Capture screenshot of desktop or window running on top using GetBackBuffer DirectX

Question

Hi I am looking to capture screenshot of the window on top or the desktop that is presently running application on top using GetBackBuffer() method in DirectX. And instead of saving I want to display the screenshot in a window.

However I am new to DirectX programming and and am unclear in the order of functions to be called. I referred to a few blogs and threads and have compiled this code:

// include the basic windows header files and the Direct3D header file
#include <windows.h>
#include <windowsx.h>
#include <d3d9.h>
//#include "iostream.h"
#include <stdio.h>

// include the Direct3D Library file
#pragma comment (lib, "d3d9.lib")

// global declarations
LPDIRECT3D9 d3d;    // the pointer to our Direct3D interface
LPDIRECT3DDEVICE9 d3ddev;    // the pointer to the device class

// function prototypes
void initD3D(HWND hWnd);    // sets up and initializes Direct3D
void render_frame(void);    // renders a single frame
void cleanD3D(void);    // closes Direct3D and releases memory

// the WindowProc function prototype
LRESULT CALLBACK WindowProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam);


// the entry point for any Windows program
int WINAPI WinMain(HINSTANCE hInstance,
    HINSTANCE hPrevInstance,
    LPSTR lpCmdLine,
    int nCmdShow)
{
    HWND hWnd;
    WNDCLASSEX wc;

    ZeroMemory(&wc, sizeof(WNDCLASSEX));

    wc.cbSize = sizeof(WNDCLASSEX);
    wc.style = CS_HREDRAW | CS_VREDRAW;
    wc.lpfnWndProc = WindowProc;
    wc.hInstance = hInstance;
    wc.hCursor = LoadCursor(NULL, IDC_ARROW);
    wc.hbrBackground = (HBRUSH)COLOR_WINDOW;
    wc.lpszClassName = L"WindowClass";

    RegisterClassEx(&wc);

    hWnd = CreateWindowEx(NULL,
        L"WindowClass",
        L"Our First Direct3D Program",
        WS_OVERLAPPEDWINDOW,
        300, 300,
        800, 600,
        NULL,
        NULL,
        hInstance,
        NULL);

    ShowWindow(hWnd, nCmdShow);

    // set up and initialize Direct3D
    initD3D(hWnd);

    // enter the main loop:

    MSG msg;

    while (TRUE)
    {
        while (PeekMessage(&msg, NULL, 0, 0, PM_REMOVE))
        {
            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }

        if (msg.message == WM_QUIT)
            break;

        render_frame();
    }

    // clean up DirectX and COM
    cleanD3D();

    return msg.wParam;
}


// this is the main message handler for the program
LRESULT CALLBACK WindowProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    switch (message)
    {
    case WM_DESTROY:
    {
        PostQuitMessage(0);
        return 0;
    } break;
    }

    return DefWindowProc(hWnd, message, wParam, lParam);
}


// this function initializes and prepares Direct3D for use
void initD3D(HWND hWnd)
{
    d3d = Direct3DCreate9(D3D_SDK_VERSION);    // create the Direct3D interface

    D3DPRESENT_PARAMETERS d3dpp;    // create a struct to hold various device information

    ZeroMemory(&d3dpp, sizeof(d3dpp));    // clear out the struct for use
    d3dpp.Windowed = TRUE;    // program windowed, not fullscreen
    d3dpp.SwapEffect = D3DSWAPEFFECT_DISCARD;    // discard old frames
    d3dpp.hDeviceWindow = hWnd;    // set the window to be used by Direct3D


    // create a device class using this information and the info from the d3dpp stuct
    d3d->CreateDevice(D3DADAPTER_DEFAULT,
        D3DDEVTYPE_HAL,
        hWnd,
        D3DCREATE_SOFTWARE_VERTEXPROCESSING,
        &d3dpp,
        &d3ddev);
}


// this is the function used to render a single frame
void render_frame(void)
{
    // clear the window to a deep blue

    IDirect3DSurface9 *offscreenSurface = 0;
    IDirect3DSurface9 *destSurface = 0;




    d3ddev->BeginScene();    // begins the 3D scene

    d3ddev->CreateOffscreenPlainSurface(800, 600, D3DFMT_A8B8G8R8, D3DPOOL_SYSTEMMEM, &offscreenSurface, NULL);
    HRESULT hr = d3ddev->GetBackBuffer(0, 0, D3DBACKBUFFER_TYPE_MONO, &offscreenSurface);
    d3ddev->GetRenderTargetData(offscreenSurface, destSurface);

    HRESULT hr1 = d3ddev->CreateRenderTarget(800, 600, D3DFMT_A8R8G8B8, D3DMULTISAMPLE_NONE, 0, TRUE, &offscreenSurface, NULL);


    // do 3D rendering on the back buffer here

    d3ddev->EndScene();    // ends the 3D scene

    d3ddev->Present(NULL, NULL, NULL, NULL);   // displays the created frame on the screen
}


// this is the function that cleans up Direct3D and COM
void cleanD3D(void)
{
    d3ddev->Release();    // close and release the 3D device
    d3d->Release();    // close and release Direct3D
}

I understand that I am doing something wrong and been trying to figure that out for the past days but I could not fix my code to make it capture screenshot of the desktop.

So please suggest me what changes I need to make in the code so that it does what it is supposed to.