Libevent 反应堆的初始化

// 默认情况下new eventbase
struct event_base *
event_base_new(void)
{
    struct event_base *base = NULL;
    //初始化系统配置
    struct event_config *cfg = event_config_new(); // 使用默认的配置方法
    if (cfg) {
        //再利用事件的系统配置去初始化base event
        base = event_base_new_with_config(cfg);
        event_config_free(cfg);
    }
    return base;
}

event_config 结构如下

struct event_config {
　　//我们知道libevent 支持很多io模型  select poll epoll devpoll win32等
    //这里的entries 是一个队列里面存放着要避免的方法 event_config_entry{ const char *avoid_method, 双向表指针}
    TAILQ_HEAD(event_configq, event_config_entry) entries;
    //初始化占用cpu个数
    int n_cpus_hint;
    //反应堆检查事件的间隔
    struct timeval max_dispatch_interval;
    //指两次事件间隔最多调用事件个数
    int max_dispatch_callbacks;
    //若优先级别低于它就不会执行上两项
    int limit_callbacks_after_prio;
    //支持后台方法的特性 如epoll 模型想要采用ET 即EV_FEATURE_ET 还用延迟关闭等选项
    enum event_method_feature require_features;

    enum event_base_config_flag flags;
};

/*
对于event_config的初始化都有相关接口
例如：
　　entries 不希望采用的后台模型  event_config_avoid_method() 它会将所有要避免的模型加入队列  event_get_supported_method 得到所有避免的模型
　 event_config_set_num_cpus_hint   设置cpu个数

  event_config_set_max_dispatch_interval(struct event_config *cfg,
    const struct timeval *max_interval, int max_callbacks,
    int min_priority);
    详细说明一下：
  Record an interval and/or a number of callbacks after which the event base
  should check for new events.  By default, the event base will run as many
  events are as activated at the higest activated priority before checking
  for new events.  If you configure it by setting max_interval, it will check
  the time after each callback, and not allow more than max_interval to
  elapse before checking for new events.  If you configure it by setting
  max_callbacks to a value >= 0, it will run no more than max_callbacks
  callbacks before checking for new events.
   记录反应堆应该监测新事件时间间隔或者回调函数的个数。 一般来说 反应堆会在检查新事件之前会运行所有的激活的事件。
   如果你配置了max_interval, 它会在每次callback之后检查时间， 保证反应堆每次去check新事件的时间间隔不允许超过最大间隔。
   设置max_callbacks ，意思就是说每次监测新事件的间隔 最多可以有max_callbacks个回调被执行。

  This option can decrease the latency of high-priority events, and
  avoid priority inversions where multiple low-priority events keep us from
  polling for high-priority events, but at the expense of slightly decreasing
  the throughput.  Use it with caution!
   这个选项能够减少潜在的高优先级事件， 避免低优先级事件阻碍我们及时处理高优先级事件，但是由于处理激活事件受到了限制
   它的吞吐量将减小， 小心使用。
  @param cfg The event_base configuration object.
  @param max_interval An interval after which Libevent should stop running
      callbacks and check for more events, or NULL if there should be
      no such interval.
  @param max_callbacks A number of callbacks after which Libevent should
      stop running callbacks and check for more events, or -1 if there
      should be no such limit.
      min_priority 事件优先级低于它  就不会执行上面俩个参数的设置
       0 表示任何优先级都会执行    若是1 就是表示优先级是1或在在1之上就会执行
  @param min_priority A priority below which max_interval and max_callbacks
      should not be enforced.  If this is set to 0, they are enforced
      for events of every priority; if it's set to 1, they're enforced
      for events of priority 1 and above, and so on.
  @return 0 on success, -1 on failure.

int event_config_require_features(struct event_config *cfg, int feature);//设置feature
int event_config_set_flag(struct event_config *cfg, int flag);
event_base_config_flag：
这个字段：
enum event_base_config_flag {
    /** Do not allocate a lock for the event base, even if we have
        locking set up.
        Setting this option will make it unsafe and nonfunctional to call
        functions on the base concurrently from multiple threads.
    */
    //不分配锁  用于是否选择多线程
    EVENT_BASE_FLAG_NOLOCK = 0x01,
    /** Do not check the EVENT_* environment variables when configuring
        an event_base  */
    //是否监测EVENT_*环境变量
    EVENT_BASE_FLAG_IGNORE_ENV = 0x02,
    //windows 选项
    EVENT_BASE_FLAG_STARTUP_IOCP = 0x04,
    /** Instead of checking the current time every time the event loop is
        ready to run timeout callbacks, check after each timeout callback.
     */
     //在超时callback之后check 当前时间  而不是在准备运行timeout callbacks时
    EVENT_BASE_FLAG_NO_CACHE_TIME = 0x08,

    /** If we are using the epoll backend, this flag says that it is
        safe to use Libevent's internal change-list code to batch up
        adds and deletes in order to try to do as few syscalls as
        possible.  Setting this flag can make your code run faster, but
        it may trigger a Linux bug: it is not safe to use this flag
        if you have any fds cloned by dup() or its variants.  Doing so
        will produce strange and hard-to-diagnose bugs.

        This flag can also be activated by setting the
        EVENT_EPOLL_USE_CHANGELIST environment variable.

        This flag has no effect if you wind up using a backend other than
        epoll.
     */
     //使用changelist 减少系统调用  但是可能是不安全的
    EVENT_BASE_FLAG_EPOLL_USE_CHANGELIST = 0x10,

    /** Ordinarily, Libevent implements its time and timeout code using
        the fastest monotonic timer that we have.  If this flag is set,
        however, we use less efficient more precise timer, assuming one is
        present.
     */
     //选则计时器
     //通常情况下，Libevent将使用最快计时器。 但是如果这个标志被设置，我们使用更低效的更精确的计时器。
    EVENT_BASE_FLAG_PRECISE_TIMER = 0x20
};
*/

//默认情况下 产生的event_config
struct event_config *
event_config_new(void)
{
    struct event_config *cfg = mm_calloc(, sizeof(*cfg));

    if (cfg == NULL)
        return (NULL);

    TAILQ_INIT(&cfg->entries);
    cfg->max_dispatch_interval.tv_sec = -;
    cfg->max_dispatch_callbacks = INT_MAX;
    //优先级低于1的话就不会有上述设置     这里的优先级别越小优先级就越大
    //那么这里默认就不使用上述设置
    cfg->limit_callbacks_after_prio = ;
    return (cfg);
}

//根据event config 初始化反应堆
struct event_base *
event_base_new_with_config(const struct event_config *cfg)
{
    int i;
    struct event_base *base;
    int should_check_environment;
    。。。。。
    根据cfg填充base的各个字段
    return (base);
}