一、事件類型 event-internal.h
/**
* @name event flags
*
* Flags to pass to event_new(), event_assign(), event_pending(), and
* anything else with an argument of the form "short events"
*/
/**@{*/
/** Indicates that a timeout has occurred. It's not necessary to pass
* this flag to event_for new()/event_assign() to get a timeout. */
// 定時事件
#define EV_TIMEOUT 0x01
/** Wait for a socket or FD to become readable */
// 讀事件
#define EV_READ 0x02
/** Wait for a socket or FD to become writeable */
// 寫事件
#define EV_WRITE 0x04
/** Wait for a POSIX signal to be raised*/
// 信號
#define EV_SIGNAL 0x08
/**
* Persistent event: won't get removed automatically when activated.
*
* When a persistent event with a timeout becomes activated, its timeout
* is reset to 0.
*/
// 永久事件，激活執行後會重新加到隊列中等待下一次激活，否則激活執行後會自動移除
#define EV_PERSIST 0x10
/** Select edge-triggered behavior, if supported by the backend. */
// 邊沿觸發，一般需要後臺方法支持
#define EV_ET 0x20
/**
* If this option is provided, then event_del() will not block in one thread
* while waiting for the event callback to complete in another thread.
*
* To use this option safely, you may need to use event_finalize() or
* event_free_finalize() in order to safely tear down an event in a
* multithreaded application. See those functions for more information.
*
* THIS IS AN EXPERIMENTAL API. IT MIGHT CHANGE BEFORE THE LIBEVENT 2.1 SERIES
* BECOMES STABLE.
**/
// 終止事件，如果設置這個選項，則event_del不會阻塞，需要使用event_finalize或者
// event_free_finalize以保證多線程安全
#define EV_FINALIZE 0x40
/**
* Detects connection close events. You can use this to detect when a
* connection has been closed, without having to read all the pending data
* from a connection.
*
* Not all backends support EV_CLOSED. To detect or require it, use the
* feature flag EV_FEATURE_EARLY_CLOSE.
**/
// 檢查事件連接是否關閉；可以使用這個選項來檢測鏈接是否關閉，而不需要讀取此鏈接所有未決數據；
#define EV_CLOSED 0x80二、事件狀態標誌 event_struct.h中
// 事件在time min_heap堆中
#define EVLIST_TIMEOUT 0x01
// 事件在已註冊事件鏈表中
#define EVLIST_INSERTED 0x02
// 目前未使用
#define EVLIST_SIGNAL 0x04
// 事件在激活鏈表中
#define EVLIST_ACTIVE 0x08
// 內部使用標記
#define EVLIST_INTERNAL 0x10
// 事件在下一次激活鏈表中
#define EVLIST_ACTIVE_LATER 0x20
// 事件已經終止
#define EVLIST_FINALIZING 0x40
// 事件初始化完成，但是哪兒都不在
#define EVLIST_INIT 0x80
// 包含所有事件狀態，用於判斷合法性的
#define EVLIST_ALL 0xff三、後臺方法 event.c
/* Array of backends in order of preference. */
後臺方法的全局靜態數組
static const struct eventop *eventops[] = {
#ifdef EVENT__HAVE_EVENT_PORTS
&evportops,
#endif
#ifdef EVENT__HAVE_WORKING_KQUEUE
&kqops,
#endif
#ifdef EVENT__HAVE_EPOLL
&epollops,
#endif
#ifdef EVENT__HAVE_DEVPOLL
&devpollops,
#endif
#ifdef EVENT__HAVE_POLL
&pollops,
#endif
#ifdef EVENT__HAVE_SELECT
&selectops,
#endif
#ifdef _WIN32
&win32ops,
#endif
NULL
};四、後臺方法特徵列表 event.h
/**
A flag used to describe which features an event_base (must) provide.
Because of OS limitations, not every Libevent backend supports every
possible feature. You can use this type with
event_config_require_features() to tell Libevent to only proceed if your
event_base implements a given feature, and you can receive this type from
event_base_get_features() to see which features are available.
*/
// 用來描述event_base必須提供的特徵值，其實是後臺方法提供的；
// 因爲OS的限制，不是所有event_base後臺方法都支持每個可能的特徵；
// 必須使用event_config_require_features（）進行配置，同時必須使用
// event_base_get_features（）查看是否支持配置的特徵。
enum event_method_feature {
/** Require an event method that allows edge-triggered events with EV_ET. */
// 邊沿觸發，高效但是容易丟消息，注意與水平觸發區分
EV_FEATURE_ET = 0x01,
/** Require an event method where having one event triggered among
* many is [approximately] an O(1) operation. This excludes (for
* example) select and poll, which are approximately O(N) for N
* equal to the total number of possible events. */
// 要求具有很多事件的後臺方法可以以近似O（1）處理事件；select和poll
// 無法提供這種特徵，它們只能提供近似O（N）的操作
EV_FEATURE_O1 = 0x02,
/** Require an event method that allows file descriptors as well as
* sockets. */
// 後臺方法可以處理包括sockets在內的各種文件描述符
EV_FEATURE_FDS = 0x04,
/** Require an event method that allows you to use EV_CLOSED to detect
* connection close without the necessity of reading all the pending data.
*
* Methods that do support EV_CLOSED may not be able to provide support on
* all kernel versions.
**/
// 要求後臺方法可以使用EV_CLOSED檢測鏈接關閉，而不需要讀完所有未決數據才能判斷
// 支持EV_CLOSED的後臺方法不是所有OS內核都支持的
EV_FEATURE_EARLY_CLOSE = 0x08
};五、event_base或者libevent工作模式 event.h
/**
A flag passed to event_config_set_flag().
These flags change the behavior of an allocated event_base.
@see event_config_set_flag(), event_base_new_with_config(),
event_method_feature
*/
// 可以使用event_config_set_flag設置以下配置。
// 這個配置可以改變event_base的行爲
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.
*/
// 非阻塞模式，多線程不安全
EVENT_BASE_FLAG_NOLOCK = 0x01,
/** Do not check the EVENT_* environment variables when configuring
an event_base */
// 這種模式下不再檢查EVENT_*環境變量
EVENT_BASE_FLAG_IGNORE_ENV = 0x02,
/** Windows only: enable the IOCP dispatcher at startup
If this flag is set then bufferevent_socket_new() and
evconn_listener_new() will use IOCP-backed implementations
instead of the usual select-based one on Windows.
*/
// 只應用於windows環境
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.
*/
// 每次event_loop準備運行timeout回調時，不再檢查當前的時間，而是
// 在每次timeout回調之後檢查
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.
*/
// 如果後臺方法是epoll，則此模式是指可以安全的使用libevent內部changelist
// 進行批量增刪而儘可能減少系統調用。這種模式可以讓代碼性能更高，
// 但是可能會引起Linux bug：如果有任何由dup（）或者他的變量克隆的fds，
// 則是不安全的。這樣做會引起奇怪並且難以檢查的bugs。此模式可以通過
// EVENT_EPOLL_USE_CHANGELIST環境變量激活。
// 此模式只有在使用epoll時可用
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.
*/
// 通常情況下，libevent使用最快的monotonic計時器實現自己的計時和超時控制；
// 此模式下，會使用性能較低但是準確性更高的計時器。
EVENT_BASE_FLAG_PRECISE_TIMER = 0x20
};六、事件關閉時的回調函數模式類型
/** @name Event closure codes
Possible values for evcb_closure in struct event_callback
@{
*/
/** A regular event. Uses the evcb_callback callback */
// 常規事件，使用evcb_callback回調
#define EV_CLOSURE_EVENT 0
/** A signal event. Uses the evcb_callback callback */
// 信號事件；使用evcb_callback回調
#define EV_CLOSURE_EVENT_SIGNAL 1
/** A persistent non-signal event. Uses the evcb_callback callback */
// 永久性非信號事件；使用evcb_callback回調
#define EV_CLOSURE_EVENT_PERSIST 2
/** A simple callback. Uses the evcb_selfcb callback. */
// 簡單回調，使用evcb_selfcb回調
#define EV_CLOSURE_CB_SELF 3
/** A finalizing callback. Uses the evcb_cbfinalize callback. */
// 結束的回調，使用evcb_cbfinalize回調
#define EV_CLOSURE_CB_FINALIZE 4
/** A finalizing event. Uses the evcb_evfinalize callback. */
// 結束事件回調，使用evcb_evfinalize回調
#define EV_CLOSURE_EVENT_FINALIZE 5
/** A finalizing event that should get freed after. Uses the evcb_evfinalize
* callback. */
// 結束事件之後應該釋放，使用evcb_evfinalize回調
#define EV_CLOSURE_EVENT_FINALIZE_FREE 6
/** @} */