How to iterate the vtable of COM coclass?

Question

How can I iterate/access the vtable of COM coclass which will implement the methods of its exposed interfaces?

I need to access the part of the vtable where all addresses of exposed methods of its interfaces are stored.

e.g. Math is COM object, its exposed interface is "Operations" and "Sum" is the method of this interface, how do I get the address of "Sum"?

Unknown Unknown · Accepted Answer · 2010-12-07T09:46:33

I'm not going to ask why are you doing it this way, but perhaps this could help... Every COM object must implement at least the IUnknown interface. Hence, the first four bytes of the COM object instance is the pointer to IUnknown object. The first four bytes of the IUnknown object (and any other object with virtual functions) is the pointer to vtbl.

(There is no error checking in this example, so please don't split hair on that subject.)

I used an instance of IReferenceClock for demonstration.

int main()
{

    CoInitialize( NULL );

    IReferenceClock* pRefClock;
    HRESULT hr = CoCreateInstance( CLSID_SystemClock, NULL, CLSCTX_INPROC_SERVER, IID_IReferenceClock, (void**)&pRefClock );

    DWORD* pIUnknownAddress = (DWORD*)pRefClock;
    DWORD* pVTBLaddress = (DWORD*)*pIUnknownAddress;

    // for example, the next interface could be accessed like this
    DWORD* pNextInterfaceAddress = ( (DWORD*)pRefClock ) + 1;
    DWORD* pNextVTBLaddress = (DWORD*)*pNextInterfaceAddress;
    // and you would access virtual functions in the same way as QueryInterface, AddRef and Release below in this example

    HRESULT (__stdcall *pQueryInterfaceFunction)(void*, REFIID, void**);
    ULONG (__stdcall *pAddRef)( void* );
    ULONG (__stdcall *pRelease)( void* );

    // IUnknown looks like this:
    // 
    // virtual HRESULT QueryInterface( REFIID riid, void** ppvObject);
    // virtual ULONG AddRef( void );
    // virtual ULONG Release( void );
    //
    // So, the first function in vtbl is QueryInterface, the second is AddRef...

    pQueryInterfaceFunction = (HRESULT (__stdcall*)(void*, REFIID, void**))*pVTBLaddress;
    pAddRef = (ULONG (__stdcall *)( void* ))*( pVTBLaddress + 1 );
    pRelease = (ULONG (__stdcall *)( void* ))*( pVTBLaddress + 2 );

    // Note: extra void* is actually this pointer.. see below that we pass pRefClock to every call

    IUnknown* pUnknown;
    UINT nRefCount;

    hr = pQueryInterfaceFunction( pRefClock, IID_IUnknown, (void**)&pUnknown );

    if( SUCCEEDED( hr ) )
    {
        nRefCount = pUnknown->Release();
        ATLTRACE( TEXT( "nRefCount = %d\n" ), nRefCount );
    }

    nRefCount = pAddRef( pRefClock );
    ATLTRACE( TEXT( "nRefCount after AddRef() call = %d\n" ), nRefCount );

    nRefCount = pRelease( pRefClock );
    ATLTRACE( TEXT( "nRefCount after Release() call = %d\n" ), nRefCount );

    nRefCount = pRefClock->Release();

    CoUninitialize();

    return 0;
}

How to iterate the vtable of COM coclass?

2 Answers