drawing on offscreen surface

[Prasanta Sadhukhan <psadhukhan gmail com>]

Hi,

I am trying to draw something on offscreen surface and then trying to render it on primary window. I started with simple primitives like rectangle but it seems to not draw anything except a blank white window. Here's my code. 
Can anyone suggest as to what I need to do to draw a rectangle on an offscreen surface and then render to primary visible window?

Thanks in advance!!

=========================
#include <gtk/gtk.h>

int main(int argc, char* argv[])
{
// Initialize GTK+
gtk_init(&argc, &argv);
GtkWidget *main_window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
gtk_window_set_default_size(GTK_WINDOW(main_window), 1200, 800);

GdkPixmap *pixmap = gdk_pixmap_new(main_window->window, 1200, 800, -1);
gdk_draw_rectangle(pixmap, main_window->style->black_gc, TRUE, 100, 100, 400, 400);
//gtk_widget_queue_draw_area(main_window, 100, 100, 400, 400);

GtkWidget *widget = gtk_image_new_from_pixmap(pixmap, NULL);
gtk_container_add(GTK_CONTAINER(main_window), widget);

// Make sure the main window and all its contents are visible
gtk_widget_show_all(main_window);
// Run the main GTK+ event loop
gtk_main();
return 0;
}

Regards
Prasanta