February 6th, 2014

If an asynchronous I/O completes synchronously, is the hEvent in the OVERLAPPED structure signaled anyway?

Raymond Chen
Raymond Chen

Yes. When an I/O completes (whether synchronously or asynchronously), the event is signaled and completion status notifications are queued. The Get­Overlapped­Result/Ex function can be used to wait on an I/O that has already completed; it will merely return immediately. If you ask Has­Overlapped­Io­Completed whether the I/O has completed, and the I/O completed synchronously, it will correctly report, “Yeah, of course it completed. Heck, it completed a long time ago!”

In other words, you can logically treat the case of an asynchronous I/O request completing synchronously as if it had completed asynchronously. It just completes asynchronously before you even blinked.

Category
Old New Thing
Topics
Code

Author

Raymond Chen

Raymond has been involved in the evolution of Windows for more than 30 years. In 2003, he began a Web site known as The Old New Thing which has grown in popularity far beyond his wildest imagination, a development which still gives him the heebie-jeebies. The Web site spawned a book, coincidentally also titled The Old New Thing (Addison Wesley 2007). He occasionally appears on the Windows Dev Docs Twitter account to tell stories which convey no useful information.

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