接下来就是将已经初始化的event注册到libevent的事件链表上，通过event_add()来实现，源码位于event.c中。
event_add()
这个函数主要完成了下面几件事：
1.将event注册到event_base的I/O多路复用要监听的事件中
2.将event注册到event_base的已注册事件链表中
3.如果传入了超时时间，则删除旧的超时时间，重新设置，并将event添加到event_base的小根堆中；
如果没有传入超时时间，则不会添加到小根堆中。
只有步骤1成功，才会执行步骤2和3；否则什么都没做，直接返回，保证不会改变event的状态。
从中还可以看到，将event添加到已注册事件链表、添加到小根堆、从活跃事件链表移除、从小根堆中移除，都是通过两个函数完成的：event_queue_insert()、event_queue_remove()
int event_add(struct event *ev, const struct timeval *tv) { struct event_base *base = ev->ev_base; //event所属的event_base const struct eventop *evsel = base->evsel; //event_base的I/O多路复用机制 void *evbase = base->evbase; //event_base的I/O多路复用机制 int res = 0; //DEBUG log.h event_debug(( "event_add: event: %p, %s%s%scall %p", ev, ev->ev_events & EV_READ ? "EV_READ " : " ", ev->ev_events & EV_WRITE ? "EV_WRITE " : " ", tv ? "EV_TIMEOUT " : " ", ev->ev_callback)); assert(!(ev->ev_flags & ~EVLIST_ALL)); /* * prepare for timeout insertion further below, if we get a * failure on any step, we should not change any state. */ //如果传入了超时时间并且event不再time小根堆上，则在小根堆上预留一个位置 //以保证如果后面有步骤失败，不会改变初始状态，保证是个原子操作 if (tv != NULL && !(ev->ev_flags & EVLIST_TIMEOUT)) { if (min_heap_reserve(&base->timeheap, //min_heap.h 1 + min_heap_size(&base->timeheap)) == -1) return (-1); /* ENOMEM == errno */ } //如果event不在已注册链表或活跃链表中， //则调用evsel->add()注册event事件到I/O多路复用监听的事件上 if ((ev->ev_events & (EV_READ|EV_WRITE|EV_SIGNAL)) && !(ev->ev_flags & (EVLIST_INSERTED|EVLIST_ACTIVE))) { res = evsel->add(evbase, ev); //将event注册到监听事件上 //注册监听事件成功，则将event注册到已注册事件链表上 if (res != -1) event_queue_insert(base, ev, EVLIST_INSERTED); //插入 } /* * we should change the timout state only if the previous event * addition succeeded. */ //前面操作都成功情况下，才能执行下面步骤 //改变超时状态 if (res != -1 && tv != NULL) { struct timeval now; /* * we already reserved memory above for the case where we * are not replacing an exisiting timeout. */ //EVLIST_TIMEOUT表明event已在定时器堆中 //则删除旧的定时器 if (ev->ev_flags & EVLIST_TIMEOUT) event_queue_remove(base, ev, EVLIST_TIMEOUT); //移除 /* Check if it is active due to a timeout. Rescheduling * this timeout before the callback can be executed * removes it from the active list. */ //如果事件是由于超时而变成活跃事件 //则从活跃事件链表中删除 if ((ev->ev_flags & EVLIST_ACTIVE) && (ev->ev_res & EV_TIMEOUT)) { /* See if we are just active executing this * event in a loop */ if (ev->ev_ncalls && ev->ev_pncalls) { /* Abort loop */ *ev->ev_pncalls = 0; //调用次数清0 } //从活跃事件链表移除 event_queue_remove(base, ev, EVLIST_ACTIVE); //移除 } gettime(base, &now); evutil_timeradd(&now, tv, &ev->ev_timeout); //为event添加超时时间 event_debug(( "event_add: timeout in %ld seconds, call %p", tv->tv_sec, ev->ev_callback)); //将event插入到小根堆中 event_queue_insert(base, ev, EVLIST_TIMEOUT); //插入 } return (res); }
event_queue_insert()
该函数根据不同的输入队列，即不同的事件，在不同的队列中插入，并增加相应的事件计数，更新event状态；
EVLIST_INSERTED：在已注册事件链表event_base.eventqueue插入
EVLIST_ACTIVE：根据event优先级，在活跃事件链表event_base.activequeues[event.ev_pri]插入
EVLIST_TIMEOUT：在小根堆event_base.timeheap中插入
void event_queue_insert(struct event_base *base, struct event *ev, int queue) { //如果event已经在活跃链表中，则返回；否则，出错 if (ev->ev_flags & queue) { /* Double insertion is possible for active events */ if (queue & EVLIST_ACTIVE) return; event_errx(1, "%s: %p(fd %d) already on queue %x", __func__, ev, ev->ev_fd, queue); } if (~ev->ev_flags & EVLIST_INTERNAL) base->event_count++; //增加注册事件数 ev->ev_flags |= queue; //改变event状态 switch (queue) { //根据不同的输入参数队列，选择在不同的事件集合中插入 case EVLIST_INSERTED: //I/O或Signal事件 TAILQ_INSERT_TAIL(&base->eventqueue, ev, ev_next); //在已注册事件链表插入 break; case EVLIST_ACTIVE: //活跃事件 base->event_count_active++; //增加活跃事件数 TAILQ_INSERT_TAIL(base->activequeues[ev->ev_pri], //在活跃事件链表插入 ev,ev_active_next); break; case EVLIST_TIMEOUT: { //定时器事件 min_heap_push(&base->timeheap, ev); //在小根堆插入 break; } default: event_errx(1, "%s: unknown queue %x", __func__, queue); } }
event_queue_remove()
和event_queue_insert()相对应，这个函数主要根据不同的输入参数，从不同的事件集合中删除事件。
void event_queue_remove(struct event_base *base, struct event *ev, int queue) { if (!(ev->ev_flags & queue)) event_errx(1, "%s: %p(fd %d) not on queue %x", __func__, ev, ev->ev_fd, queue); if (~ev->ev_flags & EVLIST_INTERNAL) base->event_count--; ev->ev_flags &= ~queue; switch (queue) { case EVLIST_INSERTED: //I/O、Signal事件 TAILQ_REMOVE(&base->eventqueue, ev, ev_next); break; case EVLIST_ACTIVE: //活跃事件 base->event_count_active--; TAILQ_REMOVE(base->activequeues[ev->ev_pri], ev, ev_active_next); break; case EVLIST_TIMEOUT: //定时器事件 min_heap_erase(&base->timeheap, ev); break; default: event_errx(1, "%s: unknown queue %x", __func__, queue); } }
event_del()
libevent还提供了event_del()这个函数，该函数从直接删除event事件，该函数就是主要通过调用event_queue_remove()函数完成删除的功能。
另外，该函数还将event从I/O多路复用监听的事件中删除。
int event_del(struct event *ev) { struct event_base *base; const struct eventop *evsel; void *evbase; event_debug(("event_del: %p, callback %p", ev, ev->ev_callback)); /* An event without a base has not been added */ if (ev->ev_base == NULL) return (-1); base = ev->ev_base; evsel = base->evsel; evbase = base->evbase; assert(!(ev->ev_flags & ~EVLIST_ALL)); /* See if we are just active executing this event in a loop */ //计数清0 if (ev->ev_ncalls && ev->ev_pncalls) { /* Abort loop */ *ev->ev_pncalls = 0; } //根据event不同的状态，从相应的event集合中删除 if (ev->ev_flags & EVLIST_TIMEOUT) event_queue_remove(base, ev, EVLIST_TIMEOUT); if (ev->ev_flags & EVLIST_ACTIVE) event_queue_remove(base, ev, EVLIST_ACTIVE); if (ev->ev_flags & EVLIST_INSERTED) { event_queue_remove(base, ev, EVLIST_INSERTED); return (evsel->del(evbase, ev)); //从I/O多路复用监听的事件中删除 } return (0); }