Access Violation Exception/Crash from C++ callback to C# function

Question

So I have a native 3rd party C++ code base I am working with (.lib and .hpp files) that I used to build a wrapper in C++/CLI for eventual use in C#.

I've run into a particular problem when switching from Debug to Release mode, in that I get an Access Violation Exception when a callback's code returns.

The code from the original hpp files for callback function format:

typedef int (*CallbackFunction) (void *inst, const void *data);

Code from the C++/CLI Wrapper for callback function format: (I'll explain why I declared two in a moment)

public delegate int ManagedCallbackFunction (IntPtr oInst, const IntPtr oData);
public delegate int UnManagedCallbackFunction (void* inst, const void* data);

--Quickly, the reason I declared a second "UnManagedCallbackFunction" is that I tried to create an "intermediary" callback in the wrapper, so the chain changed from Native C++ > C# to a version of Native C++ > C++/CLI Wrapper > C#...Full disclosure, the problem still lives, it's just been pushed to the C++/CLI Wrapper now on the same line (the return).

And finally, the crashing code from C#:

public static int hReceiveLogEvent(IntPtr pInstance, IntPtr pData)
    {
        Console.WriteLine("in hReceiveLogEvent...");
        Console.WriteLine("pInstance: {0}", pInstance);
        Console.WriteLine("pData: {0}", pData);

        // provide object context for static member function
        helloworld hw = (helloworld)GCHandle.FromIntPtr(pInstance).Target;
        if (hw == null || pData == null)
        {
            Console.WriteLine("hReceiveLogEvent: received null instance pointer or null data\n");
            return 0;
        }

        // typecast data to DataLogger object ptr
        IntPtr ip2 = GCHandle.ToIntPtr(GCHandle.Alloc(new DataLoggerWrap(pData)));
        DataLoggerWrap dlw = (DataLoggerWrap)GCHandle.FromIntPtr(ip2).Target;

        //Do Logging Stuff

        Console.WriteLine("exiting hReceiveLogEvent...");
        Console.WriteLine("pInstance: {0}", pInstance);
        Console.WriteLine("pData: {0}", pData);
        Console.WriteLine("Setting pData to zero...");
        pData = IntPtr.Zero;
        pInstance = IntPtr.Zero;
        Console.WriteLine("pData: {0}", pData);
        Console.WriteLine("pInstance: {0}", pInstance);

        return 1;
    }

All writes to the console are done and then we see the dreaded crash on the return:

Unhandled exception at 0x04d1004c in helloworld.exe: 0xC0000005: Access violation reading location 0x04d1004c.

If I step into the debugger from here, all I see is that the last entry on the call stack is: > "04d1004c()" which evaluates to a decimal value of: 80805964

Which is only interesting if you look at the console which shows:

entering registerDataLogger
pointer to callback handle: 790848
fp for callback: 2631370
pointer to inst: 790844
in hReceiveLogEvent...
pInstance: 790844
pData: 80805964
exiting hReceiveLogEvent...
pInstance: 790844
pData: 80805964
Setting pData to zero...
pData: 0
pInstance: 0

Now, I know that between debug and release some things are quite different in the Microsoft world. I am, of course worried about byte padding and initialization of variables, so if there is something I am not providing here, just let me know and I'll add to the (already long) post. I also think the managed code may NOT be releasing all ownership and then the native C++ stuff (which I don't have the code for) may be trying to delete or kill off the pData object, thus crashing the app.

More full disclosure, it all works fine (seemingly) in Debug mode!

A real head scratch issue that would appreciate any help!

Answer 1

I think the stack got crushed because of mismatching calling conventions: try out to put the attribute

 [UnmanagedFunctionPointer(CallingConvention.Cdecl)]

on the callback delegate declaration.