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 = ;
//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
+ min_heap_size(&base->timeheap)) == -)
return (-); /* 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 != -)
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 != - && 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 = ; //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(, "%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(, "%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(, "%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(, "%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 (-);
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 = ;
}
//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 ();
}