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WaitForMultipleObjectsEx function (synchapi.h)

  • Article

Waits until 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, or the time-out interval elapses.

Syntax

DWORD WaitForMultipleObjectsEx(
  [in] DWORD        nCount,
  [in] const HANDLE *lpHandles,
  [in] BOOL         bWaitAll,
  [in] DWORD        dwMilliseconds,
  [in] BOOL         bAlertable
);

Parameters

[in] nCount

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. This parameter cannot be zero.

[in] lpHandles

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 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.

The handles must have the SYNCHRONIZE access right. For more information, see Standard Access Rights.

[in] bWaitAll

If this parameter is TRUE, the function returns when the state of all objects in the lpHandles array is 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.

[in] dwMilliseconds

The time-out interval, in milliseconds. If a nonzero value is specified, the function waits until the specified objects are signaled, an I/O completion routine or APC is queued, or the interval elapses. If dwMilliseconds is zero, the function does not enter a wait state if the criteria is not met; it always returns immediately. If dwMilliseconds is INFINITE, the function will return only when the specified objects are signaled or an I/O completion routine or APC is queued.

Windows XP, Windows Server 2003, Windows Vista, Windows 7, Windows Server 2008 and Windows Server 2008 R2:  The dwMilliseconds value does include time spent in low-power states. For example, the timeout does keep counting down while the computer is asleep.

Windows 8, Windows Server 2012, Windows 8.1, Windows Server 2012 R2, Windows 10 and Windows Server 2016:  The dwMilliseconds value does not include time spent in low-power states. For example, the timeout does not keep counting down while the computer is asleep.

[in] bAlertable

If this parameter is TRUE and the thread is in the waiting state, the function returns when the system queues an I/O completion routine or APC, and the thread runs the routine or function. Otherwise, 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 value

If the function succeeds, the return value indicates the event that caused the function to return. It can be one of the following values. (Note that WAIT_OBJECT_0 is defined as 0 and WAIT_ABANDONED_0 is defined as 0x00000080L.)

Return code/value Description
WAIT_OBJECT_0 to (WAIT_OBJECT_0 + nCount– 1)
 If bWaitAll is TRUE, a return value in this range 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 signaled during the call, this is the array index of the signaled object with the smallest index value of all the signaled objects.
WAIT_ABANDONED_0 to (WAIT_ABANDONED_0 + nCount– 1)
 If bWaitAll is TRUE, a return value in this range 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. Ownership of the mutex object is granted to the calling thread, and the mutex is set to nonsignaled.

If a mutex was protecting persistent state information, you should check it for consistency.
WAIT_IO_COMPLETION
0x000000C0L
 The wait was ended by one or more user-mode asynchronous procedure calls (APC) queued to the thread.
WAIT_TIMEOUT
0x00000102L
 The time-out interval elapsed, the conditions specified by the bWaitAll parameter were not satisfied, and no completion routines are queued.
WAIT_FAILED
(DWORD)0xFFFFFFFF
 The function has failed. 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 the wait state until the conditions of the wait criteria have been met or the time-out interval elapses.

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.

When bWaitAll is FALSE, this function checks the handles in the array in order starting with index 0, until one of the objects is signaled. If multiple objects become signaled, the function returns the index of the first handle in the array whose object was signaled.

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. For more information, see the documentation for the individual synchronization objects.

To wait on more than MAXIMUM_WAIT_OBJECTS handles, use one of the following methods:

  • Create a thread to wait on MAXIMUM_WAIT_OBJECTS handles, then wait on that thread plus the other handles. Use this technique to break the handles into groups of MAXIMUM_WAIT_OBJECTS.
  • Call RegisterWaitForSingleObject or SetThreadpoolWait to wait on each handle. The thread pool waits efficiently on the handles and assigns a worker thread after the object is signaled or the time-out interval expires.
The WaitForMultipleObjectsEx function can specify handles of any of the following object types in the lpHandles array:
  • Change notification
  • Console input
  • Event
  • Memory resource notification
  • Mutex
  • Process
  • Semaphore
  • Thread
  • Waitable timer
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 the CoInitialize function. Therefore, if you have a thread that creates windows, use MsgWaitForMultipleObjects or MsgWaitForMultipleObjectsEx, rather than WaitForMultipleObjectsEx.

Requirements

Requirement Value
Minimum supported client Windows XP [desktop apps | UWP apps]
Minimum supported server Windows Server 2003 [desktop apps | UWP apps]
Target Platform Windows
Header synchapi.h (include Windows.h on Windows Server 2003, Windows Vista, Windows 7, Windows Server 2008 Windows Server 2008 R2)
Library Kernel32.lib
DLL Kernel32.dll

See also

Synchronization Functions

Wait Functions