libevent源码解析（二）event接口函数

旧城等待， 2022-05-22 10:45 265阅读 0赞

### 一.前言 ###

在上一篇中，我们提到了event结构注释里说明了可用的接口函数，即

*    event_new(), event_free(), event_assign(), event_get_assignment(),
     *    event_add(), event_del(), event_active(), event_pending(),
     *    event_get_fd(), event_get_base(), event_get_events(),
     *    event_get_callback(), event_get_callback_arg(),
     *    event_priority_set()

本文对这些接口函数逐个进行分析。这些函数实现了事件的新建、添加、删除、释放、查询、激活、闲置等等功能，即事件的基础功能实现。其实从这些接口函数的名字很容易就可以知道其作用，本文的主要目的是分析这些接口函数的实现原理和内在源码。这些接口函数均在event.c中实现。其中几个get函数其实就是简单的返回操作，所以本文掠过不提。

### 二.接口函数分析 ###

（1）event\_new()函数  
  分配并赋值新的event结构，准备用于添加和删除，即event\_add() 或 event\_del()。  
参数包括：  
  （1）base 新事件属于的事件库event\_base  
  （2）fd 文件描述符或者信号  
  （3）events 对应的控制事件: bitfield of EV\_READ, EV\_WRITE, EV\_SIGNAL, EV\_PERSIST, EV\_ET.  
  （4）callback 事件发生时的回调函数  
  （5）callback\_arg 回调函数传参  
  fd和event决定了什么情况会触发该事件，回调函数和回调传参决定了事件触发时应做什么。  
返回值：  
  返回新的结构体event，必须由event\_free()释放或者置为NULL  
  若events包括 EV\_READ, EV\_WRITE, 或 EV\_READ|EV\_WRITE，则fd是文件描述符或者套接字，并可以用于读、写。若event包括EV\_SIGNAL则fd是信号事件。若没有任何标记，则事件仅可以在超时或手动激活时(调用event\_actifve())生效。  
  代码非常的简单，源码和分析如下所示：

/*新建事件*/
    struct event *
    event_new(struct event_base *base, evutil_socket_t fd, short events, void (*cb)(evutil_socket_t, short, void *), void *arg)
    { 
    	/*创建指针*/
    	struct event *ev;
    	ev = mm_malloc(sizeof(struct event));
    
    	/*错误判断*/
    	if (ev == NULL)
    		return (NULL);
    
    	/*调用event_assign*/
    	if (event_assign(ev, base, fd, events, cb, arg) < 0) { 
    		mm_free(ev);
    		return (NULL);
    	}
    
    	return (ev);
    }

（2）event\_free()函数  
  源码和分析如下所示：

/*释放事件（包括资源释放）*/
    void
    event_free(struct event *ev)
    { 
    	/* This is disabled, so that events which have been finalized be a * valid target for event_free(). That's */
    	// event_debug_assert_is_setup_(ev);
    
    	/* make sure that this event won't be coming back to haunt us. */
    	event_d el(ev);
    	event_debug_note_teardown_(ev);/*关闭针对ev的debug*/
    	mm_free(ev);
    
    }

（3）event\_assign()函数  
  源码和分析如下所示：

/*事件的配置：赋值以及异常处理*/
    int
    event_assign(struct event *ev, struct event_base *base, evutil_socket_t fd, short events, void (*callback)(evutil_socket_t, short, void *), void *arg)
    { 
    	/*异常处理*/
    	if (!base)
    		base = current_base;
    	if (arg == &event_self_cbarg_ptr_)
    		arg = ev;
    
    	event_debug_assert_not_added_(ev);
    
    	/*将事件的堆属性赋值为已有的事件堆base*/
    	ev->ev_base = base;
    
    	/*赋值*/
    	ev->ev_callback = callback;
    	ev->ev_arg = arg;
    	ev->ev_fd = fd;
    	ev->ev_events = events;
    	ev->ev_res = 0;
    	ev->ev_flags = EVLIST_INIT;
    	ev->ev_ncalls = 0;
    	ev->ev_pncalls = NULL;
    
    	/*根据事件类型分开进行错误判断*/
    	/*信号类型*/
    	if (events & EV_SIGNAL) { 
    		/*信号类型不允许有IO类型的读写关闭标记位*/
    		if ((events & (EV_READ|EV_WRITE|EV_CLOSED)) != 0) { 
    			event_warnx("%s: EV_SIGNAL is not compatible with "
    			    "EV_READ, EV_WRITE or EV_CLOSED", __func__);
    			return -1;
    		}
    		ev->ev_closure = EV_CLOSURE_EVENT_SIGNAL;
    	} else { 
    		/*对永久事件，超时置零*/
    		if (events & EV_PERSIST) { 
    			evutil_timerclear(&ev->ev_io_timeout);
    			ev->ev_closure = EV_CLOSURE_EVENT_PERSIST;
    		} else { 
    			ev->ev_closure = EV_CLOSURE_EVENT;
    		}
    	}
    
    	min_heap_elem_init_(ev);
    
    	/*优先级：默认放在队列中间*/
    	if (base != NULL) { 
    		/* by default, we put new events into the middle priority */
    		ev->ev_pri = base->nactivequeues / 2;
    	}
    
    	/*debug功能开启*/
    	event_debug_note_setup_(ev);
    
    	return 0;
    }

（4）event\_get\_assignment()函数  
  源码和分析如下所示：

/*获取event的属性：包括事件堆event_base, IO事件的fd, 信号事件的信息，回调函数等*/
    void
    event_get_assignment(const struct event *event, struct event_base **base_out, evutil_socket_t *fd_out, short *events_out, event_callback_fn *callback_out, void **arg_out)
    { 
    	event_debug_assert_is_setup_(event);
    
    	if (base_out)
    		*base_out = event->ev_base;
    	if (fd_out)
    		*fd_out = event->ev_fd;
    	if (events_out)
    		*events_out = event->ev_events;
    	if (callback_out)
    		*callback_out = event->ev_callback;
    	if (arg_out)
    		*arg_out = event->ev_arg;
    }

（5）event\_add()函数  
  源码和分析如下所示：

/*添加事件*/
    int
    event_add(struct event *ev, const struct timeval *tv)
    { 
    	int res;
    
    	/*异常处理，检查是否有事件堆event_base*/
    	if (EVUTIL_FAILURE_CHECK(!ev->ev_base)) { 
    		event_warnx("%s: event has no event_base set.", __func__);
    		return -1;
    	}
    
    	/*加锁、调用event_add_nolock_添加事件，解锁*/
    	EVBASE_ACQUIRE_LOCK(ev->ev_base, th_base_lock);
    
    	res = event_add_nolock_(ev, tv, 0);
    
    	EVBASE_RELEASE_LOCK(ev->ev_base, th_base_lock);
    
    	return (res);
    }

其中调用的event\_add\_nolock\_()，该函数较为复杂，源码和分析如下：

/* Implementation function to add an event. Works just like event_add, * except: 1) it requires that we have the lock. 2) if tv_is_absolute is set, * we treat tv as an absolute time, not as an interval to add to the current * time */
    int
    event_add_nolock_(struct event *ev, const struct timeval *tv,
        int tv_is_absolute)
    { 
    	struct event_base *base = ev->ev_base;
    	int res = 0;
    	int notify = 0;
    
    	/*上锁判断，debug判断*/
    	EVENT_BASE_ASSERT_LOCKED(base);
    	event_debug_assert_is_setup_(ev);
    
    	event_debug((
    		 "event_add: event: %p (fd "EV_SOCK_FMT"), %s%s%s%scall %p",
    		 ev,
    		 EV_SOCK_ARG(ev->ev_fd),
    		 ev->ev_events & EV_READ ? "EV_READ " : " ",
    		 ev->ev_events & EV_WRITE ? "EV_WRITE " : " ",
    		 ev->ev_events & EV_CLOSED ? "EV_CLOSED " : " ",
    		 tv ? "EV_TIMEOUT " : " ",
    		 ev->ev_callback));
    
    	EVUTIL_ASSERT(!(ev->ev_flags & ~EVLIST_ALL));
    
    	if (ev->ev_flags & EVLIST_FINALIZING) { 
    		/* XXXX debug */
    		return (-1);
    	}
    
    	/* * 新的timer事件，调用timer heap接口在堆上预留一个位置 * 注：这样能保证该操作的原子性： * 向系统I/O机制注册可能会失败，而当在堆上预留成功后， * 定时事件的添加将肯定不会失败； * 而预留位置的可能结果是堆扩充，但是内部元素并不会改变 * prepare for timeout insertion further below, if we get a * failure on any step, we should not change any state. */
    	if (tv != NULL && !(ev->ev_flags & EVLIST_TIMEOUT)) { 
    		if (min_heap_reserve_(&base->timeheap,
    			1 + min_heap_size_(&base->timeheap)) == -1)
    			return (-1);  /* ENOMEM == errno */
    	}
    
    
    	/* If the main thread is currently executing a signal event's * callback, and we are not the main thread, then we want to wait * until the callback is done before we mess with the event, or else * we can race on ev_ncalls and ev_pncalls below. */
    #ifndef EVENT__DISABLE_THREAD_SUPPORT
    	if (base->current_event == event_to_event_callback(ev) &&
    	    (ev->ev_events & EV_SIGNAL)
    	    && !EVBASE_IN_THREAD(base)) { 
    		++base->current_event_waiters;
    		EVTHREAD_COND_WAIT(base->current_event_cond, base->th_base_lock);
    	}
    #endif
    
    	/*如果事件ev不在已注册或者激活链表中，则调用evbase注册事件 */
    	if ((ev->ev_events & (EV_READ|EV_WRITE|EV_CLOSED|EV_SIGNAL)) &&
    	    !(ev->ev_flags & (EVLIST_INSERTED|EVLIST_ACTIVE|EVLIST_ACTIVE_LATER))) { 
    	    
    	    /*判断io或者信号分类添加*/
    		if (ev->ev_events & (EV_READ|EV_WRITE|EV_CLOSED))
    			res = evmap_io_add_(base, ev->ev_fd, ev);
    		else if (ev->ev_events & EV_SIGNAL)
    			res = evmap_signal_add_(base, (int)ev->ev_fd, ev);
    		
    		/*// 注册成功，插入event到已注册链表中 */
    		if (res != -1)
    			event_queue_insert_inserted(base, ev);
    		if (res == 1) { 
    			/* evmap says we need to notify the main thread. */
    			notify = 1;
    			res = 0;
    		}
    	}
    
    	/* 准备添加定时事件 * we should change the timeout state only if the previous event * addition succeeded. */
    	if (res != -1 && tv != NULL) { 
    		struct timeval now;
    		int common_timeout;
    #ifdef USE_REINSERT_TIMEOUT
    		int was_common;
    		int old_timeout_idx;
    #endif
    
    		/* * for persistent timeout events, we remember the * timeout value and re-add the event. * * If tv_is_absolute, this was already set. */
    		if (ev->ev_closure == EV_CLOSURE_EVENT_PERSIST && !tv_is_absolute)
    			ev->ev_io_timeout = *tv;
    
    		/*EVLIST_TIMEOUT表明event已经在定时器堆中了，删除旧的*/
    #ifndef USE_REINSERT_TIMEOUT
    		if (ev->ev_flags & EVLIST_TIMEOUT) { 
    			event_queue_remove_timeout(base, ev);
    		}
    #endif
    
    		/* 如果事件已经是就绪状态则从激活链表中删除 * Check if it is active due to a timeout. Rescheduling * this timeout before the callback can be executed * removes it from the active list. */
    		if ((ev->ev_flags & EVLIST_ACTIVE) &&
    		    (ev->ev_res & EV_TIMEOUT)) { 
    			if (ev->ev_events & EV_SIGNAL) { 
    				/* See if we are just active executing * this event in a loop * 将ev_callback调用次数设置为0以终止循环 */
    				if (ev->ev_ncalls && ev->ev_pncalls) { 
    					/* Abort loop */
    					*ev->ev_pncalls = 0;
    				}
    			}
    
    			event_queue_remove_active(base, event_to_event_callback(ev));
    		}
    
    		/* 计算时间，并插入到timer根堆中 */
    		gettime(base, &now);
    
    		common_timeout = is_common_timeout(tv, base);
    #ifdef USE_REINSERT_TIMEOUT
    		was_common = is_common_timeout(&ev->ev_timeout, base);
    		old_timeout_idx = COMMON_TIMEOUT_IDX(&ev->ev_timeout);
    #endif
    
    		if (tv_is_absolute) { 
    			ev->ev_timeout = *tv;
    		} else if (common_timeout) { 
    			struct timeval tmp = *tv;
    			tmp.tv_usec &= MICROSECONDS_MASK;
    			evutil_timeradd(&now, &tmp, &ev->ev_timeout);
    			ev->ev_timeout.tv_usec |=
    			    (tv->tv_usec & ~MICROSECONDS_MASK);
    		} else { 
    			evutil_timeradd(&now, tv, &ev->ev_timeout);
    		}
    
    		event_debug((
    			 "event_add: event %p, timeout in %d seconds %d useconds, call %p",
    			 ev, (int)tv->tv_sec, (int)tv->tv_usec, ev->ev_callback));
    
    #ifdef USE_REINSERT_TIMEOUT
    		event_queue_reinsert_timeout(base, ev, was_common, common_timeout, old_timeout_idx);
    #else
    		event_queue_insert_timeout(base, ev);
    #endif
    
    		if (common_timeout) { 
    			struct common_timeout_list *ctl =
    			    get_common_timeout_list(base, &ev->ev_timeout);
    			if (ev == TAILQ_FIRST(&ctl->events)) { 
    				common_timeout_schedule(ctl, &now, ev);
    			}
    		} else { 
    			struct event* top = NULL;
    			/* See if the earliest timeout is now earlier than it * was before: if so, we will need to tell the main * thread to wake up earlier than it would otherwise. * We double check the timeout of the top element to * handle time distortions due to system suspension. */
    			if (min_heap_elt_is_top_(ev))
    				notify = 1;
    			else if ((top = min_heap_top_(&base->timeheap)) != NULL &&
    					 evutil_timercmp(&top->ev_timeout, &now, <))
    				notify = 1;
    		}
    	}
    
    	/* if we are not in the right thread, we need to wake up the loop */
    	if (res != -1 && notify && EVBASE_NEED_NOTIFY(base))
    		evthread_notify_base(base);
    
    	event_debug_note_add_(ev);
    
    	return (res);
    }

（6）event\_del()函数  
  源码和分析如下所示：

int
    event_del(struct event *ev)
    { 
    	return event_del_(ev, EVENT_DEL_AUTOBLOCK);
    }

调用的event\_del\_()如下：

static int
    event_del_(struct event *ev, int blocking)
    { 
    	int res;
    	struct event_base *base = ev->ev_base;
    
    	/*异常处理*/
    	if (EVUTIL_FAILURE_CHECK(!base)) { 
    		event_warnx("%s: event has no event_base set.", __func__);
    		return -1;
    	}
    
    	/*上锁*/
    	EVBASE_ACQUIRE_LOCK(base, th_base_lock);
    	res = event_del_nolock_(ev, blocking);
    	EVBASE_RELEASE_LOCK(base, th_base_lock);
    
    	return (res);
    }

真正的删除和添加一样，在上锁之后调用函数执行：

/** 事件删除 Helper for event_del: always called with th_base_lock held. * * "blocking" must be one of the EVENT_DEL_{BLOCK, NOBLOCK, AUTOBLOCK, * EVEN_IF_FINALIZING} values. See those for more information. */
    int
    event_del_nolock_(struct event *ev, int blocking)
    { 
    	struct event_base *base;
    	int res = 0, notify = 0;
    
    	event_debug(("event_del: %p (fd "EV_SOCK_FMT"), callback %p",
    		ev, EV_SOCK_ARG(ev->ev_fd), ev->ev_callback));
    
    	/* 异常处理，ev_base为NULL，表明ev没有被注册 * An event without a base has not been added */
    	if (ev->ev_base == NULL)
    		return (-1);
    
    	EVENT_BASE_ASSERT_LOCKED(ev->ev_base);
    
    	if (blocking != EVENT_DEL_EVEN_IF_FINALIZING) { 
    		if (ev->ev_flags & EVLIST_FINALIZING) { 
    			/* XXXX Debug */
    			return 0;
    		}
    	}
    
    	base = ev->ev_base;
    
    	EVUTIL_ASSERT(!(ev->ev_flags & ~EVLIST_ALL));
    
    	/* 终止循环 See if we are just active executing this event in a loop */
    	if (ev->ev_events & EV_SIGNAL) { 
    		if (ev->ev_ncalls && ev->ev_pncalls) { 
    			/* Abort loop */
    			*ev->ev_pncalls = 0;
    		}
    	}
    
    	if (ev->ev_flags & EVLIST_TIMEOUT) { 
    		/* 从超时队列中删除 * NOTE: We never need to notify the main thread because of a * deleted timeout event: all that could happen if we don't is * that the dispatch loop might wake up too early. But the * point of notifying the main thread _is_ to wake up the * dispatch loop early anyway, so we wouldn't gain anything by * doing it. */
    		event_queue_remove_timeout(base, ev);
    	}
    
    	/*从激活/等待激活队列中删除*/
    	if (ev->ev_flags & EVLIST_ACTIVE)
    		event_queue_remove_active(base, event_to_event_callback(ev));
    	else if (ev->ev_flags & EVLIST_ACTIVE_LATER)
    		event_queue_remove_active_later(base, event_to_event_callback(ev));
    
    	/*从对应的链表中删除事件 */
    	if (ev->ev_flags & EVLIST_INSERTED) { 
    		event_queue_remove_inserted(base, ev);
    		if (ev->ev_events & (EV_READ|EV_WRITE|EV_CLOSED))
    			res = evmap_io_del_(base, ev->ev_fd, ev);
    		else
    			res = evmap_signal_del_(base, (int)ev->ev_fd, ev);
    		if (res == 1) { 
    			/* evmap says we need to notify the main thread. */
    			notify = 1;
    			res = 0;
    		}
    		/* If we do not have events, let's notify event base so it can * exit without waiting */
    		if (!event_haveevents(base) && !N_ACTIVE_CALLBACKS(base))
    			notify = 1;
    	}
    
    	/* 多线程情况下，判断是否在该线程执行 if we are not in the right thread, we need to wake up the loop */
    	if (res != -1 && notify && EVBASE_NEED_NOTIFY(base))
    		evthread_notify_base(base);
    
    	event_debug_note_del_(ev);
    
    	/* If the main thread is currently executing this event's callback, * and we are not the main thread, then we want to wait until the * callback is done before returning. That way, when this function * returns, it will be safe to free the user-supplied argument. */
    #ifndef EVENT__DISABLE_THREAD_SUPPORT
    	if (blocking != EVENT_DEL_NOBLOCK &&
    	    base->current_event == event_to_event_callback(ev) &&
    	    !EVBASE_IN_THREAD(base) &&
    	    (blocking == EVENT_DEL_BLOCK || !(ev->ev_events & EV_FINALIZE))) { 
    		++base->current_event_waiters;
    		EVTHREAD_COND_WAIT(base->current_event_cond, base->th_base_lock);
    	}
    #endif
    
    	return (res);
    }

（7）event\_active()函数  
  源码和分析如下所示：

void
    event_active(struct event *ev, int res, short ncalls)
    { 
    	/*异常处理*/
    	if (EVUTIL_FAILURE_CHECK(!ev->ev_base)) { 
    		event_warnx("%s: event has no event_base set.", __func__);
    		return;
    	}
    
    	/*上锁*/
    	EVBASE_ACQUIRE_LOCK(ev->ev_base, th_base_lock);
    
    	event_debug_assert_is_setup_(ev);
    
    	event_active_nolock_(ev, res, ncalls);
    
    	EVBASE_RELEASE_LOCK(ev->ev_base, th_base_lock);
    }

event\_active\_nolock\_()代码如下：

/*激活事件*/
    void
    event_active_nolock_(struct event *ev, int res, short ncalls)
    { 
    	struct event_base *base;
    
    	event_debug(("event_active: %p (fd "EV_SOCK_FMT"), res %d, callback %p",
    		ev, EV_SOCK_ARG(ev->ev_fd), (int)res, ev->ev_callback));
    
    	base = ev->ev_base;
    	EVENT_BASE_ASSERT_LOCKED(base);
    
    	/*标记为终止则无法激活*/
    	if (ev->ev_flags & EVLIST_FINALIZING) { 
    		/* XXXX debug */
    		return;
    	}
    
    	/*根据标记位判断立刻激活或者稍后激活*/
    	switch ((ev->ev_flags & (EVLIST_ACTIVE|EVLIST_ACTIVE_LATER))) { 
    	default:
    	case EVLIST_ACTIVE|EVLIST_ACTIVE_LATER:
    		EVUTIL_ASSERT(0);
    		break;
    	case EVLIST_ACTIVE:
    		/* We get different kinds of events, add them together */
    		ev->ev_res |= res;
    		return;
    	case EVLIST_ACTIVE_LATER:
    		ev->ev_res |= res;
    		break;
    	case 0:
    		ev->ev_res = res;
    		break;
    	}
    
    	if (ev->ev_pri < base->event_running_priority)
    		base->event_continue = 1;
    
    	if (ev->ev_events & EV_SIGNAL) { 
    #ifndef EVENT__DISABLE_THREAD_SUPPORT
    		if (base->current_event == event_to_event_callback(ev) &&
    		    !EVBASE_IN_THREAD(base)) { 
    			++base->current_event_waiters;
    			EVTHREAD_COND_WAIT(base->current_event_cond, base->th_base_lock);
    		}
    #endif
    		ev->ev_ncalls = ncalls;
    		ev->ev_pncalls = NULL;
    	}
    
    	/*将事件添加入激活列表中*/
    	event_callback_activate_nolock_(base, event_to_event_callback(ev));
    }

（8）event\_pending()函数  
  源码和分析如下所示：

/* 检测某事件是否待发生，返回标记位 * Checks if a specific event is pending or scheduled. */
    
    int
    event_pending(const struct event *ev, short event, struct timeval *tv)
    { 
    	int flags = 0;
    
    	/*异常检测*/
    	if (EVUTIL_FAILURE_CHECK(ev->ev_base == NULL)) { 
    		event_warnx("%s: event has no event_base set.", __func__);
    		return 0;
    	}
    
    	EVBASE_ACQUIRE_LOCK(ev->ev_base, th_base_lock);
    	event_debug_assert_is_setup_(ev);
    
    	/*检查标记位*/
    	if (ev->ev_flags & EVLIST_INSERTED)
    		flags |= (ev->ev_events & (EV_READ|EV_WRITE|EV_CLOSED|EV_SIGNAL));
    	if (ev->ev_flags & (EVLIST_ACTIVE|EVLIST_ACTIVE_LATER))
    		flags |= ev->ev_res;
    	if (ev->ev_flags & EVLIST_TIMEOUT)
    		flags |= EV_TIMEOUT;
    
    	event &= (EV_TIMEOUT|EV_READ|EV_WRITE|EV_CLOSED|EV_SIGNAL);
    
    	/* 添加超时 See if there is a timeout that we should report */
    	if (tv != NULL && (flags & event & EV_TIMEOUT)) { 
    		struct timeval tmp = ev->ev_timeout;
    		tmp.tv_usec &= MICROSECONDS_MASK;
    		/* correctly remamp to real time */
    		evutil_timeradd(&ev->ev_base->tv_clock_diff, &tmp, tv);
    	}
    
    	EVBASE_RELEASE_LOCK(ev->ev_base, th_base_lock);
    
    	return (flags & event);
    }

（9）event\_priority\_set()函数  
  源码和分析如下所示：

/* 设置优先级 * Set's the priority of an event - if an event is already scheduled * changing the priority is going to fail. */
    
    int
    event_priority_set(struct event *ev, int pri)
    { 
    	event_debug_assert_is_setup_(ev);
    
    	/*优先级设置对已激活的事件无效*/
    	if (ev->ev_flags & EVLIST_ACTIVE)
    		return (-1);
    	if (pri < 0 || pri >= ev->ev_base->nactivequeues)
    		return (-1);
    
    	ev->ev_pri = pri;
    
    	return (0);
    }

### 三.小结 ###

本文分析了event的接口函数，下一篇中会分析event\_base的接口函数，并由此分析事件处理的中心部分——事件主循环，根据系统提供的事件多路分发机制执行事件循环，对已注册的就绪事件，调用注册事件的回调函数来处理事件。

--------------------

欢迎关注本人公众号，公众号会更新一些不一样的内容，相信一定会有所收获。  
![在这里插入图片描述][20201219120057415.jpg_pic_center]

[20201219120057415.jpg_pic_center]: /images/20220522/3df8db3fb7b4411c957e7eb87d975aae.png