According to MSDN ReadFile() Win32 function may incorrectly report read operation completion. When?

Question

The MSDN states in its description of ReadFile() function:

If hFile is opened with FILE_FLAG_OVERLAPPED, the lpOverlapped parameter must point to a valid and unique OVERLAPPED structure, otherwise the function can incorrectly report that the read operation is complete.

I have some applications that are violating the above recommendation and I would like to know the severity of the problem. I mean the program uses named pipe that has been created with FILE_FLAG_OVERLAPPED, but it reads from it using the following call:

ReadFile(handle, &buf, n, &n_read, NULL);

That means it passes NULL as the lpOverlapped parameter. That call should not work correctly in some circumstances according to documentation. I have spent a lot of time trying to reproduce the problem, but I was unable to! I always got all data in right place at right time. I was testing only Named Pipes though.

Would anybody know when can I expect that ReadFile() will incorrectly return and report successful completion even the data are not yet in the buffer? What would have to happen in order to reproduce the problem? Does it happen with files, pipes, sockets, consoles, or other devices? Do I have to use particular version of OS? Or particular version of reading (like register the handle to I/O completion port)? Or particular synchronization of reading and writing processes/threads?

Or when would that fail? It works for me :/

Please help!

With regards, Martin

Answer 1

Internally the system only supports asynchronous I/O. For synchronous I/O the system creates a temporary OVERLAPPED structure with hEvent = NULL;, issues an asynchronous I/O request passing in this temporary, and then waits for completion using GetOverlappedResult( bWait = TRUE ).

Recall that the hEvent of the temporary OVERLAPPED structure is NULL and pay attention to the Remarks section of GetOverlappedResult:

If the hEvent member of the OVERLAPPED structure is NULL, the system uses the state of the hFile handle to signal when the operation has been completed.

A file HANDLE is a waitable object that becomes unsignaled when an I/O operation begins, and signaled when an I/O operation ends.

Now consider a scenario where an asynchronous file HANDLE has a pending I/O request at the time you issue a synchronous I/O request. The system creates an OVERLAPPED structure and waits on the hFile HANDLE for completion. In the meantime the asynchronous I/O completes, thereby signaling the HANDLE causing the synchronous I/O to return prematurely without having actually completed.

Worse though is that by the time the asynchronous I/O that was initiated in response to the synchronous I/O request completes it will update the temporary OVERLAPPED structure that no longer exists. The result is memory corruption.

The full story can be found at The Old New Thing.

Answer 2

Seems like you are in a situation where you are deliberately calling an API in contravention of the documented best practices. In such situations all bets are off. It may work, it may not. If may work on this OS, but not on the next iteration of the OS, or the next service pack of the same OS. What happens when you port to Win64? Will it still work then?

Does calling GetLastError() (or looking at @ERR,hr in the debugger) give any value that is useful in addition to the error code?

I recommend that you call it with a valid OVERLAPPED structure, get it working and remove all doubt (and possibility of random failure). Why have possibly buggy code (and very hard to reproduce bugs) in your software when you can fix the problem easily by using a valid OVERLAPPED structure?

Answer 3

Why ask the question rather than fix the code to call the API as it was intended?

I suspect it always appears to work because, even though this is an asynchronous I/O, it completes very quickly. Depending on how you're testing for success, it's possible the function is incorrectly reporting that the operation completed, but it actually completes before you test the results.

The real test would be to do a read on the pipe before there's data to be read.

But really, you should just fix the code. If your architecture cannot handle asynchronous I/O, then remove the FILE_FLAG_OVERLAPPED from the creation of the named pipe.

Answer 4

When they say

Blockquote If hFile is opened with FILE_FLAG_OVERLAPPED, the lpOverlapped parameter must point to a valid and unique OVERLAPPED structure, otherwise the function can incorrectly report that the read operation is complete.

they mean that there's nothing in the code preventing it working, but there's also a path through their code that can produce erroneous results. Just because you can't reproduce the problem with your particular hardware does not mean there is no problem.

If you really want to reproduce this problem, leave the code as is and go on with your life. Right about the time you've forgotten all about this problem, strange behavior will surface that will not have any obvious relations to calling ReadFile. You'll spend days pulling your hair out, and the problem will appear to come and go randomly. Eventually you'll find it and kick yourself for not following the instructions. Been there, done that, no fun!

The other way to recreate the problem is to schedule an important demo for your customer. It's sure to fail then!

If you don't want to splatter your code with OVERLAPPED structures and all of the related return value checks, Waits, Events, etc, you can write a wrapper function that takes a handle from which to read, and a timeout. Simply replace your calls to ReadFile with this handy-dandy wrapper.