I can't understand the difference output of only a line differ

[Binary Chen <binary chen gmail com>]

Hi,

I tried gdk_draw_image with a small program, but the behavior is very
confusing, please help me find out whats happening.

With the sleep(1) the program shows nothing, but without it the window
will show, why?

Thanks.
Bin

#include <gtk/gtk.h>

main(int argc, char **argv)
{
        int ret;

        gtk_init(&argc, &argv);

        GtkWidget *win;
        GtkWidget *drawing;
        GdkImage *img;
        GdkGC *gc;

        win = gtk_window_new(GTK_WINDOW_TOPLEVEL);
        drawing = gtk_drawing_area_new();
        gtk_widget_set_size_request(drawing, 200, 200);
        gtk_container_add(GTK_CONTAINER(win), drawing);
        img = gdk_image_new(GDK_IMAGE_FASTEST, gdk_visual_get_system(),
                        200, 200);

        memset(img->mem, 1, 200 * 200 * 2);

        gtk_widget_show_all(win);

        gc = gdk_gc_new(drawing->window);

        while (1) {
                sleep(1);
                gdk_draw_image(drawing->window, gc, img, 0, 0, 0, 0, 200, 200);
        }

        gtk_main();

}