WaitForSingleObject

The WaitForSingleObject function returns when one of the following occurs:

• The specified object is in the signaled state.
• The time-out interval elapses.

To enter an alertable wait state, use the WaitForSingleObjectEx function. To wait for multiple objects, use the WaitForMultipleObjects.

DWORD WaitForSingleObject(
  HANDLE hHandle,        // handle to object
  DWORD dwMilliseconds   // time-out interval
);

Parameters

hHandle

[in] Handle to the object. For a list of the object types whose handles can be specified, see the following Remarks section.

If this handle is closed while the wait is still pending, the function's behavior is undefined.

Windows NT/2000: The handle must have SYNCHRONIZE access. For more information, see Standard Access Rights.

dwMilliseconds

[in] Specifies the time-out interval, in milliseconds. The function returns if the interval elapses, even if the object's state is nonsignaled. If dwMilliseconds is zero, the function tests the object's state 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.

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

Remarks

The WaitForSingleObject function checks the current state of the specified object. If the object's state is nonsignaled, the calling thread enters the wait state. It uses no processor time while waiting for the object state to become signaled or the time-out interval to elapse.

The function modifies the state of some types of synchronization objects. Modification occurs only for the object 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
• 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 WaitForSingleObject.

Requirements

  Windows NT/2000: Requires Windows NT 3.1 or later.
  Windows 95/98: Requires Windows 95 or later.
  Header: Declared in Winbase.h; include Windows.h.
  Library: Use Kernel32.lib.

See Also

Synchronization Overview, Synchronization Functions, MsgWaitForMultipleObjects, MsgWaitForMultipleObjectsEx, WaitForMultipleObjects, WaitForSingleObjectEx