MsgWaitForMultipleObjects info  Overview  Group

The MsgWaitForMultipleObjects function returns when one of the following occurs:

• Either any one or all of the specified objects are in the signaled state. The objects can include input event objects, which you specify using the dwWakeMask parameter.

• The time-out interval elapses.

Note that MsgWaitForMultipleObjects does not return if there is unread input of the specified type in the message queue after the thread has called a function to check the queue. This is because functions such as PeekMessage, GetMessage, WaitMessage, MsgWaitForMultipleObjects, and MsgWaitForMultipleObjectsEx check the queue and then change the state information for the queue so that the input is no longer considered new. A subsequent call to MsgWaitForMultipleObjects will not return until new input of the specified type arrives. The existing unread input is ignored.

DWORD MsgWaitForMultipleObjects(

Parameters

nCount

Specifies the number of object handles in the array pointed to by pHandles. The maximum number of object handles is MAXIMUM_WAIT_OBJECTS minus one.

pHandles

Points to an array of object handles. For a list of the object types whose handles can be specified, see the following Remarks section. The array can contain handles of objects of different types.

Windows NT: The handles must have SYNCHRONIZE access.

fWaitAll

Specifies the wait type. If TRUE, the function returns when the states of all objects in the pHandles array, including input events, have been set to signaled. If FALSE, the function returns when the state of any one of the objects is set to signaled. In the latter case, the return value indicates the object whose state caused the function to return.

dwMilliseconds

Specifies the time-out interval, in milliseconds. The function returns if the interval elapses, even if the criteria specified by the fWaitAll or dwWakeMask parameter have not been met. If dwMilliseconds is zero, the function tests the states of the specified objects and returns immediately. If dwMilliseconds is INFINITE, the function’s time-out interval never elapses.

dwWakeMask

Specifies input types for which an input event object handle will be added to the array of object handles. This parameter can be any combination of the following values:

Value	Meaning
QS_ALLINPUT	Any message is in the queue.
QS_HOTKEY	A WM_HOTKEY message is in the queue.
QS_INPUT	An input message is in the queue.
QS_KEY	A WM_KEYUP, WM_KEYDOWN, WM_SYSKEYUP, or WM_SYSKEYDOWN message is in the queue.
QS_MOUSE	A WM_MOUSEMOVE message or mouse-button message (WM_LBUTTONUP, WM_RBUTTONDOWN, and so on).
QS_MOUSEBUTTON	A mouse-button message (WM_LBUTTONUP, WM_RBUTTONDOWN, and so on).
QS_MOUSEMOVE	A WM_MOUSEMOVE message is in the queue.
QS_PAINT	A WM_PAINT message is in the queue.
QS_POSTMESSAGE	A posted message (other than those just listed) is in the queue.
QS_SENDMESSAGE	A message sent by another thread or application is in the queue.
QS_TIMER	A WM_TIMER message is in the queue.

Return Values

If the function succeeds, the return value indicates the event that caused the function to return. The successful return value is one of the following:

If the function fails, the return valueis 0xFFFFFFFF. To get extended error information, call GetLastError.

Remarks

The MsgWaitForMultipleObjects function determines whether the wait criteria have been met. If the criteria have not been met, the calling thread enters an efficient wait state, using very little processor time while waiting for the conditions of the wait criteria to be met.

The function does not modify the states of the specified objects until the states of all objects have been set to signaled. For example, a mutex can be signaled, but the thread does not get ownership until the states of the other objects have also been set to signaled. In the meantime, some other thread may get ownership of the mutex, thereby setting its state to nonsignaled.

When fWaitAll is TRUE, the function’s wait is completed only when the states of all objects have been set to signaled, including the input events specified by dwWaskMask. Therefore, setting fWaitAll to TRUE prevents input from being processed until the state of all objects in the pHandles array have been set to signaled. For this reason, if you set fWaitAll to TRUE, you should use a short timeout value in dwMilliseconds. If you have a thread that creates windows waiting for all objects in the pHandles array, including input events specified by dwWakeMask, with no timeout interval, the system will deadlock. This is because threads that create windows must process messages. DDE sends message to all windows in the system. Therefore, if a thread creates windows, do not set the fWaitAll parameter to TRUE in calls to MsgWaitForMultipleObjects made from that thread.

Before returning, a wait function modifies the state of some types of synchronization objects. Modification occurs only for the object or objects whose signaled state caused the function to return. For example, the count of a semaphore object is decreased by one.

The MsgWaitForMultipleObjects function can specify handles of any of the following object types in the pHandles array:

In some circumstances, you can specify a handle of a file, named pipe, or communications device as a synchronization object in lpHandles. However, their use for this purpose is discouraged.

See Also

CancelWaitableTimer, CreateEvent, CreateFile, CreateMutex, CreateProcess, CreateRemoteThread, CreateSemaphore, CreateThread, FindFirstChangeNotification, GetStdHandle, MsgWaitForMultipleObjectsEx OpenEvent, OpenMutex, OpenProcess, OpenSemaphore, OpenWaitableTimer, PulseEvent, ResetEvent, SetEvent

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