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

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  1. POAccess Violation Exception/Crash from C++ callback to C# function
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    <p>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#. </p> <p>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. </p> <p>The code from the original hpp files for callback function format:</p> <pre><code>typedef int (*CallbackFunction) (void *inst, const void *data); </code></pre> <p>Code from the C++/CLI Wrapper for callback function format: (I'll explain why I declared two in a moment)</p> <pre><code>public delegate int ManagedCallbackFunction (IntPtr oInst, const IntPtr oData); public delegate int UnManagedCallbackFunction (void* inst, const void* data); </code></pre> <p>--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).</p> <p>And finally, the crashing code from C#:</p> <pre><code>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; } </code></pre> <p>All writes to the console are done and then we see the dreaded crash on the return:</p> <blockquote> <p>Unhandled exception at 0x04d1004c in helloworld.exe: 0xC0000005: Access violation reading location 0x04d1004c.</p> </blockquote> <p>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</p> <p>Which is only interesting if you look at the console which shows: </p> <pre><code>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 </code></pre> <p>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.</p> <p>More full disclosure, it all works fine (seemingly) in Debug mode!</p> <p>A real head scratch issue that would appreciate any help!</p>
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