Question

I’ve got an first chance exception in ppltasks.h’s _ContextCallback that I just don’t understand.

My project is a mixed WinRT/native application. The application is made up of multiple sub-projects, one of them uses WinRT. The WinRT code is in a static library running in a different thread from the main thread (and it’s not in my power to change that). The WinRT code is copied from a generated Direct3D metro app in Visual Studio 2012 RC.

The code attempts to load one of the simple shaders:

void CubeRenderer::CreateDeviceResources()
{
    Direct3DBase::CreateDeviceResources();

    // going to crash while attempting to load a file.
    auto loadVSTask = DX::ReadDataAsync("SimpleVertexShader.cso");

The file exists, at the correct location, and proven to work.

The code crashes here. _M_context._M_pContextCallback is invalid.

   void _Reset()
    {
        if (_M_context._M_captureMethod != _S_captureDeferred && _M_context._M_pContextCallback != nullptr)
        {
            _M_context._M_pContextCallback->Release();
        }
    }

Output window gives:
First-chance exception at 0x00E34C89 in MyApplication.exe: 0xC0000005: Access violation reading location 0xCDCDCDCD.

Stack trace looks like:

MyApplication.exe!Concurrency::details::_ContextCallback::_Reset() Line 620 C++
MyApplication.exe!Concurrency::details::_ContextCallback::operator=(const Concurrency::details::_ContextCallback & _Src) Line 563 C++
MyApplication.exe!Concurrency::task_continuation_context::operator=(const Concurrency::task_continuation_context & __that) C++
MyApplication.exe!Concurrency::task::_ContinuationTaskHandle,std::integral_constant,Concurrency::details::_TypeSelectorAsyncOperation>::_ContinuationTaskHandle,std::integral_constant,Concurrency::details::_TypeSelectorAsyncOperation>(const std::shared_ptr > & _AncestorImpl, const std::shared_ptr > & ContinuationImpl, const DX::ReadDataAsync::_l3:: & _Func, const Concurrency::task_continuation_context & _Context, Concurrency::details::_TaskInliningMode _InliningMode) Line 3292 C++
MyApplication.exe!Concurrency::task::ThenImpl >(const DX::ReadDataAsync::_l3:: & _Func, Concurrency::details::_CancellationTokenState * _PTokenState, const Concurrency::task_continuation_context & _ContinuationContext, bool _Aggregating, Concurrency::details::_TaskInliningMode InliningMode) Line 3584 C++
MyApplication.exe!Concurrency::task::then< >(const DX::ReadDataAsync::_l3:: & _Func) Line 2882 C++
MyApplication.exe!DX::ReadDataAsync(Platform::String ^ filename) Line 42 C++
MyApplication.exe!CubeRenderer::[Direct3DBase]::CreateDeviceResources() Line 30 C++

So there’s something wrong with the _ContextCallback, and that’s where I get stuck.

• What is this context callback? It seems to have something to do with passing from a task to another.
• If it’s invalid, how could that have happened?
• If it’s simply just not setup properly, how to I properly set it up?

I can get it to work, but it involves changing all the sub-projects in the application to use WinRT. Unfortunately this is not considered an acceptable solution as the code in those other sub-projects are also outside of my control. I also would like to try to understand what’s actually going on here.

Any insights or ideas? Thanks!

Answer 1

I’ve added a reference to ppltasks.h in my main cpp file. It makes the problem go away, but I have no idea why.