XCB: Catch Substructure Redirect Error

static int redirect(xcb_connection_t* conn)
{
    xcb_screen_iterator_t iter;
    uint32_t root_redirect_values[] = {
        XCB_EVENT_MASK_ENTER_WINDOW | XCB_EVENT_MASK_LEAVE_WINDOW |
        XCB_EVENT_MASK_PROPERTY_CHANGE | XCB_EVENT_MASK_COLOR_MAP_CHANGE |
        XCB_EVENT_MASK_SUBSTRUCTURE_REDIRECT |
        XCB_EVENT_MASK_BUTTON_PRESS | XCB_EVENT_MASK_BUTTON_RELEASE |
        XCB_EVENT_MASK_KEY_PRESS
    };

    /* this assume conn has been initialized */
    iter = xcb_setup_roots_iterator(xcb_get_setup(conn));

    while (iter.rem) {
        xcb_screen_t* screen = iter.data;
        xcb_void_cookie_t cookie;
        xcb_generic_error_t* error;

        cookie = xcb_change_window_attributes_checked(conn, screen->root, XCB_CW_EVENT_MASK, root_redirect_values);
        xcb_flush(conn);
        if (error = xcb_request_check(conn, cookie)) {
            /*
             * Unable to redirect... another window manager is running...?
             */
            fprintf(stderr, "Unable to redirect, another window manager is running...\nerror code: %d\n",
                    error->error_code);
            return false;
        }
        fprintf(stdout, "Redirecting root window of screen %p succeeded...\n", screen);

        /* do some initial setup for this screen */

        xcb_screen_next(&iter);
    }
    return true;
}