在之前博文libevent之Reactor模式中，我们知道Reactor模式中一个重要的组件就是事件多路分发机制（event demultiplexer）。而在libevent中，对事件多路分发机制的支持依赖于操作系统支持的多路复用机制（select、poll、epoll等）。

eventop

　　libevent定义了一个顶层的结构体eventop（event option），来抽象不同操作系统支持的多路复用机制：

 // <event_internal.h> 1 /** Structure to define the backend of a given event_base. */
 2 struct eventop {
 3     /** The name of this backend. */
 4     const char *name;
 5     /** Function to set up an event_base to use this backend.  It should
 6      * create a new structure holding whatever information is needed to
 7      * run the backend, and return it.  The returned pointer will get
 8      * stored by event_init into the event_base.evbase field.  On failure,
 9      * this function should return NULL. */
10     void *(*init)(struct event_base *);
11     /** Enable reading/writing on a given fd or signal.  ‘events‘ will be
12      * the events that we‘re trying to enable: one or more of EV_READ,
13      * EV_WRITE, EV_SIGNAL, and EV_ET.  ‘old‘ will be those events that
14      * were enabled on this fd previously.  ‘fdinfo‘ will be a structure
15      * associated with the fd by the evmap; its size is defined by the
16      * fdinfo field below.  It will be set to 0 the first time the fd is
17      * added.  The function should return 0 on success and -1 on error.
18      */
19     int (*add)(struct event_base *, evutil_socket_t fd, short old, short events, void *fdinfo);
20     /** As "add", except ‘events‘ contains the events we mean to disable. */
21     int (*del)(struct event_base *, evutil_socket_t fd, short old, short events, void *fdinfo);
22     /** Function to implement the core of an event loop.  It must see which
23         added events are ready, and cause event_active to be called for each
24         active event (usually via event_io_active or such).  It should
25         return 0 on success and -1 on error.
26      */
27     int (*dispatch)(struct event_base *, struct timeval *);
28     /** Function to clean up and free our data from the event_base. */
29     void (*dealloc)(struct event_base *);
30     /** Flag: set if we need to reinitialize the event base after we fork.
31      */
32     int need_reinit;
33     /** Bit-array of supported event_method_features that this backend can
34      * provide. */
35     enum event_method_feature features;
36     /** Length of the extra information we should record for each fd that
37         has one or more active events.  This information is recorded
38         as part of the evmap entry for each fd, and passed as an argument
39         to the add and del functions above.
40      */
41     size_t fdinfo_len;
42 };

　　可以看到，该结构体已经声明（非定义）了多路分发机制常备的Reactor初始化、事件添加、事件移除、事件分发及Reactor清理函数，而且均以函数指针的方式定义，便于复用。

　　如epoll对该结构体的一个复用：

 1 static void *epoll_init(struct event_base *);
 2 static int epoll_dispatch(struct event_base *, struct timeval *);
 3 static void epoll_dealloc(struct event_base *);
 4
 5 static const struct eventop epollops_changelist = {
 6     "epoll (with changelist)",
 7     epoll_init,
 8     event_changelist_add,
 9     event_changelist_del,
10     epoll_dispatch,
11     epoll_dealloc,
12     1, /* need reinit */
13     EV_FEATURE_ET|EV_FEATURE_O1,
14     EVENT_CHANGELIST_FDINFO_SIZE
15 };

实际调用的多路复用机制

　　libevent用了一个数组来存储其所支持的多路复用机制：

 1 // <event.c>
 2 /* Array of backends in order of preference. */
 3 static const struct eventop *eventops[] = {
 4 #ifdef _EVENT_HAVE_EVENT_PORTS
 5     &evportops,
 6 #endif
 7 #ifdef _EVENT_HAVE_WORKING_KQUEUE
 8     &kqops,
 9 #endif
10 #ifdef _EVENT_HAVE_EPOLL
11     &epollops,
12 #endif
13 #ifdef _EVENT_HAVE_DEVPOLL
14     &devpollops,
15 #endif
16 #ifdef _EVENT_HAVE_POLL
17     &pollops,
18 #endif
19 #ifdef _EVENT_HAVE_SELECT
20     &selectops,
21 #endif
22 #ifdef WIN32
23     &win32ops,
24 #endif
25     NULL
26 };

　　通过这个程序，我们可以知道libevent是通过宏定义来确定当前操作系统是否支持某中多路复用机制，并且按顺序选择系统支持的机制。

　　另外，我们如果想知道程序当前所支持的多路复用机制，我们可以调用函数event_get_supported_methods：

 1 // <event.c>
 2 const char **
 3 event_get_supported_methods(void)
 4 {
 5     static const char **methods = NULL;
 6     const struct eventop **method;
 7     const char **tmp;
 8     int i = 0, k;
 9
10     /* count all methods */
11     for (method = &eventops[0]; *method != NULL; ++method) {
12         ++i;
13     }
14
15     /* allocate one more than we need for the NULL pointer */
16     tmp = mm_calloc((i + 1), sizeof(char *));
17     if (tmp == NULL)
18         return (NULL);
19
20     /* populate the array with the supported methods */
21     for (k = 0, i = 0; eventops[k] != NULL; ++k) {
22         tmp[i++] = eventops[k]->name;
23     }
24     tmp[i] = NULL;
25
26     if (methods != NULL)
27         mm_free((char**)methods);
28
29     methods = tmp;
30
31     return (methods);
32 }

　　而要想知道程序实际调用的是哪一种多路复用机制，我们可以调用函数event_get_method得到：

1 // <event.c>
2 const char *
3 event_get_method(void)
4 {
5     return (current_base->evsel->name);
6 }