. I am thinking of using CreateThread(). Is there an easier way? Is it a good idea to use global variable in multithread programming? Is it a good idea to output global variables t1 and t2 (system time) from thread 1 and 2 respectively, and compare them in the third thread? One problem with this approach might be that since both threads 1 and 2 are doing the same thing, it takes let's say T second for them to complete their task and therefore there is always a fixed lag between these threads which might be longer that what we want. And as a result, thread 3 would never find x1 , y1 , x2 and y2 which are obtained very close in time!

NOTE: I am using windows 7, visual studio 2010, programming language C++.

Answer 1

Thread 1 can SetEvent(event1) and thread 2 can SetEvent(event2). Thread 3 uses WaitForMultipleObjects to wait until both events have been set. WaitForMultipleObjects suspends the calling thread and then returns when both events are set.