Env variables SDL_VIDEO_WINDOW_POS and SDL_VIDEO_CENTERED usage?

SDL Development
1

Hi,
I am developing a GTK application with a drawing area widget in which
a mediaplayer’s video output is displayed. I use the environment
variable SDL_WINDOWID
to initialise the SDL to use the window of the drawing area.

       long sdl_xwid;
       sdl_xwid = GDK_DRAWABLE_XID (screen->window); //where

screen is a drawing area widget

        char SDL_windowhack[32];
        sprintf (SDL_windowhack, "SDL_WINDOWID=%ld", sdl_xwid);
        putenv (SDL_windowhack);

The Gtk window size is 320x240. If the media file’s resolution is
176x144, i would like to orient its display to the center of the
GTK(now SDL’s) window.
For this i set the following environment variables

        char SDL_windowpos[32];
        sprintf (SDL_windowpos, "SDL_VIDEO_WINDOW_POS=center");
        putenv (SDL_windowpos);

        char SDL_windowcenter[32];
        sprintf(SDL_windowcenter, "SDL_VIDEO_CENTERED=1");
        putenv (SDL_windowcenter);

But still the video is displayed on the top left corner of the window.
How do i get the video displayed in the center? Kindly Help.

Thanks,
Unna