WaitForSingleObject on a file handle?

Question

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

For reasons not worth explaining I would like to use WaitForSingleObject() to wait on a HANDLE that i've created with _get_osfhandle(fd), where fd comes from a regular call to _open(). Is this possible?

I have tried it in practice, and on some machines it works as expected (the HANDLE is always in the signaled state because you can read more data from it), and on some machines WaitForSingleObject() will block indefinitely if you let it.

The MSDN page for WaitForSingleObject() says that the only supported things that it handles are "change notifications, console input, events, memory resource notifications, mutex, processes, semaphores, threads, and waitable timers."

Additionally, would it be different if I used CreateFile() instead of _get_osfhandle() on a CRT file descriptor?

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