What's the purpose of SignalObjectAndWait regards there is SetEvent and WaitForSingleObject?

My understanding is that this single function is more efficient in the way that it avoid the following scenario.

The SignalObjectAndWait function provides a more efficient way to signal one object and then wait on another compared to separate function calls such as SetEvent followed by WaitForSingleObject.

When you you SetEvent and another [esp. higher priority] thread is waiting on this event, it might so happen that thread scheduler takes control away from the signaling thread. When the thread receives control back, the only thing that it does is the following WaitForSingleObject call, thus wasting context switch for such a tiny thing.

Using SignalObjectAndWait you hint the kernel by saying "hey, I will be waiting for another event anyway, so if it makes any difference for you don't excessively bounce with context switches back and forth".

The purpose, as MSDN explains is to ensure that the thread is in a Wait State BEFORE the event is signalled. If you call WaitForSingleObject, the thread is in a waitstate, but you can't call that BEFORE calling SetEvent, since that will cause SetEvent to happen only AFTER the wait has finished - which is pointless if nothing else is calling SetEvent.

As you know, Microsoft gives the following example of why we may ever need SignalObjectAndWait if we already need separate SetEvent and WaitForSingleObject (quote the Microsoft example):

A thread can use the SignalObjectAndWait function to ensure that a worker thread is in a wait state before signaling an object. For example, a thread and a worker thread may use handles to event objects to synchronize their work. The thread executes code such as the following:

dwRet = WaitForSingleObject(hEventWorkerDone, INFINITE);if( WAIT_OBJECT_0 == dwRet)  SetEvent(hEventMoreWorkToDo);

The worker thread executes code such as the following:

dwRet = SignalObjectAndWait(hEventWorkerDone,                            hEventMoreWorkToDo,                            INFINITE,                             FALSE);

This algorithm flow is flawed and should never be used. We do not need such a perplexing mechanism where the threads notify each other until we are in a “Race condition”. Microsoft itself in this example creates the Race Condition. The worker thread should just wait for an event and take tasks from a list, while the thread that generates tasks should just add tasks to this list and signal the event. So, we just need one event, not two as in the above Microsoft example. The list has to be protected by a critical section. The thread that generates tasks should not wait for the worker thread to complete the tasks. If there are tasks that require to notify somebody on their completion, the tasks should send the notifications by themselves. In other words, it is the task who will notify the thread on completion -- it is not the thread who will specifically wait for the jobs thread until it finishes processing all the tasks.

Such a flawed design, as in the Microsoft Example, creates imperative for such monsters like atomic SignalObjectAndWait and atomic PulseEvent -- function that ultimately lead to doom.

Here is an algorithm how can you achieve you goal set in your question. The goal is achieved with just plain and simple events, and simple function SetEvent and WaitForSingleObject - no other functions needed.

1. Create one common auto-reset event for all job threads to signal that there is a task (tasks) available; and also create per-thread auto-reset events, one event for each job thread.
2. Multiple job treads, once finished running all the jobs, all wait for this common auto-reset “task available” event using WaitForMultipleObjects - it waits two event - the common event and the own thread event.
3. The scheduler thread puts new (pending) jobs to the list.
4. The jobs list access has to be protected by EnterCriticalSection/LeaveCriticalSection, so no one ever accesses this list the other way.
5. Each of the job threads, after completing one job, before starting to wait for the auto-reset “task available” event and its own event, checks the pending jobs list. If the list is not empty, get one job from the list (remove it from the list) and execute it.
6. There have to be another list protected by critical section – waiting jobs thread list.
7. Before each jobs tread starts waiting, i.e. before it calls WaitForMultipleObjects, it adds itself to the “waiting” list. On exit from wait, it removes itself from this waiting list.
8. When the scheduler thread puts new (pending) jobs to the jobs list, it first enters critical section of the jobs list and then of the treads list - so two critical sections are entered simultaneously. The jobs threads, however, may never enter both critical sections simultaneously.
9. If there is just one job pending, the scheduler sets the common auto-reset event to the signaled state (call SetEvent) -- it doesn’t matter which of the sleeping job threads will pick up the job.
10. If there are two or more jobs pending, it would not signal the common event, but will count how many threads are waiting. If there are at least as many threads waiting as there are the jobs, signal the own event of that number of threads as there are events, and leave the remaining thread to continue their sleeping.
11. If there are more jobs than waiting threads, signal the own event for each of the waiting thread.
12. After the scheduler thread has signaled all the events, it leaves the critical sections - first of the thread list, and then of the jobs list.
13. After the scheduler thread has signaled all the events needed for the particular case, it goes to sleep itself, i.e. calls WaitForSingleObject with its own sleep event (that is also an auto-reset event that should be signaled whenever a new job appears).
14. Since the jobs threads will not start to sleep until the whole jobs list is depleted, you will no longer need the scheduler thread again. The scheduler thread will only be needed later, when a new jobs appears, not when a job is finished by the jobs thread.

Important: this scheme is based purely on auto-reset events. You won’t ever need to call ResetEvent. All the functions that are needed are: SetEvent and WaitForMultipleObjects (or WaitForSingleObject). No atomic event operation is needed.

Please note: when I wrote that a thread sleeps, it doesn't call "Sleep" API call - it will never be needed, it just is in the "wait" state as a result of calling WaitForMultipleObjects (or WaitForSingleObject).

As you know, auto-reset event, and the SetEvent and WaitForMultipleObjects function are very reliable. They exist since NT 3.1. You may always architect such a program logic that will solely rely on these simple functions -- so you would not ever need complex and unreliable functions that presume atomic operations, like PulseEvent or SignalObjectAndWait. By the way, SignalObjectAndWait did only appear in Windows NT 4.0, while SetEvent and WaitForMultipleObjects did exist from the initial version of Win32 – NT 3.1.