invalidOverlappedToPinvoke MDA

The invalidOverlappedToPinvoke managed debugging assistant (MDA) is activated when an overlapped pointer that was not created on the garbage collection heap is passed to specific Win32 functions.

Note

By default, this MDA is activated only if the platform invoke call is defined in your code and the debugger reports the JustMyCode status of each method. A debugger that does not understand JustMyCode (such as MDbg.exe with no extensions) will not activate this MDA. This MDA can be enabled for those debuggers by using a configuration file and explicitly settting justMyCode="false" in the .mda.config file (<invalidOverlappedToPinvoke enable="true" justMyCode="false"/>).

Symptoms

Crashes or unexplainable heap corruptions.

Cause

An overlapped pointer that was not created on the garbage collection heap is passed to specific operating system functions.

The following table shows the functions that this MDA tracks.

Module Function
HttpApi.dll HttpReceiveHttpRequest
IpHlpApi.dll NotifyAddrChange
kernel32.dll ReadFile
kernel32.dll ReadFileEx
kernel32.dll WriteFile
kernel32.dll WriteFileEx
kernel32.dll ReadDirectoryChangesW
kernel32.dll PostQueuedCompletionStatus
MSWSock.dll ConnectEx
WS2_32.dll WSASend
WS2_32.dll WSASendTo
WS2_32.dll WSARecv
WS2_32.dll WSARecvFrom
MQRT.dll MQReceiveMessage

The potential for heap corruption is high for this condition because the AppDomain making the call might unload. If the AppDomain unloads, the application code will either free the memory for the overlapped pointer, causing corruption when the operation finishes, or the code will leak the memory, causing difficulties later.

Resolution

Use an Overlapped object, calling the Pack method to get a NativeOverlapped structure that can be passed to the function. If the AppDomain unloads, the CLR waits until the asynchronous operation completes before freeing the pointer.

Effect on the Runtime

This MDA had no effect on the CLR.

Output

The following is an example of output from this MDA.

An overlapped pointer (0x00ea3430) that was not allocated on the GC heap was passed via Pinvoke to the Win32 function 'WriteFile' in module 'KERNEL32.DLL'. If the AppDomain is shut down, this can cause heap corruption when the async I/O completes. The best solution is to pass a NativeOverlapped structure retrieved from a call to System.Threading.Overlapped.Pack(). If the AppDomain exits, the CLR will keep this structure alive and pinned until the I/O completes.

Configuration

<mdaConfig>  
  <assistants>  
    <invalidOverlappedToPinvoke/>  
  </assistants>  
</mdaConfig>  

See Also

MarshalAsAttribute
Diagnosing Errors with Managed Debugging Assistants
Interop Marshaling