WaitForMultipleObjects

The WaitForMultipleObjects function returns when one of the following occurs: 

DWORD WaitForMultipleObjects(
  DWORD nCount,             // number of handles in the handle array
  CONST HANDLE *lpHandles,  // pointer to the object-handle array
  BOOL fWaitAll,            // 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
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 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. This value can be one of the following.

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.

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

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

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 WaitForMultipleObjects.

Windows CE: Windows CE does not support waiting for semaphores, change notification objects, console input, and timers.

QuickInfo

  Windows NT: Requires version 3.1 or later.
  Windows: Requires Windows 95 or later.
  Windows CE: Requires version 1.0 or later.
  Header: Declared in winbase.h.
  Import Library: Use kernel32.lib.

See Also

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