| Platform SDK: DLLs, Processes, and Threads |
The Win32® API provides a set of wait functions to allow a thread to block its own execution. The wait functions do not return until the specified criteria have been met. The type of wait function determines the set of criteria used. When a wait function is called, it checks 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.
There are four types of wait functions:
The SignalObjectAndWait, WaitForSingleObject, and WaitForSingleObjectEx functions require a handle to one synchronization object. These functions return when one of the following occurs:
The SignalObjectAndWait function enables the calling thread to atomically set the state of an object to signaled and wait for the state of another object to be set to signaled.
The WaitForMultipleObjects, WaitForMultipleObjectsEx, MsgWaitForMultipleObjects, and MsgWaitForMultipleObjectsEx functions enable the calling thread to specify an array containing one or more synchronization object handles. These functions return when one of the following occurs:
The MsgWaitForMultipleObjects and MsgWaitForMultipleObjectsEx function allow you to specify input event objects in the object handle array. This is done when you specify the type of input to wait for in the thread's input queue.
For example, a thread could use MsgWaitForMultipleObjects to block its execution until the state of a specified object has been set to signaled and there is mouse input available in the thread's input queue. The thread can use the GetMessage or PeekMessage function to retrieve the input.
When waiting for the states of all objects to be set to signaled, these multiple-object functions do not modify the states of the specified objects until the states of all objects have been set signaled. For example, the state of a mutex object can be signaled, but the calling thread does not get ownership until the states of the other objects specified in the array have also been set to signaled. In the meantime, some other thread may get ownership of the mutex object, thereby setting its state to nonsignaled.
The MsgWaitForMultipleObjectsEx, SignalObjectAndWait, WaitForMultipleObjectsEx, and WaitForSingleObjectEx functions differ from the other wait functions in that they can optionally perform an alertable wait operation. In an alertable wait operation, the function can return when the specified conditions are met, but it can also return if the system queues an I/O completion routine or an APC for execution by the waiting thread. For more information about alertable wait operations and I/O completion routines, see Synchronization and Overlapped Input and Output. For more information about APCs, see Asynchronous Procedure Calls.
The RegisterWaitForSingleObject function differs from the other wait functions in that the wait operation is performed by a thread from the thread pool. When the specified conditions are met, the callback function is executed by a worker thread from the thread pool.
By default, a registered wait operation is a multiple-wait operation. The system resets the timer every time the event is signaled (or the time-out interval elapses) until you call the UnregisterWaitEx function to cancel the operation. To specify that a wait operation should be executed only once, set the dwFlags parameter of RegisterWaitForSingleObject to WT_EXECUTEONLYONCE.
The wait functions can modify the states of some types of synchronization objects. Modification occurs only for the object or objects whose signaled state caused the function to return. Wait functions can modify the states of synchronization objects as follows:
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 the other wait functions.