Windows’ kernel I/O manager will serialize I/O requests automatically if you didn’t have the flag FILE_FLAG_OVERLAPPED in CreateFile() call, as the page described. This fact haven’t be mentioned in M$’s documentation, and there is few information about this on the web.

Conclusion. If you want to

  • Open a handle
  • and access it from multiple threads simultaneously

You have to

  • Call CreateFile() with FILE_FLAG_OVERLAPPED set
  • Call ReadFile() and WriteFile() in overlapped way.

Only when the handle is accessed in overlapped mode, the I/O Manager doesn’t wait for the completion of previous system call. The I/O Manager serialize operations in system call level, on kernel part. So you won’t even see the IRP.

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