WaitForMultipleObjectsEx

The WaitForMultipleObjectsEx function returns when one of the following occurs:

• Either any one or all of the specified objects are in the signaled state.
• An I/O completion routine or asynchronous procedure call (APC) is queued to the thread.
• The time-out interval elapses.

DWORD WaitForMultipleObjectsEx(
  DWORD nCount,             // number of handles in handle array
  CONST HANDLE *lpHandles,  // points to the object-handle array
  BOOL fWaitAll,            // wait flag
  DWORD dwMilliseconds,     // time-out interval in milliseconds
  BOOL bAlertable           // alertable wait flag
);
 

Parameters

nCount

Specifies the number of object handles to wait for in the array pointed to by lpHandles. The maximum number of object handles is MAXIMUM_WAIT_OBJECTS.

lpHandles

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.

Windows NT: 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

Specifies the wait type. If TRUE, the function returns when the states all objects in the lpHandles array are 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 bWaitAll parameter are not met and no completion routines or APCs are queued. If dwMilliseconds is zero, the function tests the states of the specified objects and checks for queued completion routines or APCs and then returns immediately. If dwMilliseconds is INFINITE, the function's time-out interval never elapses.

bAlertable

Specifies whether the function returns when the system queues an I/O completion routine or APC. If TRUE, the function returns and the completion routine or APC function is executed. If FALSE, the function does not return and the completion routine or APC function is not executed.

A completion routine is queued when the ReadFileEx or WriteFileEx function in which it was specified has completed. The wait function returns and the completion routine is called only if bAlertable is TRUE and the calling thread is the thread that initiated the read or write operation. An APC is queued when you call QueueUserAPC.

Return Values

If the function succeeds, the return value indicates the event that caused the function to return. This value can be one of the following.

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

Remarks

The WaitForMultipleObjectsEx 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 criteria to be met.

When fWaitAll 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. 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 WaitForMultipleObjectsEx function can specify handles of any of the following object types in the lpHandles array:

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

For more information, see Synchronization Objects.

Use caution when calling 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. A thread that uses a wait function with no time-out interval may cause the system to become deadlocked. 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 WaitForMultipleObjectsEx.

QuickInfo

  Windows NT: Requires version 3.1 or later.
  Windows: Requires Windows 95 or later.
  Windows CE: Unsupported.
  Header: Declared in winbase.h.
  Import Library: Use kernel32.lib.

See Also

Synchronization Overview, Synchronization Functions, CancelWaitableTimer, CreateEvent, CreateFile, CreateMutex, CreateProcess, CreateRemoteThread, CreateSemaphore, CreateWaitableTimer, CreateThread, FindFirstChangeNotification, GetStdHandle, MsgWaitForMultipleObjects, MsgWaitForMultipleObjectsEx, OpenEvent, OpenMutex, OpenProcess, OpenSemaphore, OpenWaitableTimer, PulseEvent, QueueUserAPC, ReadFileEx, ResetEvent, SetEvent, SetWaitableTimer, WriteFileEx