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WaitForSingleObject info  Overview  Group

The WaitForSingleObject function returns when one of the following occurs:

    HANDLE hHandle,

// handle of object to wait for

    DWORD dwMilliseconds 

// time-out interval in milliseconds



Identifies the object. For a list of the object types whose handles can be specified, see the following Remarks section.

Windows NT: The handle must have SYNCHRONIZE access. For more information, see Access Masks and Access Rights.

Specifies the time-out interval, in milliseconds. The function returns if the interval elapses, even if the objectís state is nonsignaled. If dwMilliseconds is zero, the function tests the objectís state and returns immediately. If dwMilliseconds is INFINITE, the functionís 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 on success is one of the following values:




The specified object is a mutex object that was not released by the thread that owned the mutex object before the owning thread terminated. Ownership of the mutex object is granted to the calling thread, and the mutex is set to nonsignaled.


The state of the specified object is signaled.


The time-out interval elapsed, and the objectís state is nonsignaled.


The WaitForSingleObject function checks the current state of the specified object. If the objectís state is nonsignaled, the calling thread enters an efficient wait state. The thread consumes very little processor time while waiting for the object state to become signaled or the time-out interval to elapse.

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 WaitForSingleObject function can wait for the following objects:



Change notification

The FindFirstChangeNotification function returns the handle. A change notification objectís 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 objectís state is signaled when there is unread input in the consoleís input buffer, and it is nonsignaled when the input buffer is empty.


The CreateEvent or OpenEvent function returns the handle. An event objectís state is set explicitly to signaled by the SetEvent or PulseEvent function. A manual-reset event objectís state must be reset explicitly to nonsignaled by the ResetEvent function. For an auto-reset event object, the wait function resets the objectís state to nonsignaled before returning. Event objects are also used in overlapped operations, in which the state is set by the system.


The CreateMutex or OpenMutex function returns the handle. A mutex objectís 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 mutexís state to nonsignaled when ownership is granted.


The CreateProcess or OpenProcess function returns the handle. A process objectís state is signaled when the process terminates.


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.


The CreateProcess, CreateThread, or CreateRemoteThread function returns the handle. A thread objectís state is signaled when the thread terminates.


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 WaitForSingleObject.

See Also

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

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