Why is the CancelIo function failing?

Question

I wrote the code below. I am calling this method to write to three different hard disks using three separate threads. One of the CancelIo calls is failing (returns false).

I assume that the disk is OK (the hardware is fine).

How can this be?

(After changing the disk I see it happening again on another disk.)

  public void foo(byte[] bufferToWrite)
  {
      unsafe
      {

            NativeOverlapped overlapped = new NativeOverlapped()
            {
                  EventHandle = eventHandle,
                  OffsetLow   = ( int )( s       & 0xffffffff ),
                  OffsetHigh  = ( int )( s >> 32 & 0xffffffff )
            };

            GCHandle gch = GCHandle.Alloc( bufferToWrite, GCHandleType.Pinned );
            IntPtr ptr = new IntPtr( ( void* )gch.AddrOfPinnedObject() );

            WriteFile( handle, ptr, length, ref bytesWritten, &overlapped );

            dwResult = WaitForSingleObject( EventHandle, 30000 );
            if(dwResult== TIME_OUT)
            {
                   Debug.Assert( CancelIo ( handle ) );

            }
      }
  }

This looks like a mistake: `if( bResult == ERROR_SUCCESS == || bResult == ERROR_IO_PENDING )` ?? — hmjd, May 03 '12 at 20:52
what do you mean i never know ? i see in runtime ( in debug ) that i get assart — Yanshof, May 03 '12 at 21:23
If (bResult == ERROR_SUCCESS), do you need to wait on the event handle? If the write completes 'immediately', would the hEvent object be signaled? — Martin James, May 03 '12 at 21:30
@Yanshof - I've seen questions where there was an obvious error in the code as posted, but others, like yours, where it's a cut and paste error. You have to double check. — ChrisF, May 03 '12 at 21:50
Oh no, he actually did it: http://stackoverflow.com/questions/10279372/is-it-possible-to-call-writefile-and-the-application-will-wait-for-callback-fore — Hans Passant, May 04 '12 at 00:23

score 2 · Accepted Answer · answered May 03 '12 at 21:47

2

CancelIo fails if there are no outstanding IOs. Your IO might complete after 30001ms, just after the event wait ended with a timeout.

This race condition cannot be fixed. I know no better solution than to ignore the return value.

answered May 03 '12 at 21:47

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And there would be no outstanding IOs if the WriteFile failed, which is probably what's happening. He needs to check the WriteFile return code and not call WaitForSingleObject if it fails. – Carey Gregory May 03 '12 at 22:51
..not call WFSO unless the writeFile fails and (GetLastError Result == ERROR_IO_PENDING). – Martin James May 04 '12 at 12:56

Why is the CancelIo function failing?

1 Answers1