FD_ZERO(fd_set *fdset);
//clear all bits in fdsetvoidFD_SET(
intfd, fd_set *fdset);
//turn on the bit for fd in fdsetvoidFD_CLR(
intfd, fd_set *fdset);
//turn off the bit for fd in fdsetintFD_ISSET(
intfd, fd_set *fdset);
//is the bit for fd on in fdset?来源: http://www.cnblogs.com/lengender-12/p/6838235.html