OVERLAPPED Win32 结构中的 hEvent 成员
当使用异步I/O（或者Win32术语中的“重叠”I/O）时，我们需要处理OVERLAPPED结构及其hEvent成员。如果I/O函数会延迟读或写操作，我们将得到一个ERROR_IO_PENDING错误代码，然后我们将使用WaitForXxxEvent函数等待异步操作完成，然后我们将调用GetOverlappedResult。
但是，如果 I/O 操作立即完成，我们将不会得到 ERROR_IO_PENDING，并且在读取操作中，我们的读取缓冲区将立即被填充。但是 OVERLAPPED::hEvent 成员呢？会被设置为有信号状态吗？我还没有找到对此的明确说法。
这个问题可能看起来毫无意义（如果我知道操作已经完成，为什么要处理该事件？），但是我有一个模仿重叠模式的库，并且我需要具有相同的确切行为。
正如 edgar.holleis 在 他的评论，Raymond Chen在他的博客中解释了这一点： 链接
如果异步 I/O 同步完成，那么 OVERLAPPED 结构中的 hEvent 是否仍会发出信号？
是的。
当 I/O 完成时（无论是同步还是异步）， 发出事件信号并排队完成状态通知。这
GetOverlappedResult/Ex函数可用于等待 I/O 已经完成；它只会立即返回。如果你问 HasOverlappedIoCompleted I/O 是否已完成，以及 I/O 同步完成，它会正确报告，“是的，当然 完全的。哎呀，它很久以前就完成了！”换句话说，你可以逻辑地处理异步的情况 I/O 请求同步完成，就像已完成一样 异步地。它只是在您之前异步完成 眨眼。
When asynchronous I/O (or "overlapped" I/O in Win32 jargon) is used, we need to deal with the OVERLAPPED structure and his hEvent member. If the I/O function will delay the read or write operation, we will get an ERROR_IO_PENDING error code, then we will wait the asynchronous operation to complete with a WaitForXxxEvent function, then we will call GetOverlappedResult.
However, if the I/O operation is immediately completed, we will not get ERROR_IO_PENDING, and in a read operation our read buffer will be filled immediately. But what about the OVERLAPPED::hEvent member? Will it be set to signaled state? I've not found a clear statement about that.
This question may seem pointless (why deal with the event if I know that the operation is already completed?), however I have a library that mimics the overlapped pattern and I need to have the same exact behavior.
As pointed by edgar.holleis in his comment, Raymond Chen explained this in his blog: Link
If an asynchronous I/O completes synchronously, is the hEvent in the OVERLAPPED structure signaled anyway?
Yes.
When an I/O completes (whether synchronously or asynchronously), the
event is signaled and completion status notifications are queued. TheGetOverlappedResult/Exfunction 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.
如果你对这篇内容有疑问，欢迎到本站社区发帖提问 参与讨论，获取更多帮助，或者扫码二维码加入 Web 技术交流群。
绑定邮箱获取回复消息
由于您还没有绑定你的真实邮箱，如果其他用户或者作者回复了您的评论，将不能在第一时间通知您！
发布评论
评论（1）
不，不会。我花了很长时间才弄清楚这一点；）
No it won't. It took me ages to figure that one out the hard way ;)