How to set Xlib window background transpatrent?

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Hi:
    I want to create a window that has transparent background.How to do?I use XSetBackground(display, gc, 0), the background is black.I change the depth of the screen to 32.The resault is still black.Here is my code:
        
        display = XOpenDisplay(getenv("DISPALY"));
        screen = DefaultScreen(display);
        depth = DefaultDepth(display,screen);
        printf("depth: %d\n", depth);
        rootwindow = RootWindow(display,screen);


        XVisualInfo vinfo;
        XMatchVisualInfo(display, DefaultScreen(display), 32, TrueColor, &vinfo);


        XSetWindowAttributes attr;
        attr.colormap = XCreateColormap(display, DefaultRootWindow(display), vinfo.visual, AllocNone);
        attr.border_pixel = 0;
        attr.background_pixel = 0; 


        window = XCreateWindow(display, DefaultRootWindow(display), 0, 0, 1440, 900, 0, vinfo.depth, InputOutput,
                        vinfo.visual, CWColormap | CWBorderPixel | CWBackPixel, &attr);
     
        gc = XCreateGC (display, window, 0, NULL);
       XSetBackground(display, gc, 0L);
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