What is the cleanest way to create a timeout for a while loop?

Question

Windows API/C/C++

1. ....  
2. ....
3. ....    
4.    while (flag1 != flag2)
5.    {
6.      SleepEx(100,FALSE);   
        //waiting for flags to be equal (flags are set from another thread).
7.    }
8. .....
9. .....  

If the flags don't equal each other after 7 seconds, I would like to continue to line 8.

Any help is appreciated. Thanks.

Answer 1

If you are waiting for a particular flag to be set or a time to be reached, a much cleaner solution may be to use an auto / manual reset event. These are designed for signalling conditions between threads and have very rich APIs designed on top of them. For instance you could use the WaitForMultipleObjects API which takes an explicit timeout value.

Answer 2

Do not poll for the flags to change. Even with a sleep or yield during the loop, this just wastes CPU cycles.

Instead, get the thread which sets the flags to signal you that they've been changed, probably using an event. Your wait on the event takes a timeout, which you can tweak to allow waiting of 7 seconds total.

For example:

Thread1:

 flag1 = foo;
 SetEvent(hEvent);


 Thread2:

 DWORD timeOutTotal = 7000;  // 7 second timeout to start.
 while (flag1 != flag2 && timeOutTotal > 0)
 {
     // Wait for flags to change
     DWORD start = GetTickCount();

     WaitForSingleObject(hEvent, timeOutTotal);

     DWORD end = GetTickCount();

    // Don't let timeOutTotal accidently dip below 0.
    if ((end - start) > timeOutTotal)
    {
        timeOutTotal = 0;
    }
    else
    {
       timeOutTotal -= (end - start);
    }

 }