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Overlapped Operations
Overlapped operations enable a thread to execute a time-consuming I/O
operation in the background, leaving the thread free to perform other tasks. To enable
overlapped I/O operations on a communications resource, the thread must specify
the FILE_FLAG_OVERLAPPED flag in the CreateFile function when the handle is opened. To execute the ReadFile or WriteFile function as an overlapped operation, the calling thread must specify a
pointer to an OVERLAPPED structure.The OVERLAPPED structure must contain a handle to a manual-reset (not an auto-reset) event
object. The system sets the state of the event object to not-signaled when a
call to the I/O function returns before the operation has been completed. The
system sets the state of the event object to signaled when the operation has been
completed. The thread uses a wait function to check the current state of the
event object or to wait for its state to be signaled.
The ReadFileEx and WriteFileEx functions can be performed only as overlapped operations. The calling thread
specifies a pointer to the FileIOCompletionRoutine function, which is executed when the overlapped operation is completed. The
completion routine is executed only if the calling thread performs an alertable
operation.
For more information about event objects, wait functions, alertable waits, and
completion routines, see Synchronization.
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