WaitForSingleObjectEx

The WaitForSingleObjectEx function returns when one of the following occurs:

• The specified object is in the signaled state.
• An I/O completion routine or asynchronous procedure call (APC) is queued to the thread.
• The time-out interval elapses.

To wait for multiple objects, use the WaitForMultipleObjectsEx.

DWORD WaitForSingleObjectEx(
  HANDLE hHandle,        // handle to object
  DWORD dwMilliseconds,  // time-out interval
  BOOL bAlertable        // alertable option
);

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 and no completion routines or APCs are queued. If dwMilliseconds is zero, the function tests the object's state 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

[in] 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 -1. To get extended error information, call GetLastError.

Remarks

The WaitForSingleObjectEx function determines whether the wait criteria have been met. If the criteria have not been met, the calling thread enters the wait state. It uses no processor time while waiting for the criteria to be met.

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 WaitForSingleObjectEx 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 WaitForSingleObjectEx.

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, WaitForMultipleObjectsEx