c++ - Not releasing filter com object causing a crash

Question

I am using Viveks capture filter with vlc (http://tmhare.mvps.org/downloads/vcam.zip) to emulate a capture source. When the filter is open and I close vlc I get a crash. The stack trace indicates that 2 COM objects still exist (Im guessing the filter and pin) which should be released before a CoUninitialize call. My problem is I am not sure where to release the filter and pin COM objects, I have a destructor for the fitler and pin but they are never called when vlc closes. Someone who had a similar problem (Unreleased DirectShow CSource filter makes program crash at process shutdown).

This is the important registering part of the dll.

STDAPI RegisterFilters( BOOL bRegister )
{
    HRESULT hr = NOERROR;
    WCHAR achFileName[MAX_PATH];
    char achTemp[MAX_PATH];
    ASSERT(g_hInst != 0);

    if( 0 == GetModuleFileNameA(g_hInst, achTemp, sizeof(achTemp))) 
        return AmHresultFromWin32(GetLastError());

    MultiByteToWideChar(CP_ACP, 0L, achTemp, lstrlenA(achTemp) + 1, 
                   achFileName, NUMELMS(achFileName));

    hr = CoInitialize(0);
    if(bRegister)
    {
        hr = AMovieSetupRegisterServer(CLSID_VirtualCam, L"Virtual Cam", achFileName, L"Both", L"InprocServer32");
    }

    if( SUCCEEDED(hr) )
    {
        IFilterMapper2 *fm = 0;

        hr = CreateComObject( CLSID_FilterMapper2, IID_IFilterMapper2, fm );
        if( SUCCEEDED(hr) )
        {
            if(bRegister)
            {
                IMoniker *pMoniker = 0;
                REGFILTER2 rf2;
                rf2.dwVersion = 1;
                rf2.dwMerit = MERIT_DO_NOT_USE;
                rf2.cPins = 1;
                rf2.rgPins = &AMSPinVCam;
                hr = fm->RegisterFilter(CLSID_VirtualCam, L"Virtual Cam", &pMoniker, &CLSID_VideoInputDeviceCategory, NULL, &rf2);
            }
            else
            {
                hr = fm->UnregisterFilter(&CLSID_VideoInputDeviceCategory, 0, CLSID_VirtualCam);
            }
        }  

      // release interface
      //
      if(fm)
          fm->Release();
    }

    if( SUCCEEDED(hr) && !bRegister )
         hr = AMovieSetupUnregisterServer( CLSID_VirtualCam );

    CoFreeUnusedLibraries();
    CoUninitialize();
    return hr;
}

STDAPI DllRegisterServer()
{
    return RegisterFilters(TRUE);
}

STDAPI DllUnregisterServer()
{
    return RegisterFilters(FALSE);
}

extern "C" BOOL WINAPI DllEntryPoint(HINSTANCE, ULONG, LPVOID);

BOOL APIENTRY DllMain(HANDLE hModule, DWORD  dwReason, LPVOID lpReserved)
{
    return DllEntryPoint((HINSTANCE)(hModule), dwReason, lpReserved);
}

The important filter part

CVCamStream::CVCamStream(HRESULT *phr, CVCam *pParent, LPCWSTR pPinName) :
CSourceStream(NAME("Virtual Cam"),phr, pParent, pPinName), m_pParent(pParent)
{
    // Set the default media type as 320x240x24@15
    GetMediaType(4, &m_mt);
}

CVCamStream::~CVCamStream()
{
    m_pParent->Release();
} 

Answer 1

Leaked COM references are a sort of hard to nail down. Assuming that COM client - in your case VLC - is doing everything right (which might be not the case, but it's okay to start with this assumption), the problem is on your code. It is typically one of the two:

1. You are dealing with raw pointers and there is somewhere no matching Release for AddRef done earlier
2. There are circular references and object keep each other alive

As you already see you have two objects floating around, a good strategy would be to identify what classes are they exactly, and trace reference counter changes to see where is the lost IUnknown::Release.