/* these may evaluate ev multiple times, and the other arguments at most once */
/* either use ev_init + ev_TYPE_set, or the ev_TYPE_init macro, below, to first initialise a watcher */
#define ev_init(ev,cb_) do { \
((ev_watcher *)(void *)(ev))->active = \
((ev_watcher *)(void *)(ev))->pending = ; \
ev_set_priority ((ev), ); \
ev_set_cb ((ev), cb_); \
} while ()
当event_loop初始化后，就可以把自己想要做的时间丢给loop啦，具体怎么搞呢，可以调用ev_init这个宏，尼玛，又是宏！
其实关键的一句就是
ev_set_cb ((ev), cb_); 就是把ev_io的回调设置为cb_啦。 ok，就是这样！ 其中，
ev_watcher 是神马玩意呢？
/* shared by all watchers */
#define EV_WATCHER(type) \
int active; /* private */ \
int pending; /* private */ \
EV_DECL_PRIORITY /* private */ \ //int priority
EV_COMMON /* rw */ \ //void* Data
EV_CB_DECLARE (type) /* private */ //声明了第一个参数为type的回调
#define EV_WATCHER_LIST(type) \
EV_WATCHER (type) \
struct ev_watcher_list *next; /* private */
是指一个watcher链表