目錄

• 00. 目錄
• 01. event_base_loop函數(shù)
• 02. event_base_dispatch函數(shù)
• 03. event_base_loopexit函數(shù)
• 04. event_base_loopbreak函數(shù)
• 05. event_base_got_exit函數(shù)
• 06. event_base_got_break函數(shù)
• 07. event_base_dump_events函數(shù)
• 08. event_base_dump_events函數(shù)
• 09. 廢棄的事件循環(huán)函數(shù)
• 10.參考

00. 目錄

聲明： 該博客來源于傳智播客C++學院相關培訓參考手冊

01. event_base_loop函數(shù)

一旦有了一個已經(jīng)注冊了某些事件的event_base（關于如何創(chuàng)建和注冊事件請看筆記四），就需要讓libevent等待事件并且通知事件的發(fā)生。

event_base_loop函數(shù)

/**Wait for events to become active, and run their callbacks.This is a more flexible version of event_base_dispatch().By default, this loop will run the event base until either there are no morepending or active events, or until something calls event_base_loopbreak() orevent_base_loopexit(). You can override this behavior with the 'flags'argument.@param eb the event_base structure returned by event_base_new() orevent_base_new_with_config()@param flags any combination of EVLOOP_ONCE | EVLOOP_NONBLOCK@return 0 if successful, -1 if an error occurred, or 1 if we exited becauseno events were pending or active.@see event_base_loopexit(), event_base_dispatch(), EVLOOP_ONCE,EVLOOP_NONBLOCK*/ int event_base_loop(struct event_base *base, int flag); //while(1) { .... } 功能:等待事件被觸發(fā), 然后執(zhí)行對應的回調函數(shù) 參數(shù):base: event_base_new的返回值flag: 標志 返回值:成功: 0成功 1表示沒有事件觸發(fā)失敗: -1

循環(huán)相關標志:

/** @name Loop flagsThese flags control the behavior of event_base_loop().*/ /**@{*/ /** Block until we have an active event, then exit once all active events* have had their callbacks run. */ #define EVLOOP_ONCE 0x01 /** Do not block: see which events are ready now, run the callbacks* of the highest-priority ones, then exit. */ #define EVLOOP_NONBLOCK 0x02 /**@}*/

默認情況下，event_base_loop（）函數(shù)運行event_base直到其中沒有已經(jīng)注冊的事件為止。執(zhí)行循環(huán)的時候，函數(shù)重復地檢查是否有任何已經(jīng)注冊的事件被觸發(fā)（比如說，讀事件的文件描述符已經(jīng)就緒，可以讀取了；或者超時事件的超時時間即將到達）。如果有事件被觸發(fā)，函數(shù)標記被觸發(fā)的事件為“激活的”，并且執(zhí)行這些事件。

在flags參數(shù)中設置一個或者多個標志就可以改變event_base_loop（）的行為。如果設置了EVLOOP_ONCE，循環(huán)將等待某些事件成為激活的，執(zhí)行激活的事件直到?jīng)]有更多的事件可以執(zhí)行，然會返回。如果設置了EVLOOP_NONBLOCK，循環(huán)不會等待事件被觸發(fā)：循環(huán)將僅僅檢測是否有事件已經(jīng)就緒，可以立即觸發(fā)，如果有，則執(zhí)行事件的回調。

完成工作后，如果正常退出，event_base_loop（）返回0；如果因為后端中的某些未處理錯誤而退出，則返回-1。

為幫助大家理解，這里給出event_base_loop（）的算法概要：

while (any events are registered with the loop) {if (EVLOOP_NONBLOCK was set, or any events are already active)If any registered events have triggered, mark them active.elseWait until at least one event has triggered, and mark it active.for (p = 0; p < n_priorities; ++p {if (any event with priority of p is active) {Run all active events with priority of p.break; /* Do not run any events of a less important priority */}}if (EVLOOP_ONCE was set or EVLOOP_NONBLOCK was set)break; }

02. event_base_dispatch函數(shù)

event_base_dispatch（）等同于沒有設置標志的event_base_loop（）。所以，event_base_dispatch（）將一直運行，直到?jīng)]有已經(jīng)注冊的事件了，或者調用了event_base_loopbreak（）或者event_base_loopexit（）為止。

/**Event dispatching loopThis loop will run the event base until either there are no more pending oractive, or until something calls event_base_loopbreak() orevent_base_loopexit().@param base the event_base structure returned by event_base_new() orevent_base_new_with_config()@return 0 if successful, -1 if an error occurred, or 1 if we exited becauseno events were pending or active.@see event_base_loop()*/ int event_base_dispatch(struct event_base *base); 等價于沒有設置標志的 event_base_loop函數(shù)

event_base_dispatch（）將一直運行，直到?jīng)]有已經(jīng)注冊的事件了，或者調用了event_base_loopbreak（）或者event_base_loopexit（）為止。

這些函數(shù)定義在<event2/event.h>中，從libevent 1.0版就存在了。

03. event_base_loopexit函數(shù)

/**Exit the event loop after the specified timeThe next event_base_loop() iteration after the given timer expires willcomplete normally (handling all queued events) then exit withoutblocking for events again.Subsequent invocations of event_base_loop() will proceed normally.@param eb the event_base structure returned by event_init()@param tv the amount of time after which the loop should terminate,or NULL to exit after running all currently active events.@return 0 if successful, or -1 if an error occurred@see event_base_loopbreak()*/ int event_base_loopexit(struct event_base *base, const struct timeval *tv); 功能:讓event_base在給定時間之后停止循環(huán)。 參數(shù):base event_base_new的返回值tv 表示延時的時間，如果為NULL 立即停止循環(huán)，沒有延時返回值:成功: 0成功 失敗: -1

注意:

如果event_base當前正在執(zhí)行任何激活事件的回調，則回調會繼續(xù)運行，直到運行完所有激活事件的回調之才退出。

04. event_base_loopbreak函數(shù)

/**Abort the active event_base_loop() immediately.event_base_loop() will abort the loop after the next event is completed;event_base_loopbreak() is typically invoked from this event's callback.This behavior is analogous to the "break;" statement.Subsequent invocations of event_loop() will proceed normally.@param eb the event_base structure returned by event_init()@return 0 if successful, or -1 if an error occurred@see event_base_loopexit()*/ int event_base_loopbreak(struct event_base *base); 功能:讓event_base立即停止循環(huán)。 參數(shù):base event_base_new的返回值 返回值:成功: 0成功 失敗: -1

這些函數(shù)聲明在<event2/event.h>中。event_break_loopexit()函數(shù)首次在libevent 1.0c版本中實現(xiàn)；event_break_loopbreak()首次在libevent 1.4.3版本中實現(xiàn)。

注意:

event_base_loopbreak（）讓event_base立即退出循環(huán)。它與event_base_loopexit（base,NULL）的不同在于，如果event_base當前正在執(zhí)行激活事件的回調，它將在執(zhí)行完當前正在處理的事件后立即退出。

event_base_loopexit(base,NULL)和event_base_loopbreak(base)在事件循環(huán)沒有運行時的行為不同：前者安排下一次事件循環(huán)在下一輪回調完成后立即停止（就好像帶EVLOOP_ONCE標志調用一樣）；后者卻僅僅停止當前正在運行的循環(huán)，如果事件循環(huán)沒有運行，則沒有任何效果。

官方參考示例一: 立即退出循環(huán)

#include <event2/event.h>/* Here's a callback function that calls loopbreak */ void cb(int sock, short what, void *arg) {struct event_base *base = arg;event_base_loopbreak(base); }void main_loop(struct event_base *base, evutil_socket_t watchdog_fd) {struct event *watchdog_event;/* Construct a new event to trigger whenever there are any bytes toread from a watchdog socket.? When that happens, we'll call thecb function, which will make the loop exit immediately withoutrunning any other active events at all.*/watchdog_event = event_new(base, watchdog_fd, EV_READ, cb, base);event_add(watchdog_event, NULL);event_base_dispatch(base); }

官方參考示例二: 執(zhí)行事件循環(huán)10秒，然后退出

#include <event2/event.h>void run_base_with_ticks(struct event_base *base) {struct timeval ten_sec;ten_sec.tv_sec = 10;ten_sec.tv_usec = 0;/* Now we run the event_base for a series of 10-second intervals, printing"Tick" after each.? For a much better way to implement a 10-secondtimer, see the section below about persistent timer events. */while (1) {/* This schedules an exit ten seconds from now. */event_base_loopexit(base, &ten_sec);event_base_dispatch(base);puts("Tick");} }

測試代碼: 每隔3秒鐘輸出一個字符串

#include <stdio.h> #include <event.h>int main(void) {struct event_base *base = NULL;struct timeval tmo = {3, 0}; base = event_base_new();if (NULL == base){ printf("event_base_new failded...\n");return 1;}while(1){ //設置三秒鐘退出事件循環(huán)event_base_loopexit(base, &tmo); //進入事件循環(huán)event_base_dispatch(base);printf("hello itcast\n");}event_base_free(base);return 0; }

05. event_base_got_exit函數(shù)

有時候需要知道對event_base_dispatch（）或者event_base_loop（）的調用是正常退出的，還是因為調用event_base_loopexit（）或者event_base_break（）而退出的。可以調用下述函數(shù)來確定是否調用了loopexit或者break函數(shù)。

/**Checks if the event loop was told to exit by event_loopexit().This function will return true for an event_base at every point afterevent_loopexit() is called, until the event loop is next entered.@param eb the event_base structure returned by event_init()@return true if event_base_loopexit() was called on this event base,or 0 otherwise@see event_base_loopexit()@see event_base_got_break()*/ int event_base_got_exit(struct event_base *base); 功能:判斷循環(huán)是否因為調用event_base_loopexit（）或者event_base_break（）而退出的時候返回true，否則返回false。下次啟動事件循環(huán)的時候，這些值會被重設。 參數(shù):base event_base_new的返回值 返回值:true 循環(huán)是因為調用對應的函數(shù)而退出0 其它情況

參考示例:

#include <stdio.h> #include <event.h>int main(void) {struct event_base *base = NULL;struct timeval tmo = {3, 0}; base = event_base_new();if (NULL == base){ printf("event_base_new failded...\n");return 1;} while(1){ event_base_loopexit(base, &tmo);event_base_dispatch(base);printf("hello itcast\n");//如果事件循環(huán)因為event_base_loopexit而退出 返回trueif (event_base_got_exit(base)){printf("event_base_got_exit return true\n");}} event_base_free(base);return 0; }

06. event_base_got_break函數(shù)

有時候需要知道對event_base_dispatch（）或者event_base_loop（）的調用是正常退出的，還是因為調用event_base_loopexit（）或者event_base_break（）而退出的。可以調用下述函數(shù)來確定是否調用了loopexit或者break函數(shù)。

/**Checks if the event loop was told to abort immediately by event_loopbreak().This function will return true for an event_base at every point afterevent_loopbreak() is called, until the event loop is next entered.@param eb the event_base structure returned by event_init()@return true if event_base_loopbreak() was called on this event base,or 0 otherwise@see event_base_loopbreak()@see event_base_got_exit()*/ int event_base_got_break(struct event_base *base); 功能:判斷循環(huán)是否因為調用event_base_loopexit（）或者event_base_break（）而退出的時候返回true，否則返回false。下次啟動事件循環(huán)的時候，這些值會被重設。 參數(shù):base event_base_new的返回值 返回值:true 循環(huán)是因為調用對應的函數(shù)而退出0 其它情況

07. event_base_dump_events函數(shù)

有時候需要在事件回調中獲取當前時間的近似視圖，但不想調用gettimeofday()（可能是因為OS將gettimeofday()作為系統(tǒng)調用實現(xiàn)，而你試圖避免系統(tǒng)調用的開銷）。

在回調中，可以請求libevent開始本輪回調時的當前時間視圖。

void event_base_dump_events(struct event_base *, FILE *);/** Sets 'tv' to the current time (as returned by gettimeofday()),looking at the cached value in 'base' if possible, and callinggettimeofday() or clock_gettime() as appropriate if there is nocached time.Generally, this value will only be cached while actuallyprocessing event callbacks, and may be very inaccuate if yourcallbacks take a long time to execute.Returns 0 on success, negative on failure.*/ int event_base_gettimeofday_cached(struct event_base *base,struct timeval *tv); 如果event_base當前正在執(zhí)行回調，event_base_gettimeofday_cached()函數(shù)設置tv_out參數(shù)的值為緩存的時間。否則，函數(shù)調用evutil_gettimeofday()獲取真正的當前時間。成功時函數(shù)返回0，失敗時返回負數(shù)。返回值:成功 0失敗 負數(shù)

注意:

注意，因為libevent在開始執(zhí)行回調的時候時間值會被緩存，所以這個值至少是有一點不精確。如果回調執(zhí)行很長時間，這個值將非常不精確。

這個函數(shù)是libevent 2.0.4-alpha新引入的。

08. event_base_dump_events函數(shù)

為幫助調試程序（或者調試libevent），有時候可能需要已經(jīng)加入到event_base的所有事件及其狀態(tài)的完整列表。調用event_base_dump_events()可以將這個列表輸出到指定的文件中。

這個列表是人可讀的，未來版本的libevent將會改變其格式。

void event_base_dump_events(struct event_base *base, FILE *file); 功能:轉儲event_base的狀態(tài)到文件中。 參數(shù):base event_base_new的返回值file FILE類型指針返回值:無

這個函數(shù)在libevent 2.0.1-alpha版本中引入。

09. 廢棄的事件循環(huán)函數(shù)

前面已經(jīng)討論過，老版本的libevent 具有“當前”event_base的概念。

本文討論的某些事件循環(huán)函數(shù)具有操作當前event_base的變體。除了沒有base參數(shù)外，這些函數(shù)跟當前新版本函數(shù)的行為相同。

2.0版本之前的event_base是不支持鎖的，所以這些函數(shù)并不是完全線程安全的：不允許在執(zhí)行事件循環(huán)的線程之外的其他線程中調用*_loopbreak()或者_loopexit()*函數(shù)。

10.參考

相關書籍： http://www.wangafu.net/~nickm/libevent-book/Ref2_eventbase.html

官方參考網(wǎng)站： https://www.monkey.org/~provos/libevent/doxygen-2.0.1/index.html

轉載于:https://www.cnblogs.com/szitcast/p/10975682.html

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