Obtain a screen pointer by the screen number with xcb.

continue-do-while.c

// Find XCB screen.
int default_screen_number = XDefaultScreen(display);

xcb_screen_t *screen = NULL;
xcb_screen_iterator_t iter = xcb_setup_roots_iterator(xcb_get_setup(connection));
int index = 0;
do {
    if (index == default_screen_number) {
        screen = iter.data;
        break;
    }
    if (iter.rem > 0) {
        index += 1;
        xcb_screen_next(&iter);
        continue;
    }
} while (0);

if (!screen) {
    fprintf(stderr, "Failed to find screen from screen number.\n");
    exit_code = -1;
    break;
}

Implementation of xcb_screen_next:

void
xcb_screen_next (xcb_screen_iterator_t *i)
{
    xcb_screen_t *R = i->data;
    xcb_generic_iterator_t child;
    child.data = (xcb_screen_t *)(((char *)R) + xcb_screen_sizeof(R));
    i->index = (char *) child.data - (char *) i->data;
    --i->rem;
    i->data = (xcb_screen_t *) child.data;
}

xcb_screen_sizeof doesn't always return the same number, it can vary depending on the type of screen.

:)