[libevent]evsignal_add()

intevsignal_add(struct event *ev){int evsignal;struct event_base *base = ev->ev_base;struct evsignal_info *sig = &ev->ev_base->sig;if (ev->ev_events & (EV_READ|EV_WRITE))event_errx(1, "%s: EV_SIGNAL incompatible use", __func__);evsignal = EVENT_SIGNAL(ev);//获取事件的信号值assert(evsignal >= 0 && evsignal < NSIG);if (TAILQ_EMPTY(&sig->evsigevents[evsignal])) {//如果信号链表未注册这个信号的话，注册这个信号并保存之前的回调函数event_debug(("%s: %p: changing signal handler", __func__, ev));if (_evsignal_set_handler(    base, evsignal, evsignal_handler) == -1)return (-1);/* catch signals if they happen quickly */evsignal_base = base;if (!sig->ev_signal_added) {//第一次注册信号事件的话要将socket的读事件注册if (event_add(&sig->ev_signal, NULL))return (-1);sig->ev_signal_added = 1;}}/* multiple events may listen to the same signal */TAILQ_INSERT_TAIL(&sig->evsigevents[evsignal], ev, ev_signal_next);//将该事件插入到信号链表return (0);}