[libevent]evsignal_init()

intevsignal_init(struct event_base *base){int i;/*  * Our signal handler is going to write to one end of the socket * pair to wake up our event loop.  The event loop then scans for * signals that got delivered. */if (evutil_socketpair(// 创建socket对    AF_UNIX, SOCK_STREAM, 0, base->sig.ev_signal_pair) == -1) {#ifdef WIN32/* Make this nonfatal on win32, where sometimes people   have localhost firewalled. */event_warn("%s: socketpair", __func__);#elseevent_err(1, "%s: socketpair", __func__);#endifreturn -1;}FD_CLOSEONEXEC(base->sig.ev_signal_pair[0]);//在exec之后关闭这个文件描述符FD_CLOSEONEXEC(base->sig.ev_signal_pair[1]);base->sig.sh_old = NULL;base->sig.sh_old_max = 0;base->sig.evsignal_caught = 0;memset(&base->sig.evsigcaught, 0, sizeof(sig_atomic_t)*NSIG);/* initialize the queues for all events */for (i = 0; i < NSIG; ++i)TAILQ_INIT(&base->sig.evsigevents[i]);        evutil_make_socket_nonblocking(base->sig.ev_signal_pair[0]);event_set(&base->sig.ev_signal, base->sig.ev_signal_pair[1],//设置socket读事件，但是并未注册要base里面EV_READ | EV_PERSIST, evsignal_cb, &base->sig.ev_signal);base->sig.ev_signal.ev_base = base;base->sig.ev_signal.ev_flags |= EVLIST_INTERNAL;//这个标志表示这个读事件不算做在事件链表中，故就算它注册了事件链表无事件的话也会终止循环return 0;}