关于获取一个union的内部字段

Question

有如下一段代码，为何在外部可以访问ev_ncalls而不需要经过_ev和ev_signal两层封装呢？
event的结构如下

struct event {
	union {
		/* used for io events */
		struct {
			TAILQ_ENTRY(event) ev_io_next;
			struct timeval ev_timeout;
		} ev_io;

		/* used by signal events */
		struct {
			TAILQ_ENTRY(event) ev_signal_next;
			short ev_ncalls;
			/* Allows deletes in callback */
			short *ev_pncalls;
		} ev_signal;
	} _ev;

};

struct event *ev;
int ncall = ev->ev_ncalls;

Answer 1

看来要自问自答了，看代码不仔细。。。
在event-internal.h里面有

#define ev_ncalls   _ev.ev_signal.ev_ncalls
#define ev_pncalls  _ev.ev_signal.ev_pncalls