游戏窗口的屏幕截图

如何在c++中截屏全屏游戏？我知道这可以用directx完成，我已经写了一个简单的代码，可以截图一个窗口，但我不知道如何获得游戏的HWND。这是我的密码。

#include <d3dx9.h>
#include <d3dx9tex.h>
#include <stdio.h>
#pragma comment(lib,"d3d9.lib")
#pragma comment(lib,"d3dx9.lib")
int main()
{
    IDirect3DSurface9 *surface;
    IDirect3DDevice9 *g_pd3dDevice;
    IDirect3D9 *g_pD3D;
    D3DDISPLAYMODE d3ddm;
    D3DPRESENT_PARAMETERS d3dpp;
    HWND hWnd = GetConsoleWindow();
    if((g_pD3D = Direct3DCreate9(D3D_SDK_VERSION)) == NULL)
    {
        printf("fail 1");
    }
    if(FAILED(g_pD3D->GetAdapterDisplayMode(D3DADAPTER_DEFAULT, &d3ddm)))
    {
        printf("fail 2");
    }
    ZeroMemory(&d3dpp, sizeof(D3DPRESENT_PARAMETERS));
    d3dpp.Windowed = TRUE;
    d3dpp.SwapEffect = D3DSWAPEFFECT_DISCARD;
    d3dpp.BackBufferFormat = d3ddm.Format;
    d3dpp.BackBufferHeight = d3ddm.Height;
    d3dpp.BackBufferWidth = d3ddm.Width;
    if(FAILED(g_pD3D->CreateDevice(D3DADAPTER_DEFAULT,
                                   D3DDEVTYPE_HAL, hWnd,
                                   D3DCREATE_SOFTWARE_VERTEXPROCESSING,
                                   &d3dpp, &g_pd3dDevice)))
    {
        printf("fail 3");
    }
    g_pd3dDevice->CreateOffscreenPlainSurface(800, 600, D3DFMT_A8R8G8B8,     D3DPOOL_SCRATCH, &surface, NULL);
    g_pd3dDevice->GetFrontBufferData(0, surface);
    D3DXSaveSurfaceToFile("c:\tmp\output.jpg", D3DXIFF_JPG, surface, NULL, NULL);
    surface->Release();
    return 0;
}