如何理解libevent的事件循环

#include <stdio.h>
#include "event2/event.h"
#include <unistd.h>

void stdin_callback(evutil_socket_t fd,short what,void *arg)
{
    static int i = 0;
    char buf[1024];
    struct event* base = (struct event_base *)arg;
    printf("stdin_callback start\n");
    int ret  = read(fd,buf,sizeof(buf));
    buf[ret] = '\0';
    printf("buf %s\n",buf);
    printf("stdin_callback end\n");
    if(++i >=1)
    {
        //event_base_loopbreak(base);
        
        //event_base_loopexit(base,NULL);
        
        struct timeval timeout = {5,0};
        event_base_loopexit(base,&timeout); //5秒之后退出
    }
}

int main()
{
    struct event_base *base = event_base_new();
    struct event *ev0 = event_new(base,STDIN_FILENO,0,NULL.NULL);
    event_assign(ev0,base,STDIN_FILENO,EV_READ|EV_PERSIST,stdin_callback,base);
    event_add(ev0,NULL);
         
    struct event *ev1 = event_new(base,STDIN_FILENO,0,NULL.NULL);
    event_assign(ev1,base,STDIN_FILENO,EV_READ|EV_PERSIST,stdin_callback,base);
    event_add(ev1,NULL);

    /*event loop*/
    event_base_loop(base,0);//效果等同于 event_base_dispatch(base);
    
    //event_base_loop(base,EVLOOP_ONCE);
    
    //event_base_loop(base,EVLOOP_NONBLOCK);
    
    if(event_base_got_break(base))
        puts("event base loop break");
    else if(event_base_got_exit(base))
        puts("event base loop exit");
    
    event_base_free(base);
    event_free(ev0);
    event_free(ev1);
    return 0;

}