libevent中的信号处理

libevent中将信号集成到event_base_loop事件循环中，通过socketpair转换成I/O事件，本文主要介绍相关的转换。

1、将信号转成I/O

采用socket pair方式，一个socket对，一个用于写，一个用于读，具体工作方式如下

linux环境下直接使用socketpair就可以，而在不支持socketpair情况就需要自己实现socket对,流程图为

libevent中是通过evutil_socketpair来实现的。

2、集成到event_base_loop事件循环中

创建socketpair后，将读socket注册到event_base实例中，当信号发生时，会将signo写入写socket,此时会触发读事件，然后调用signo对应的事件加入到激活事件链表中。

其处理框架为

3、信号初始化

其初始化代码为

int
evsig_init(struct event_base *base)
{/** 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.*///创建socket对if (evutil_socketpair(AF_UNIX, SOCK_STREAM, 0, base->sig.ev_signal_pair) == -1) {
#ifdef WIN32/* Make this nonfatal on win32, where sometimes peoplehave localhost firewalled. */event_sock_warn(-1, "%s: socketpair", __func__);
#elseevent_sock_err(1, -1, "%s: socketpair", __func__);
#endifreturn -1;}//设置socket对的FD_CLOEXEC标志evutil_make_socket_closeonexec(base->sig.ev_signal_pair[0]);evutil_make_socket_closeonexec(base->sig.ev_signal_pair[1]);base->sig.sh_old = NULL;base->sig.sh_old_max = 0;//设置非阻塞evutil_make_socket_nonblocking(base->sig.ev_signal_pair[0]);evutil_make_socket_nonblocking(base->sig.ev_signal_pair[1]);//设置读socket的回调event_assign(&base->sig.ev_signal, base, base->sig.ev_signal_pair[1],EV_READ | EV_PERSIST, evsig_cb, base);base->sig.ev_signal.ev_flags |= EVLIST_INTERNAL;event_priority_set(&base->sig.ev_signal, 0);base->evsigsel = &evsigops;return 0;
}

4、注册

注册信号事件是通过event_add,在设置信号事件时，会将事件的ev_events包含EV_SIGNAL,所以在event_add_internal函数中会调用evmap_signal_add将事件添加到signo对应的事件链表中，同时会调用evsig_add来添加信号处理函数，其代码

static int
evsig_add(struct event_base *base, evutil_socket_t evsignal, short old, short events, void *p)
{struct evsig_info *sig = &base->sig;(void)p;EVUTIL_ASSERT(evsignal >= 0 && evsignal < NSIG);/* catch signals if they happen quickly */EVSIGBASE_LOCK();if (evsig_base != base && evsig_base_n_signals_added) {event_warnx("Added a signal to event base %p with signals ""already added to event_base %p.  Only one can have ""signals at a time with the %s backend.  The base with ""the most recently added signal or the most recent ""event_base_loop() call gets preference; do ""not rely on this behavior in future Libevent versions.",base, evsig_base, base->evsel->name);}evsig_base = base;evsig_base_n_signals_added = ++sig->ev_n_signals_added;evsig_base_fd = base->sig.ev_signal_pair[0];EVSIGBASE_UNLOCK();event_debug(("%s: %d: changing signal handler", __func__, (int)evsignal));if (_evsig_set_handler(base, (int)evsignal, evsig_handler) == -1) {  //设置信号处理函数goto err;}if (!sig->ev_signal_added) {  //读socket是否注册到event_base中，如果没有，就注册到event_base中if (event_add(&sig->ev_signal, NULL))goto err;sig->ev_signal_added = 1;}return (0);err:EVSIGBASE_LOCK();--evsig_base_n_signals_added;--sig->ev_n_signals_added;EVSIGBASE_UNLOCK();return (-1);
}

在介绍之前，先说明evsig_info结构体

struct evsig_info {/* Event watching ev_signal_pair[1] */struct event ev_signal;  //socketpair的读socket事件/* Socketpair used to send notifications from the signal handler */evutil_socket_t ev_signal_pair[2];//socketpair/* True iff we've added the ev_signal event yet. */int ev_signal_added; //读socket事件添加标志/* Count of the number of signals we're currently watching. */int ev_n_signals_added;//添加的信号个数/* Array of previous signal handler objects before Libevent started* messing with them.  Used to restore old signal handlers. */
#ifdef _EVENT_HAVE_SIGACTIONstruct sigaction **sh_old; //先前的信息处理函数
#elseev_sighandler_t **sh_old;
#endif/* Size of sh_old. */int sh_old_max;//先前的最大信号值
};

信号处理函数是通过_evsig_set_handler来实现的

int
_evsig_set_handler(struct event_base *base,int evsignal, void (__cdecl *handler)(int))
{
#ifdef _EVENT_HAVE_SIGACTIONstruct sigaction sa;
#elseev_sighandler_t sh;
#endifstruct evsig_info *sig = &base->sig;void *p;/** resize saved signal handler array up to the highest signal number.* a dynamic array is used to keep footprint on the low side.*/if (evsignal >= sig->sh_old_max) { //添加的信号值大于先前的最大信号值，就重新分配先前的信号处理函数指针数组int new_max = evsignal + 1;event_debug(("%s: evsignal (%d) >= sh_old_max (%d), resizing",__func__, evsignal, sig->sh_old_max));p = mm_realloc(sig->sh_old, new_max * sizeof(*sig->sh_old));if (p == NULL) {event_warn("realloc");return (-1);}memset((char *)p + sig->sh_old_max * sizeof(*sig->sh_old),0, (new_max - sig->sh_old_max) * sizeof(*sig->sh_old));sig->sh_old_max = new_max;sig->sh_old = p;}/* allocate space for previous handler out of dynamic array */sig->sh_old[evsignal] = mm_malloc(sizeof *sig->sh_old[evsignal]);if (sig->sh_old[evsignal] == NULL) {event_warn("malloc");return (-1);}/* save previous handler and setup new handler *///注册新的信号处理函数，同时将先前的信号处理函数保存在sh_old数组中
#ifdef _EVENT_HAVE_SIGACTIONmemset(&sa, 0, sizeof(sa));sa.sa_handler = handler;sa.sa_flags |= SA_RESTART;sigfillset(&sa.sa_mask);if (sigaction(evsignal, &sa, sig->sh_old[evsignal]) == -1) {event_warn("sigaction");mm_free(sig->sh_old[evsignal]);sig->sh_old[evsignal] = NULL;return (-1);}
#elseif ((sh = signal(evsignal, handler)) == SIG_ERR) {  event_warn("signal");mm_free(sig->sh_old[evsignal]);sig->sh_old[evsignal] = NULL;return (-1);}*sig->sh_old[evsignal] = sh;
#endifreturn (0);
}

所有的信号处理函数都是evsig_handler，将信号值写入写socket中，触发event_base中事件循环的读事件

static void __cdecl
evsig_handler(int sig)
{int save_errno = errno;
#ifdef WIN32int socket_errno = EVUTIL_SOCKET_ERROR();
#endifev_uint8_t msg;if (evsig_base == NULL) {event_warnx("%s: received signal %d, but have no base configured",__func__, sig);return;}#ifndef _EVENT_HAVE_SIGACTIONsignal(sig, evsig_handler);
#endif/* Wake up our notification mechanism */msg = sig;send(evsig_base_fd, (char*)&msg, 1, 0);  //发送信号值errno = save_errno;
#ifdef WIN32EVUTIL_SET_SOCKET_ERROR(socket_errno);
#endif
}

5、事件触发

读事件触发后，调用evsig_cb，接收信号值，统计信号触发次数，读取信号对应的回调，添加到激活链表中

static void
evsig_cb(evutil_socket_t fd, short what, void *arg)
{static char signals[1024];ev_ssize_t n;int i;int ncaught[NSIG];struct event_base *base;base = arg;memset(&ncaught, 0, sizeof(ncaught));while (1) {n = recv(fd, signals, sizeof(signals), 0);  //接收信号值if (n == -1) {int err = evutil_socket_geterror(fd);if (! EVUTIL_ERR_RW_RETRIABLE(err))event_sock_err(1, fd, "%s: recv", __func__);break;} else if (n == 0) {/* XXX warn? */break;}//统计信号触发次数for (i = 0; i < n; ++i) {ev_uint8_t sig = signals[i];if (sig < NSIG)ncaught[sig]++;}}EVBASE_ACQUIRE_LOCK(base, th_base_lock);//将事件添加到激活链表中for (i = 0; i < NSIG; ++i) {if (ncaught[i])evmap_signal_active(base, i, ncaught[i]);}EVBASE_RELEASE_LOCK(base, th_base_lock);
}

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