WaitForSingleObject on a file handle?

Question

What happens when you call WaitForSingleObject() on a handle you\'ve created with CreateFile() or _get_osfhandle()?

For reason

Answer 1

Don't do it. As you can see, it has undefined behavior.

Even when the behavior is defined, it's defined in such a way as to be relatively not useful unless you don't like writing additional code. It is signaled when any asynchronous I/O operation on that handle completes, which does not generalize to tracking which I/O operation finished.

Why are you trying to wait on a file handle? Clearly the intent matters when you are doing something that isn't even supported well enough to not block indefinitely.

Answer 2

I found the following links. The concensus seems to me, don't do it.

• Asynch IO explorer

Waiting on a file handle

When an I/O operation is started on an asynchronous handle, the handle goes into a non-signaled state. Therefore, when used in the context of a WaitForSingleObject or WaitForMultipleObjects operation, the file handle will become signaled when the I/O operation completes. However, Microsoft actively discourages this technique; it does not generalize if there exists more than one pending I/O operation; the handle would become signaled if any I/O operation completed. Therefore, although this technique is feasible, it is not considered best practice.

• Egghead Cafe:

Use ReadDirectoryChangesW in overlapped mode. WaitForSingleObject can wait on the event in the OVERLAPPED struct.

You can also use the API WaitForSingleObject() to wait on a file change if you use the following change notification function:
FindFirstChangeNotification()
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/fileio/fs/findfirstchangenotification.asp http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dllproc/base/waitforsingleobject.asp

An interesting note on "evilness" of ReadDirectoryChangesW:
http://blogs.msdn.com/ericgu/archive/2005/10/07/478396.aspx