[libevent]event_base_new()

struct event_base *
event_base_new(void)
{
int i;
struct event_base *base; //相当于一个reactor，声明后分配空间

if ((base = calloc(1, sizeof(struct event_base))) == NULL)
event_err(1, "%s: calloc", __func__);

detect_monotonic(); //通过调用clock_gettime()来检测系统是否支持monotonic时钟类型
gettime(base, &base->event_tv);//将当前时间赋值给event_tv

min_heap_ctor(&base->timeheap);//初始化小根堆
TAILQ_INIT(&base->eventqueue);//初始化链表
base->sig.ev_signal_pair[0] = -1;//初始化socket对
base->sig.ev_signal_pair[1] = -1;

base->evbase = NULL;
for (i = 0; eventops[i] && !base->evbase; i++) {//选择统一事件的方法，如epoll
base->evsel = eventops[i];

base->evbase = base->evsel->init(base);//选择完之后调用epoll_init()
}

if (base->evbase == NULL)
event_errx(1, "%s: no event mechanism available", __func__);

if (evutil_getenv("EVENT_SHOW_METHOD"))            //打印一些信息而已，没什么用
event_msgx("libevent using: %s\n",
base->evsel->name);

/* allocate a single active event queue */
event_base_priority_init(base, 1);          //设置优先级的最高级，这里只有1个优先级

return (base);
}