libevent(3)

信号signal事件：

见博客http://www.cnblogs.com/hustcat/archive/2010/08/31/1814022.html分析

socket pair
Libevent通过socketpair，将signal事件与I/O事件完美的统一起来。Socketpair，简单的说就一对socket，一端用于写，一端用于读。工作方式如下

为了与I/O事件统一起来，libevent内部使用了一个针对read socket的读事件。

Socketpair的创建
与信号事件的初始化工作都是在evsignal_init中完成的，而evsignal_init通过调用evutil_socketpair创建socketpair。对于Unix平台，有socketpair系统调用；对于Windows，则相对复杂一些，具体见evutil_socketpair函数的实现。

Socketpair used to send notifications from the signal handler

evsig_info结构体分析

struct evsig_info {
	/* Event watching ev_signal_pair[1] */
	struct event ev_signal;
	/* Socketpair used to send notifications from the signal handler */
	evutil_socket_t ev_signal_pair[2];
	/* True iff we've added the ev_signal event yet. */
	int ev_signal_added;
	/* Count of the number of signals we're currently watching. */
	int ev_n_signals_added;

	/* Array of previous signal handler objects before Libevent started
	 * messing with them.  Used to restore old signal handlers. */
#ifdef EVENT__HAVE_SIGACTION
	struct sigaction **sh_old;
#else
	ev_sighandler_t **sh_old;
#endif
	/* Size of sh_old. */
	int sh_old_max;
};