就如libevent官网上所写的“libevent - an event notification library”，libevent就是一个基于事件通知机制的库，可以看出event是整个库的核心。event就是Reactor框架中的事件处理程序组件（event_handler），它提供了函数接口，供Reactor在事件发生时调用，以执行相应的事件处理，通常它会绑定一个有效的句柄。

event结构体

　　event结构体定义在<event2/event_struct.h>中：

 1 struct event {
 2     TAILQ_ENTRY(event) ev_active_next;
 3     TAILQ_ENTRY(event) ev_next;
 4     /* for managing timeouts */
 5     union {
 6         TAILQ_ENTRY(event) ev_next_with_common_timeout;
 7         int min_heap_idx;
 8     } ev_timeout_pos;
 9     evutil_socket_t ev_fd;
10
11     struct event_base *ev_base;
12
13     union {　　　　　　　　　　　　　　　　// I/O事件和信号事件不能同时设置
14         /* used for io events */
15         struct {
16             TAILQ_ENTRY(event) ev_io_next;
17             struct timeval ev_timeout;
18         } ev_io;
19
20         /* used by signal events */
21         struct {
22             TAILQ_ENTRY(event) ev_signal_next;
23             short ev_ncalls;
24             /* Allows deletes in callback */
25             short *ev_pncalls;
26         } ev_signal;
27     } _ev;
28
29     short ev_events;
30     short ev_res;        /* result passed to event callback */
31     short ev_flags;
32     ev_uint8_t ev_pri;    /* smaller numbers are higher priority */
33     ev_uint8_t ev_closure;
34     struct timeval ev_timeout;
35
36     /* allows us to adopt for different types of events */
37     void (*ev_callback)(evutil_socket_t, short, void *arg);
38     void *ev_arg;
39 };

　　在原文档中，作者对event结构体做了详细的解释（英文）：

/**
 * @struct event
 *
 * Structure to represent a single event.
 *
 * An event can have some underlying condition it represents: a socket
 * becoming readable or writeable (or both), or a signal becoming raised.
 * (An event that represents no underlying condition is still useful: you
 * can use one to implement a timer, or to communicate between threads.)
 *
 * Generally, you can create events with event_new(), then make them
 * pending with event_add().  As your event_base runs, it will run the
 * callbacks of an events whose conditions are triggered.  When you
 * longer want the event, free it with event_free().
 *
 * In more depth:
 *
 * An event may be "pending" (one whose condition we are watching),
 * "active" (one whose condition has triggered and whose callback is about
 * to run), neither, or both.  Events come into existence via
 * event_assign() or event_new(), and are then neither active nor pending.
 *
 * To make an event pending, pass it to event_add().  When doing so, you
 * can also set a timeout for the event.
 *
 * Events become active during an event_base_loop() call when either their
 * condition has triggered, or when their timeout has elapsed.  You can
 * also activate an event manually using event_active().  The even_base
 * loop will run the callbacks of active events; after it has done so, it
 * marks them as no longer active.
 *
 * You can make an event non-pending by passing it to event_del().  This
 * also makes the event non-active.
 *
 * Events can be "persistent" or "non-persistent".  A non-persistent event
 * becomes non-pending as soon as it is triggered: thus, it only runs at
 * most once per call to event_add().  A persistent event remains pending
 * even when it becomes active: you‘ll need to event_del() it manually in
 * order to make it non-pending.  When a persistent event with a timeout
 * becomes active, its timeout is reset: this means you can use persistent
 * events to implement periodic timeouts.
 *
 * This should be treated as an opaque structure; you should never read or
 * write any of its fields directly.  For backward compatibility with old
 * code, it is defined in the event2/event_struct.h header; including this
 * header may make your code incompatible with other versions of Libevent.
 *
 * @see 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()
 */

　　下面简单解释一下结构体中重要字段的含义：
　　1）ev_events：说明要监听的事件类型（event支持I/O、超时和信号3种事件类型，），它的值可由以下字段位与而成：

 1 /**
 2  * @name event flags
 3  *
 4  * Flags to pass to event_new(), event_assign(), event_pending(), and
 5  * anything else with an argument of the form "short events"
 6  */
 7 /**@{*/
 8 /** Indicates that a timeout has occurred.  It‘s not necessary to pass
 9  * this flag to event_for new()/event_assign() to get a timeout. */
10 #define EV_TIMEOUT    0x01
11 /** Wait for a socket or FD to become readable */
12 #define EV_READ       0x02
13 /** Wait for a socket or FD to become writeable */
14 #define EV_WRITE      0x04
15 /** Wait for a POSIX signal to be raised*/
16 #define EV_SIGNAL     0x08
17 /**
18  * Persistent event: won‘t get removed automatically when activated.
19  *
20  * When a persistent event with a timeout becomes activated, its timeout
21  * is reset to 0.
22  */
23 #define EV_PERSIST    0x10
24 /** Select edge-triggered behavior, if supported by the backend. */
25 #define EV_ET         0x20
26 /**@}*/

　　2）ev_next、ev_active_next、ev_next_with_common_timeout、ev_io_next和ev_signal_next都是双向链表节点指针。它们是libevent对不同事件类型和在不同的时期，对事件的管理时使用到的字段。

　　3）min_heap_idx或ev_next_with_common_timeout指明超时事件在小根堆中的索引或在timeout list中的位置。

　　4）ev_base该事件所属的反应堆实例，这是一个event_base结构体。

　　5）ev_fd，对于I/O事件，是绑定的文件描述符；对于signal事件，是绑定的信号。

　　6）eb_flags：libevent用于标记event信息的字段，表明其当前的状态，可能的值有：

1 #define EVLIST_TIMEOUT  0x01 　// event在time堆中
2 #define EVLIST_INSERTED 0x02 　// event在已注册事件链表中
3 #define EVLIST_SIGNAL   0x04 　// 未见使用
4 #define EVLIST_ACTIVE   0x08 　// event在激活链表中
5 #define EVLIST_INTERNAL 0x10 　// 内部使用标记
6 #define EVLIST_INIT     0x80 　// event已被初始化

　　7）ev_callback，event的回调函数，被ev_base调用，执行事件处理程序，这是一个函数指针，原型为：

void (*ev_callback)(int fd, short events, void *arg)

　　其中参数fd对应于ev_fd；events对应于ev_events；arg对应于ev_arg；
　　8）ev_arg：void*，表明可以是任意类型的数据，在设置event时指定；

　　9）ev_ncalls：事件就绪执行时，调用ev_callback的次数，通常为1；

　　10）ev_pncalls：指针，通常指向ev_ncalls或者为NULL；

　　11）ev_res：记录了当前激活事件的类型；

libevent对event的管理

　　libevent对event的管理如下图所示：

　　每次当有事件event转变为就绪状态时，libevent就会把它移入到active event list[priority]中，其中priority是event的优先级；接着libevent会根据自己的调度策略选择就绪事件，调用其cb_callback() 函数执行事件处理，并根据就绪的句柄和事件类型填充cb_callback函数的参数。

事件属性设置接口函数

　　在libevent，有几个函数可以用于设置事件的属性：

　　1. event_set

　　用于设置event属性的event_set函数实际上是调用了event_assign。

1 void
2 event_set(struct event *ev, evutil_socket_t fd, short events,
3       void (*callback)(evutil_socket_t, short, void *), void *arg)
4 {
5     int r;
6     r = event_assign(ev, current_base, fd, events, callback, arg);
7     EVUTIL_ASSERT(r == 0);
8 }

　　而event_assign函数的定义为：

 1 int
 2 event_assign(struct event *ev, struct event_base *base, evutil_socket_t fd, short events, void (*callback)(evutil_socket_t, short, void *), void *arg)
 3 {
 4     if (!base)
 5         base = current_base;
 6
 7     _event_debug_assert_not_added(ev);
 8
 9     ev->ev_base = base;
10
11     ev->ev_callback = callback;
12     ev->ev_arg = arg;
13     ev->ev_fd = fd;
14     ev->ev_events = events;
15     ev->ev_res = 0;
16     ev->ev_flags = EVLIST_INIT;
17     ev->ev_ncalls = 0;
18     ev->ev_pncalls = NULL;
19
20     if (events & EV_SIGNAL) {
21         if ((events & (EV_READ|EV_WRITE)) != 0) {
22             event_warnx("%s: EV_SIGNAL is not compatible with "
23                 "EV_READ or EV_WRITE", __func__);
24             return -1;
25         }
26         ev->ev_closure = EV_CLOSURE_SIGNAL;
27     } else {
28         if (events & EV_PERSIST) {
29             evutil_timerclear(&ev->ev_io_timeout);
30             ev->ev_closure = EV_CLOSURE_PERSIST;
31         } else {
32             ev->ev_closure = EV_CLOSURE_NONE;
33         }
34     }
35
36     min_heap_elem_init(ev);
37
38     if (base != NULL) {
39         /* by default, we put new events into the middle priority */
40         ev->ev_pri = base->nactivequeues / 2;
41     }
42
43     _event_debug_note_setup(ev);
44
45     return 0;
46 }

　　其中，参数为：

　　另外，我们也可以在创建新事件的时候设定事件属性，具体函数是event_new。而event_new实际也是调用了event_assign来实现的，不同的是event_new需要先给事件分配空间：

 1 struct event *
 2 event_new(struct event_base *base, evutil_socket_t fd, short events, void (*cb)(evutil_socket_t, short, void *), void *arg)
 3 {
 4     struct event *ev;
 5     ev = mm_malloc(sizeof(struct event));
 6     if (ev == NULL)
 7         return (NULL);
 8     if (event_assign(ev, base, fd, events, cb, arg) < 0) {
 9         mm_free(ev);
10         return (NULL);
11     }
12
13     return (ev);
14 }

　　2. event_base_set

　　在默认情况下，事件event会被注册到一个全局event_base指针current_base。使用该函数可以指定不同的event_base。如果一个进程中存在多个libevent实例，必须要调用该函数为event设置不同的event_base。

　　该函数的定义如下：

 1 int
 2 event_base_set(struct event_base *base, struct event *ev)
 3 {
 4     /* Only innocent events may be assigned to a different base */
 5     if (ev->ev_flags != EVLIST_INIT)
 6         return (-1);
 7
 8     _event_debug_assert_is_setup(ev);
 9
10     ev->ev_base = base;
11     ev->ev_pri = base->nactivequeues/2;
12
13     return (0);
14 }

　　3. event_priority_set

　　在默认情况下，所有的event的优先级都被设定为 active event list 长度的一半（nactivequeues / 2）。该函数可用于设定event的优先级。优先级的数值越小，表示优先级越高。另外，函数event_base_priority_init可用于设定优先级的最大值。

　　event_priority_set的函数定义如下：

 1 /*
 2  * Set‘s the priority of an event - if an event is already scheduled
 3  * changing the priority is going to fail.
 4  */
 5
 6 int
 7 event_priority_set(struct event *ev, int pri)
 8 {
 9     _event_debug_assert_is_setup(ev);
10
11     if (ev->ev_flags & EVLIST_ACTIVE)
12         return (-1);
13     if (pri < 0 || pri >= ev->ev_base->nactivequeues)
14         return (-1);
15
16     ev->ev_pri = pri;
17
18     return (0);
19 }

事件相关的其他常用接口函数

　　1. event_new

　　创建事件（涉及内存分配）。

　　1. event_add

　　添加事件到event_base。

　　2. event_del

　　将事件从监听列表中移除。

　　3. event_free

　　释放由event_new创建的事件（内存）。从其定义可看出其与event_del的区别：

 1 void
 2 event_free(struct event *ev)
 3 {
 4     _event_debug_assert_is_setup(ev);
 5
 6     /* make sure that this event won‘t be coming back to haunt us. */
 7     event_del(ev);
 8     _event_debug_note_teardown(ev);
 9     mm_free(ev);
10
11 }

　　3. event_callback_fn

　　事件的回调函数，用于执行具体的I/O操作。其定义如下：

 1 /**
 2    A callback function for an event.
 3
 4    It receives three arguments:
 5
 6    @param fd An fd or signal
 7    @param events One or more EV_* flags
 8    @param arg A user-supplied argument.
 9
10    @see event_new()
11  */
12 typedef void (*event_callback_fn)(evutil_socket_t, short, void *);

关于超时和信号事件的特殊接口函数

　　为了方便对超时和信号事件的处理，libevent特别为它们定义了接口函数（实际是对通用函数的封装）。

　　超时事件：

 1 /**
 2    @name evtimer_* macros
 3
 4     Aliases for working with one-shot timer events */
 5 /**@{*/
 6 #define evtimer_assign(ev, b, cb, arg)  7     event_assign((ev), (b), -1, 0, (cb), (arg))
 8 #define evtimer_new(b, cb, arg)           event_new((b), -1, 0, (cb), (arg))
 9 #define evtimer_add(ev, tv)               event_add((ev), (tv))
10 #define evtimer_del(ev)                   event_del(ev)
11 #define evtimer_pending(ev, tv)           event_pending((ev), EV_TIMEOUT, (tv))
12 #define evtimer_initialized(ev)           event_initialized(ev)
13 /**@}*/

　　信号事件：

 1 /**
 2    @name evsignal_* macros
 3
 4    Aliases for working with signal events
 5  */
 6 /**@{*/
 7 #define evsignal_add(ev, tv)        event_add((ev), (tv))
 8 #define evsignal_assign(ev, b, x, cb, arg)             9     event_assign((ev), (b), (x), EV_SIGNAL|EV_PERSIST, cb, (arg))
10 #define evsignal_new(b, x, cb, arg)                11     event_new((b), (x), EV_SIGNAL|EV_PERSIST, (cb), (arg))
12 #define evsignal_del(ev)            event_del(ev)
13 #define evsignal_pending(ev, tv)    event_pending((ev), EV_SIGNAL, (tv))
14 #define evsignal_initialized(ev)    event_initialized(ev)
15 /**@}*/

参考资料

　　libevent源码深度剖析五