MsgWaitForMultipleObjects

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.

To enter an alertable wait state, use the MsgWaitForMultipleObjectsEx function.

Note  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, GetQueueStatus, and WaitMessage 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 (received prior to the last time the thread checked the queue) is ignored.

DWORD MsgWaitForMultipleObjects(
  DWORD nCount,          // number of handles in array
  CONST HANDLE pHandles, // object-handle array
  BOOL fWaitAll,         // wait option
  DWORD dwMilliseconds,  // time-out interval
  DWORD dwWakeMask       // input-event type
);

Parameters

nCount

[in] 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

[in] Pointer 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. It may not contain the multiple copies of the same handle.

If one of these handles is closed while the wait is still pending, the function's behavior is undefined.

Windows NT/2000: The handles must have SYNCHRONIZE access. For more information, see Standard Access Rights.

Windows 95: No handle may be a duplicate of another handle created using DuplicateHandle.

fWaitAll

[in] Specifies the wait type. If TRUE, the function returns when the states of all objects in the pHandles array have been set to signaled and an input event has been received. If FALSE, the function returns when the state of any one of the objects is set to signaled or an input event has been received. In this case, the return value indicates the object whose state caused the function to return.

dwMilliseconds

[in] 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

[in] 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_ALLEVENTS	An input, WM_TIMER, WM_PAINT, WM_HOTKEY, or posted message is in the queue. This value is a combination of QS_INPUT, QS_POSTMESSAGE, QS_TIMER, QS_PAINT, and QS_HOTKEY.
QS_ALLINPUT	Any message is in the queue. This value is a combination of QS_INPUT, QS_POSTMESSAGE, QS_TIMER, QS_PAINT, QS_HOTKEY, and QS_SENDMESSAGE.
QS_ALLPOSTMESSAGE	A posted message is in the queue. This value is cleared when you call GetMessage or PeekMessage, whether or not you are filtering messages.
QS_HOTKEY	A WM_HOTKEY message is in the queue.
QS_INPUT	An input message is in the queue. This value is a combination of QS_MOUSE and QS_KEY.
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). This value is a combination of QS_MOUSEMOVE and QS_MOUSEBUTTON.
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 is in the queue. This value is cleared when you call GetMessage or PeekMessage without filtering messages.
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 value is -1. 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 the wait state. It uses no processor time while waiting for the conditions of the wait criteria to be met.

When fWaitAll is TRUE, 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 and an input event has been received. 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.

The 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. When fWaitAll is FALSE, and multiple objects are in the signaled state, the function chooses one of the objects to satisfy the wait; the states of the objects not selected are unaffected.

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

• Change notification
• Console input
• Event
• Job
• Mutex
• Process
• Semaphore
• Thread
• Waitable timer

For more information, see Synchronization Objects.

The QS_ALLPOSTMESSAGE and QS_POSTMESSAGE flags differ in when they are cleared. QS_POSTMESSAGE is cleared when you call GetMessage or PeekMessage, whether or not you are filtering messages. QS_ALLPOSTMESSAGE is cleared when you call GetMessage or PeekMessage without filtering messages (wMsgFilterMin and wMsgFilterMax are 0). This can be useful when you call PeekMessage multiple times to get messages in different ranges.

Requirements

  Windows NT/2000: Requires Windows NT 3.1 or later.
  Windows 95/98: Requires Windows 95 or later.
  Header: Declared in Winuser.h; include Windows.h.
  Library: Use User32.lib.

See Also

Synchronization Overview, Synchronization Functions, MsgWaitForMultipleObjectsEx