Directx accessing backbuffer - c++

Question

I am trying to access the screen data from backbuffer using directx. However with the code below, the pointer to the pixel data has all 0.
I am really not sure if I am checking the data correctly. I am running this on Visual studio 2013, and using break points to check the pointer b. And its value is 0, all the time.
I am aware that I am not using IDirect3DDevice9::GetRenderTargetData method. This cause me to try to access the data in the device's memory I suppose. Would it prevent me from getting correct pixel data ?
d3dManager = new D3DManager(NULL, 600, 600);
IDirect3DDevice9 *device = d3dManager->getDevice();
IDirect3DSurface9 *ppBackBuffer = NULL;
HRESULT result = device->GetBackBuffer(
0,
0,
D3DBACKBUFFER_TYPE_MONO,
&ppBackBuffer
);
if (FAILED(result))
{
printf("vuhu");
return 1;
}
D3DSURFACE_DESC pDesc;
ppBackBuffer->GetDesc(&pDesc);
HANDLE *handle = NULL;
device->CreateOffscreenPlainSurface(pDesc.Width, pDesc.Height, pDesc.Format, D3DPOOL_SYSTEMMEM, &ppBackBuffer, handle);
D3DLOCKED_RECT lockedRectangle;
ppBackBuffer->LockRect(&lockedRectangle, NULL, D3DLOCK_DONOTWAIT);
void* bits = lockedRectangle.pBits;
int *a = (int*)(bits);
int *b = a + 120;

Answer 1

When you call CreateOffscreenPlainSurface here, and you pass in ppBackBuffer, it gets overwritten because it's creating new surface that's empty by default. In fact, you just leaked your reference to the backbuffer you obtained from GetBackBuffer.
D3DSURFACE_DESC pDesc;
ppBackBuffer->GetDesc(&pDesc);
HANDLE *handle = NULL;
device->CreateOffscreenPlainSurface(pDesc.Width, pDesc.Height, pDesc.Format, D3DPOOL_SYSTEMMEM, &ppBackBuffer, handle);
You need to actually call GetRenderTargetData to copy the data from the backbuffer (which is likely in video memory that is completely inaccessible to the CPU) to a surface you can read from the CPU.
BTW, check the HRESULTs of your calls, particularly CreateOffscreenPlainSurface and LockRect.
EDIT: If what you mean to do is create a new surface with the same properties as the backbuffer, then you should do:
D3DSURFACE_DESC pDesc;
ppBackBuffer->GetDesc(&pDesc);
ppBackBuffer->Release(); // <--- Let go of our reference to the back buffer surface
HANDLE *handle = NULL;
device->CreateOffscreenPlainSurface(pDesc.Width, pDesc.Height, pDesc.Format, D3DPOOL_SYSTEMMEM, &ppBackBuffer, handle);
There's nothing at all in your code that is actually moving the contents of the back buffer into the new surface. You are just reusing the variable IDirect3DSurface9 *ppBackBuffer.