* \param n the highest-numbered descriptor in any of the two sets, plus 1
* \param readfds fds that should be checked for readability
* \param writefds fds that should be checked for writablility
* \param n the highest-numbered descriptor in any of the two sets, plus 1
* \param readfds fds that should be checked for readability
* \param writefds fds that should be checked for writablility