fd_set rset_stack, wset_stack, xset_stack;
  fd_set *rset = &rset_stack, *wset = &wset_stack, *xset = &xset_stack;

  if (nfds > FD_SETSIZE)
    {
      size_t nelems = __DARWIN_howmany(nfds, __DARWIN_NFDBITS);
      size_t size = sizeof (struct { int32_t fds_bits[nelems] });
      rset = g_malloc0 (size);
      wset = g_malloc0 (size);
      xset = g_malloc0 (size);
    }

  …

  if (nfds > FD_SETSIZE)
    {
      g_free (rset);
      g_free (wset);
      g_free (xset);
    }