Question

I am using WaitForSingleObject() function for implementing wait in my program.

WaitForSingleObject(eventToBeSigaled, timeOut);

all of us know that this function wait for the event to be signaled for the specified amount of time.

But I want to know that what happens when the event has already singled before entering to this call, at that time is the wait will fail(WAIT_FAILED)? Please answer this with proper reason. I want to know this little deeper.

Answer 1

Answer to your first question: if the event is already signaled, your Wait() would return immediately returning WAIT_OBJECT_0.

Second question: One of the circumstances WAIT_FAILED is returned is if the event handle is closed when Wait() is called. In this case, the OS scheduler won’t be able to process the Wait() call and hence returns WAIT_FAILED.

Note that at the end of the day, the Wait() functions are a means for the user threads to pass the CPU back to the OS until a certain condition is met. Depending on your needs, you use one of the OS primitives as a condition (semaphore, mutex, events, etc). The OS scheduler in turn checks this condition to determine if your worker thread should be given CPU time slice thereby ensuring that all threads (and hence all processes) get a fair share of the system resources.