Libevent(2.1.8)中的事件结构体
这里的libevent版本为 2.1.8 。 libevent中事件的结构体struct event，定义在event_struct.h 中， 这里我们简单看一下：
struct event {
struct event_callback ev_evcallback; //事件的回调函数
/* for managing timeouts */
union {
TAILQ_ENTRY(event) ev_next_with_common_timeout;// 链表形式管理定时 之前只用最小堆管理超时
int min_heap_idx; //最小堆 管理超时事件
} ev_timeout_pos;// 二选一
evutil_socket_t ev_fd; // io事件时为关心的文件描述符，信号事件时为信号值
struct event_base *ev_base;// 所属事件堆的实例
union {
/* used for io events */
struct {
LIST_ENTRY (event) ev_io_next;
struct timeval ev_timeout;
} ev_io; //这部分指io事件时 所关心的同一描述符下，所有IO事件的链表
/* used by signal events */
struct {
LIST_ENTRY (event) ev_signal_next;
short ev_ncalls;
/* Allows deletes in callback */
short *ev_pncalls;
} ev_signal;//同一信号下信号事件链表
} ev_;
short ev_events; //事件的类型 可读可写 超时等
short ev_res; /* result passed to event callback */
struct timeval ev_timeout; //事件超时时间
};
这里可以看到这个版本下的event 结构体比之前的好像少了不少东西， 如之前的结构：
//event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(ev_fd)做为回调函数的参数.
struct event {
//已注册事件队列入口
TAILQ_ENTRY (event) ev_next;
//已激活事件队列入口
TAILQ_ENTRY (event) ev_active_next;
//信号事件队列入口 不过在以前的分析中可以看到，将来这个队列中的内容会被添加到ev_active_next队列中。
TAILQ_ENTRY (event) ev_signal_next;
//表示该event在定时器事件最小堆min_heap的索引
unsigned int min_heap_idx; /* for managing timeouts */
//该事件所属的反应堆实例
struct event_base *ev_base;
//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.
int ev_fd; //表示事件类型: I/O,定时器或者信号
short ev_events; //事件就绪执行时,将要调用ev_callback 的次数,通常为1
short ev_ncalls; //该事件的超时时间,在定时器最小堆min_heap操作中作为节点值进行比较.
struct timeval ev_timeout; //该事件的优先级,越小越优先.
int ev_pri; /* smaller numbers are higher priority */ //该事件被激活时的回调函数
void (*ev_callback)(int, short, void *arg); //该事件的标记信息,表示其当前的状态,即它在哪个链表中
int ev_flags; ... //其他成员.
};
这里可以看到在老版本的libevent 中event结构体中有回调函数， 参数，优先级参数， 新版本中都被定义在了struct event_callback ev_evcallback;中 如下：
struct event_callback {
TAILQ_ENTRY(event_callback) evcb_active_next;
short evcb_flags; //选择回调类型 由以下四种
ev_uint8_t evcb_pri; /* smaller numbers are higher priority */
ev_uint8_t evcb_closure;
/* allows us to adopt for different types of events */
union {
void (*evcb_callback)(evutil_socket_t, short, void *);
void (*evcb_selfcb)(struct event_callback *, void *);
void (*evcb_evfinalize)(struct event *, void *);
void (*evcb_cbfinalize)(struct event_callback *, void *);
} evcb_cb_union;
void *evcb_arg;
};
比起老版本 新版本又丢弃了TAILQ_ENTRY(event) ev_next; 已注册事件队列入口 ev_active_next 已激活队列入口 ev_signal_next 信号事件队列入口等结构。
Libevent 事件堆实例结构体：
新版本的struct base_event 如下：
struct event_base {
/** Function pointers and other data to describe this event_base's
* backend. */
const struct eventop *evsel;//具体事件处理模型结构(select /poll/epoll)指针
/** Pointer to backend-specific data. */
void *evbase;//指向IO复用的真正操作 通过evsel来初始化
/** List of changes to tell backend about at next dispatch. Only used
* by the O(1) backends. */
struct event_changelist changelist; //？？？？？？？后续
/** Function pointers used to describe the backend that this event_base
* uses for signals */
const struct eventop *evsigsel;//信号操作
/** Data to implement the common signal handelr code. */
struct evsig_info sig;
/** Number of virtual events */
int virtual_event_count; //？？？？？？后续
/** Maximum number of virtual events active */
int virtual_event_count_max; //？？？？？
/** Number of total events added to this event_base */
int event_count; //事件总数
/** Maximum number of total events added to this event_base */
int event_count_max;//事件堆最多的事件数量
/** Number of total events active in this event_base */
int event_count_active;
/** Maximum number of total events active in this event_base */
int event_count_active_max;
/** Set if we should terminate the loop once we're done processing
* events. */
int event_gotterm; //处理完事件后是否终止
/** Set if we should terminate the loop immediately */
int event_break; //是否马上终止事件循环
/** Set if we should start a new instance of the loop immediately. */
int event_continue; //是否马上开始实例
/** The currently running priority of events */
int event_running_priority; //当前running 事件的优先级
/** Set if we're running the event_base_loop function, to prevent
* reentrant invocation. */
int running_loop; //是否正在loop event
/** Set to the number of deferred_cbs we've made 'active' in the
* loop. This is a hack to prevent starvation; it would be smarter
* to just use event_config_set_max_dispatch_interval's max_callbacks
* feature */
int n_deferreds_queued;
/* Active event management. */
/** An array of nactivequeues queues for active event_callbacks (ones
* that have triggered, and whose callbacks need to be called). Low
* priority numbers are more important, and stall higher ones.
*/
//已激活的回调事件队列 即event_callback 结构的队列
struct evcallback_list *activequeues;
/** The length of the activequeues array */
int nactivequeues; //激活队列长度
/** A list of event_callbacks that should become active the next time
* we process events, but not this time. */
struct evcallback_list active_later_queue;// 下次激活的事件队列？
/* common timeout logic */
//超时事件链表与小根堆配合使用
/** An array of common_timeout_list* for all of the common timeout
* values we know. */
struct common_timeout_list **common_timeout_queues;
/** The number of entries used in common_timeout_queues */
int n_common_timeouts;
/** The total size of common_timeout_queues. */
int n_common_timeouts_allocated;
/** Mapping from file descriptors to enabled (added) events */
struct event_io_map io;//存储io 事件
/** Mapping from signal numbers to enabled (added) events. */
struct event_signal_map sigmap;//存储信号事件
/** Priority queue of events with timeouts. */
struct min_heap timeheap;//小根堆
/** Stored timeval: used to avoid calling gettimeofday/clock_gettime
* too often. */
struct timeval tv_cache;
struct evutil_monotonic_timer monotonic_timer;
/** Difference between internal time (maybe from clock_gettime) and
* gettimeofday. */
struct timeval tv_clock_diff;
/** Second in which we last updated tv_clock_diff, in monotonic time. */
time_t last_updated_clock_diff;
#ifndef EVENT__DISABLE_THREAD_SUPPORT //多线程支持
/* threading support */
/** The thread currently running the event_loop for this base */
unsigned long th_owner_id;
/** A lock to prevent conflicting accesses to this event_base */
void *th_base_lock;
/** A condition that gets signalled when we're done processing an
* event with waiters on it. */
void *current_event_cond;
/** Number of threads blocking on current_event_cond. */
int current_event_waiters;
#endif
/** The event whose callback is executing right now */
struct event_callback *current_event;
/** Flags that this base was configured with */
//这里后续的event config 会讲到
enum event_base_config_flag flags;
struct timeval max_dispatch_time;
int max_dispatch_callbacks;
int limit_callbacks_after_prio;
/* Notify main thread to wake up break, etc. */
//多线程 主线程loop过程中一般会阻塞
//若这时添加或者删除事件的话， 就需要唤醒阻塞的主线程， 加入了一个未决状态
/** True if the base already has a pending notify, and we don't need
* to add any more. */
int is_notify_pending;
/** A socketpair used by some th_notify functions to wake up the main
* thread. */
evutil_socket_t th_notify_fd[];
/** An event used by some th_notify functions to wake up the main
* thread. */
struct event th_notify;
/** A function used to wake up the main thread from another thread. */
int (*th_notify_fn)(struct event_base *base);//唤醒主线程的函数指针
/** Saved seed for weak random number generator. Some backends use
* this to produce fairness among sockets. Protected by th_base_lock. */
struct evutil_weakrand_state weakrand_seed;
/** List of event_onces that have not yet fired. */
LIST_HEAD(once_event_list, event_once) once_events;
};
对于事件的存储 老版本中用队列来存储， 在新版本中使用哈希表map 如event_io_map , event_signal_map结构。还有一些多线程支持的参数后续会看到它们的用途。后续还会详细解释其中的各个字段含义。