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struct event {
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    // 外部操作ev_evcallback时，是基于宏定义操作的
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    #define ev_pri ev_evcallback.evcb_pri
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    #define ev_flags ev_evcallback.evcb_flags
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    #define ev_closure ev_evcallback.evcb_closure
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    #define ev_callback ev_evcallback.evcb_cb_union.evcb_callback
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    #define ev_arg ev_evcallback.evcb_arg
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    struct event_callback ev_evcallback;
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    /* for managing timeouts */
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    union {
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        TAILQ_ENTRY(event) ev_next_with_common_timeout;
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        int min_heap_idx;
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    } ev_timeout_pos;
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    evutil_socket_t ev_fd;      // 文件句柄
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    struct event_base *ev_base;
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    union {
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        /* used for io events */
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        struct {
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            LIST_ENTRY (event) ev_io_next;
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            struct timeval ev_timeout;
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        } ev_io;
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        /* used by signal events */
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        struct {
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            LIST_ENTRY (event) ev_signal_next;
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            short ev_ncalls;
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            short *ev_pncalls;
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        } ev_signal;
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    } ev_;
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    short ev_events;            // 事件集合
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    struct timeval ev_timeout;
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};

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// 获取信号事件的信号值
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#define event_get_signal(ev) ((int)event_get_fd(ev))
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// 获取信号事件的信号值，或者IO事件的描述符
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evutil_socket_t event_get_fd(const struct event *ev);
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// 获取事件所属的event_base
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struct event_base *event_get_base(const struct event *ev);
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// 获取事件类型（EV_READ, EV_WRITE等）
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short event_get_events(const struct event *ev);
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// 获取回调函数
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event_callback_fn event_get_callback(const struct event *ev);
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// 获取回调参数
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void *event_get_callback_arg(const struct event *ev);
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// 获取libevent版本
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const char *event_get_version(void);
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ev_uint32_t event_get_version_number(void);
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// 检测事件是否被初始化
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int event_initialized(const struct event *ev);
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/* 检测事件是否处于pending状态
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 * @param ev      事件
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 * @param events  请求的事件类型：EV_TIMEOUT|EV_READ|EV_WRITE|EV_SIGNAL
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 * @param tv      如果不为NULL，且事件有超时，设置超时时间
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 * @return        如果事件pending返回true，否则返回0
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 */
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int event_pending(const struct event *ev, short events, struct timeval *tv);
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/* 提取一个给定事件的所有参数 */
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void event_get_assignment(const struct event *event,
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    struct event_base **base_out, evutil_socket_t *fd_out, short *events_out,
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    event_callback_fn *callback_out, void **arg_out);
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/* 获取当前正在运行的事件，线程不安全 */
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struct event* event_base_get_running_event(struct event_base * base);