WaitForMultipleObjects info Overview Group

The WaitForMultipleObjects function returns when one of the following occurs:

Either any one or all of the specified objects are in the signaled state.
The time-out interval elapses.
DWORD WaitForMultipleObjects(

DWORD nCount,	// number of handles in the object handle array
CONST HANDLE *lpHandles,	// pointer to the object-handle array
BOOL bWaitAll,	// wait flag
DWORD dwMilliseconds	// time-out interval in milliseconds
);

Parameters

nCount: Specifies the number of object handles in the array pointed to by lpHandles. The maximum number of object handles is MAXIMUM_WAIT_OBJECTS.
lpHandles: 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. For more information, see Access Masks and Access Rights.
bWaitAll: Specifies the wait type. If TRUE, the function returns when the state all objects in the lpHandles array is signaled. If FALSE, the function returns when the state of any one of the objects set to is 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 conditions specified by the bWaitAll parameter are not 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.

Return Values

If the function succeeds, the return value indicates the event that caused the function to return.

If the function fails, the return value is WAIT_FAILED. To get extended error information, call GetLastError.

The return value upon success is one of the following values:

Value	Meaning
WAIT_OBJECT_0 to (WAIT_OBJECT_0 + nCount - 1)	If bWaitAll is TRUE, the return value indicates that the state of all specified objects is signaled. If bWaitAll is FALSE, the return value minus WAIT_OBJECT_0 indicates the lpHandles array index of the object that satisfied the wait. If more than one object became signalled during the call, this is the array index of the signalled object with the smallest index value of all the signalled objects.
WAIT_ABANDONED_0 to (WAIT_ABANDONED_0 + nCount - 1)	If bWaitAll is TRUE, the return value indicates that the state of all specified objects is signaled and at least one of the objects is an abandoned mutex object. If bWaitAll is FALSE, the return value minus WAIT_ABANDONED_0 indicates the lpHandles array index of an abandoned mutex object that satisfied the wait.
WAIT_TIMEOUT	The time-out interval elapsed and the conditions specified by the bWaitAll parameter are not satisfied.

Remarks

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

When bWaitAll is TRUE, the function’s wait operation is completed only when the states of all objects have been set to signaled. 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 are also set to signaled. In the meantime, some other thread may get ownership of the mutex, thereby setting its state to nonsignaled.

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 WaitForMultipleObjects function can specify handles of any of the following object types in the lpHandles array:

Object	Description
Change notification	The FindFirstChangeNotification function returns the handle. A change notification object’s state is signaled when a specified type of change occurs within a specified directory or directory tree.
Console input	The handle is returned by the CreateFile function when the CONIN$ value is specified, or by the GetStdHandle function. The object’s state is signaled when there is unread input in the console’s input buffer, and it is nonsignaled when the input buffer is empty.
Event	The CreateEvent or OpenEvent function returns the handle. An event object’s state is set explicitly to signaled by the SetEvent or PulseEvent function. A manual-reset event object’s state must be reset explicitly to nonsignaled by the ResetEvent function. For an auto-reset event object, the wait function resets the object’s state to nonsignaled before returning. Event objects are also used in overlapped operations, in which the state is set by the system.
Mutex	The CreateMutex or OpenMutex function returns the handle. A mutex object’s state is signaled when it is not owned by any thread. The wait function requests ownership of the mutex for the calling thread, changing the mutex’s state to nonsignaled when ownership is granted.
Process	The CreateProcess or OpenProcess function returns the handle. A process object’s state is signaled when the process terminates.
Semaphore	The CreateSemaphore or OpenSemaphore function returns the handle. A semaphore object maintains a count between zero and some maximum value. Its state is signaled when its count is greater than zero and nonsignaled when its count is zero. If the current state is signaled, the wait function decreases the count by one.
Thread	The CreateProcess, CreateThread, or CreateRemoteThread function returns the handle. A thread object’s state is signaled when the thread terminates.
Timer	The CreateWaitableTimer or OpenWaitableTimer function returns the handle. Activate the timer by calling the SetWaitableTimer function. The state of an active timer is signaled when it reaches its due time. You can deactivate the timer by calling the CancelWaitableTimer function. The state of an active timer is signaled when it reaches its due time. You can deactivate the timer by calling the CancelWaitableTimer function.

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.

Wait Functions and Creating Windows

You have to be careful when using the wait functions and code that directly or indirectly creates windows. If a thread creates any windows, it must process messages. Message broadcasts are sent to all windows in the system. If you have a thread that uses a wait function with no time-out interval, the system will deadlock. Two examples of code that indirectly creates windows are DDE and COM CoInitialize. Therefore, if you have a thread that creates windows, use MsgWaitForMultipleObjects or MsgWaitForMultipleObjectsEx, rather than WaitForMultipleObjects.