【传智播客】Libevent学习笔记(二)：创建event_base

00. 目录

声明： 该博客来源于传智播客C++学院相关培训参考手册

01. 简介

​ 使用libevent函数之前需要分配一个或者多个event_base结构体。每个event_base结构体持有一个事件集合，可以检测以确定哪个事件是激活的。

​ 如果设置event_base使用锁，则可以安全地在多个线程中访问它。然而，其事件循环只能运行在一个线程中。如果需要用多个线程检测IO，则需要为每个线程使用一个event_base。

每个event_base都有一种用于检测哪种事件已经就绪的“方法”，或者说后端。可以识别的方法有：

• select
• poll
• epoll
• kqueue
• devpoll
• evport
• win32

​ 用户可以用环境变量禁止某些特定的后端。比如说，要禁止kqueue后端，可以设置EVENT_NOKQUEUE环境变量。如果要用编程的方法禁止后端，请看下面关于event_config_avoid_method（）的说明。

02. 创建默认的event_base

event_base_new（）函数分配并且返回一个新的具有默认设置的event_base。函数会检测环境变量，返回一个到event_base的指针。如果发生错误，则返回NULL。选择各种方法时，函数会选择操作系统支持的最快方法。

函数相关说明：

/**
 * Create and return a new event_base to use with the rest of Libevent.
 *
 * @return a new event_base on success, or NULL on failure.
 *
 * @see event_base_free(), event_base_new_with_config()
 */

struct event_base *event_base_new(void);
功能: 
	创建一个默认属性的struct event_base对象
参数:
	无
返回值:
	成功: 返回struct event_base结构体指针
	失败: NULL

event_base_new（）函数声明在*中，首次出现在libevent 1.4.3*版。

大多数情况下， 我们创建默认的event_base就可以满足我们的需求。

03. 创建复杂的event_base

要对取得什么类型的event_base有更多的控制，就需要使用event_config。

event_config是一个容纳event_base配置信息的不透明结构体。需要event_base时，将event_config传递给event_base_new_with_config（）。

相关头文件: libevent-2.0.22-stable/include/event2/event.h

3.1 event_config_new函数

/**
   Allocates a new event configuration object.

   The event configuration object can be used to change the behavior of
   an event base.

   @return an event_config object that can be used to store configuration, or
     NULL if an error is encountered.
   @see event_base_new_with_config(), event_config_free(), event_config
*/
struct event_config *event_config_new(void);
功能：
	分配一个event_config
参数:
	无
返回值:
	成功：返回struct event_config结构体指针
	失败：NULL

3.2 event_base_new_with_config函数

/**
  Initialize the event API.

  Use event_base_new_with_config() to initialize a new event base, taking
  the specified configuration under consideration.  The configuration object
  can currently be used to avoid certain event notification mechanisms.

  @param cfg the event configuration object
  @return an initialized event_base that can be used to registering events,
     or NULL if no event base can be created with the requested event_config.
  @see event_base_new(), event_base_free(), event_init(), event_assign()
*/
struct event_base *event_base_new_with_config(const struct event_config * cfg);
功能：
	初始化一个新的event_base
参数：
	cfg event_config结构体指针 
返回值：
	成功：返回一个可以用于注册事件的初始化好的event_base对象
    失败：NULL

3.3 event_config_free函数

/**
   Deallocates all memory associated with an event configuration object

   @param cfg the event configuration object to be freed.
*/
void event_config_free(struct event_config *cfg);
功能:
	释放event_config对象
参数:
	cfg event_config_new函数的返回值
返回值：
	无

3.4 event_config_avoid_method函数

/**
   Enters an event method that should be avoided into the configuration.

   This can be used to avoid event mechanisms that do not support certain
   file descriptor types, or for debugging to avoid certain event
   mechanisms.  An application can make use of multiple event bases to
   accommodate incompatible file descriptor types.

   @param cfg the event configuration object
   @param method the name of the event method to avoid
   @return 0 on success, -1 on failure.
*/
int event_config_avoid_method(struct event_config *cfg, const char *method);
功能:
	可以通过名字让libevent避免使用特定的可用后端。
参数:
	cfg event_config对象指针
	method 避免使用特定的后端
	
返回值:
	成功 0
    失败 -1

3.5 event_config_require_features函数

/**
   Enters a required event method feature that the application demands.

   Note that not every feature or combination of features is supported
   on every platform.  Code that requests features should be prepared
   to handle the case where event_base_new_with_config() returns NULL, as in:
   
     event_config_require_features(cfg, EV_FEATURE_ET);
     base = event_base_new_with_config(cfg);
     if (base == NULL) {
       // We can't get edge-triggered behavior here.
       event_config_require_features(cfg, 0);
       base = event_base_new_with_config(cfg);
     }
   

   @param cfg the event configuration object
   @param feature a bitfield of one or more event_method_feature values.
          Replaces values from previous calls to this function.
   @return 0 on success, -1 on failure.
   @see event_method_feature, event_base_new_with_config()
*/
int event_config_require_features(struct event_config *cfg, int feature);
功能:
	设置libevent不使用不能提供所有指定特征的后端。
参数:
	cfg event_config结构体指针
	feature 指定特征的后端
返回值:
	成功 0
    失败 -1

指定特征的后端

/**
   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.
*/
enum event_method_feature {
    /** Require an event method that allows edge-triggered events with EV_ET. */
    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. */
    EV_FEATURE_O1 = 0x02,
    /** Require an event method that allows file descriptors as well as
     * sockets. */
    EV_FEATURE_FDS = 0x04
};

event_config_require_features（）可识别的特征值有：

• EV_FEATURE_ET：要求支持边沿触发的后端。

• EV_FEATURE_O1：要求添加、删除单个事件，或者确定哪个事件激活的操作是O（1）复杂度的后端。

• EV_FEATURE_FDS：要求支持任意文件描述符，而不仅仅是套接字的后端。

3.6 event_config_set_flag函数

/**
 * Sets one or more flags to configure what parts of the eventual event_base
 * will be initialized, and how they'll work.
 *
 * @see event_base_config_flags, event_base_new_with_config()
 **/
int event_config_set_flag(struct event_config *cfg, int flag);
功能:
	设置让libevent在创建event_base时设置一个或者多个将在下面介绍的运行时标志。
参数:
	cfg event_config结构体指针
	flag 指定的标志
返回值:
	成功 0
    失败 -1

对应的标志有

/**
   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
 */
enum event_base_config_flag {
    /** Do not allocate a lock for the event base, even if we have
        locking set up. */
    EVENT_BASE_FLAG_NOLOCK = 0x01,
    /** Do not check the EVENT_* environment variables when configuring
        an event_base  */
    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.
     */
    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.
     */
    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 settnig the
        EVENT_EPOLL_USE_CHANGELIST environment variable.

        This flag has no effect if you wind up using a backend other than
        epoll.
     */
    EVENT_BASE_FLAG_EPOLL_USE_CHANGELIST = 0x10
};

event_config_set_flag（）可识别的选项值有：

• EVENT_BASE_FLAG_NOLOCK：不要为event_base分配锁。设置这个选项可以为event_base节省一点用于锁定和解锁的时间，但是让在多个线程中访问event_base成为不安全的。

• EVENT_BASE_FLAG_IGNORE_ENV：选择使用的后端时，不要检测EVENT_*环境变量。使用这个标志需要三思：这会让用户更难调试你的程序与libevent的交互。

• EVENT_BASE_FLAG_STARTUP_IOCP：仅用于Windows，让libevent在启动时就启用任何必需的IOCP分发逻辑，而不是按需启用。

• EVENT_BASE_FLAG_NO_CACHE_TIME：不是在事件循环每次准备执行超时回调时检测当前时间，而是在每次超时回调后进行检测。注意：这会消耗更多的CPU时间。

• EVENT_BASE_FLAG_EPOLL_USE_CHANGELIST：告诉libevent，如果决定使用epoll后端，可以安全地使用更快的基于changelist的后端。epoll-changelist后端可以在后端的分发函数调用之间，同样的fd多次修改其状态的情况下，避免不必要的系统调用。但是如果传递任何使用dup（）或者其变体克隆的fd给libevent，epoll-changelist后端会触发一个内核bug，导致不正确的结果。在不使用epoll后端的情况下，这个标志是没有效果的。也可以通过设置EVENT_EPOLL_USE_CHANGELIST环境变量来打开epoll-changelist选项。

注意:

设置event_config，请求OS不能提供的后端是很容易的。比如说，对于libevent 2.0.1-alpha，在Windows中是没有O（1）后端的；在Linux中也没有同时提供EV_FEATURE_FDS和EV_FEATURE_O1特征的后端。如果创建了libevent不能满足的配置，event_base_new_with_config（）会返回NULL。

---

3.7 event_config_set_num_cpus_hint函数

函数当前仅在Windows上使用IOCP时有用，虽然将来可能在其他平台上有用。这个函数告诉event_config在生成多线程event_base的时候，应该试图使用给定数目的CPU。注意这仅仅是一个提示：event_base使用的CPU可能比你选择的要少。

/**
 * Records a hint for the number of CPUs in the system. This is used for
 * tuning thread pools, etc, for optimal performance.  In Libevent 2.0,
 * it is only on Windows, and only when IOCP is in use.
 *
 * @param cfg the event configuration object
 * @param cpus the number of cpus
 * @return 0 on success, -1 on failure.
 */
int event_config_set_num_cpus_hint(struct event_config *cfg, int cpus);
功能:
	设置试图给定数目的CPU。
参数:
	cfg event_config结构体指针
	cpus CPU数量
返回值:
	成功 0
    失败 -1

官方参考示例:

struct event_config *cfg;
struct event_base *base;
int i;
 
/* My program wants to use edge-triggered events if at all possible.  So
   I'll try to get a base twice: Once insisting on edge-triggered IO, and
   once not. */
for (i=0; i<2; ++i) {
    cfg = event_config_new();
 
    /* I don't like select. */
    event_config_avoid_method(cfg, "select");
 
    if (i == 0)
        event_config_require_features(cfg, EV_FEATURE_ET);
 
    base = event_base_new_with_config(cfg);
    event_config_free(cfg);
    if (base)
        break;
    /* If we get here, event_base_new_with_config() returned NULL.  If
       this is the first time around the loop, we'll try again without
       setting EV_FEATURE_ET.  If this is the second time around the
       loop, we'll give up. */
}

总述:

以上函数和类型在中声明。

EVENT_BASE_FLAG_IGNORE_ENV标志首次出现在2.0.2-alpha版本。event_config_set_num_cpus_hint（）函数是2.0.7-rc版本新引入的。本节的其他内容首次出现在2.0.1-alpha版本。

04. 检查event_base的后端

有时候需要检查event_base支持哪些特征，或者当前使用哪种方法。

4.1 event_get_supported_methods函数

/**
   Gets all event notification mechanisms supported by Libevent.

   This functions returns the event mechanism in order preferred by
   Libevent.  Note that this list will include all backends that
   Libevent has compiled-in support for, and will not necessarily check
   your OS to see whether it has the required resources.

   @return an array with pointers to the names of support methods.
     The end of the array is indicated by a NULL pointer.  If an
     error is encountered NULL is returned.
*/
const char **event_get_supported_methods(void);
功能:
	获取当前系统支持的后端。
参数:
	无
返回值:
	成功 返回一个指针，指向libevent支持的方法名字数组
    失败 NULL

参考程序:

#include 
#include 

int main(void)
{
    int i = 0;


    const char **pstr = event_get_supported_methods();
    if (NULL == pstr)
    {   
        printf("event_get_supported_methods failed...\n"); 
        return 1;
    }   

    printf("Libevent version: %s\n", event_get_version());
    for (i = 0; NULL != pstr[i]; i++) 
    {   
        printf("\t%s\n", pstr[i]); 
    }   

    return 0;
}

执行结果:

注意:

这个函数返回libevent被编译以支持的方法列表。然而libevent运行的时候，操作系统可能不能支持所有方法。比如说，可能OS X版本中的kqueue的bug太多，无法使用。

4.2 event_base_get_method函数

/**
 Get the kernel event notification mechanism used by Libevent.

 @param eb the event_base structure returned by event_base_new()
 @return a string identifying the kernel event mechanism (kqueue, epoll, etc.)
 */
const char *event_base_get_method(const struct event_base *base);
功能:
	获取当前base使用的后端。
参数:
	base event_base对象
返回值:
	成功 返回一个指针，指向使用的后端。
    失败 NULL。

参考示例:

#include 
#include 

int main(void)
{
    struct event_base *base = NULL;
    //创建一个对象
    base = event_base_new();
    if (NULL == base)
    {   
        printf("event_base_new failded...\n");
        return 1;
    }   

    printf("methods: %s\n", event_base_get_method(base));

    //释放对象
    event_base_free(base);

    return 0;
}

执行结果:

4.3 event_base_get_features函数

/**
   Return a bitmask of the features implemented by an event base.  This
   will be a bitwise OR of one or more of the values of
   event_method_feature

   @see event_method_feature
 */
int event_base_get_features(const struct event_base *base);
功能:
	返回event_base支持的特征的比特掩码。
参数:
	base event_base对象
返回值:
	成功 特征的比特掩码。
    失败 0。

参考代码:

#include 
#include 

int main(void)
{
    struct event_base *base = NULL;

    enum event_method_feature f;

    base = event_base_new();
    if (NULL == base)
    {
        printf("event_base_new failded...\n");
        return 1;
    }

    f = event_base_get_features(base);
    if ((f & EV_FEATURE_ET))
        printf("  Edge-triggered events are supported.");
    if ((f & EV_FEATURE_O1))
        printf("  O(1) event notification is supported.");
    if ((f & EV_FEATURE_FDS))
        printf("  All FD types are supported.");
    printf("\n");
    event_base_free(base);

    return 0;
}

执行结果:

05. 释放event_base

使用完event_base之后，使用event_base_free（）进行释放。

/**
  Deallocate all memory associated with an event_base, and free the base.

  Note that this function will not close any fds or free any memory passed
  to event_new as the argument to callback.

  @param eb an event_base to be freed
 */
void event_base_free(struct event_base *base);
功能:
	释放base
参数:
	base event_base对象
返回值:
	无

参考代码:

#include 
#include 

int main(void)
{
    struct event_base *base = NULL;

	//创建base对象
    base = event_base_new();
    if (NULL == base)
    {   
        printf("event_base_new failded...\n");
        return 1;
    }   
    
	//释放base
    event_base_free(base);

    return 0;
}

注意:

这个函数不会释放当前与event_base关联的任何事件，或者关闭他们的套接字，或者释放任何指针。

event_base_free（）定义在*中，首次由libevent 1.2*实现。

06. 设置event_base的优先级

​ libevent支持为事件设置多个优先级。然而，event_base默认只支持单个优先级。可以调用event_base_priority_init*（）设置event_base的优先级数目。

event_base_priority_init函数

/**
  Set the number of different event priorities

  By default Libevent schedules all active events with the same priority.
  However, some time it is desirable to process some events with a higher
  priority than others.  For that reason, Libevent supports strict priority
  queues.  Active events with a lower priority are always processed before
  events with a higher priority.

  The number of different priorities can be set initially with the
  event_base_priority_init() function.  This function should be called
  before the first call to event_base_dispatch().  The
  event_priority_set() function can be used to assign a priority to an
  event.  By default, Libevent assigns the middle priority to all events
  unless their priority is explicitly set.

  Note that urgent-priority events can starve less-urgent events: after
  running all urgent-priority callbacks, Libevent checks for more urgent
  events again, before running less-urgent events.  Less-urgent events
  will not have their callbacks run until there are no events more urgent
  than them that want to be active.

  @param eb the event_base structure returned by event_base_new()
  @param npriorities the maximum number of priorities
  @return 0 if successful, or -1 if an error occurred
  @see event_priority_set()
 */
int event_base_priority_init(struct event_base *base, int pri);
功能:
	设置base优先级
参数:
	base event_base对象
	pri 这个数目至少是1。每个新的事件可用的优先级将从0（最高）pri-1（最低）。
返回值:
	成功 0
	失败 -1
        
/** Largest number of priorities that Libevent can support. */
#define EVENT_MAX_PRIORITIES 256
常量EVENT_MAX_PRIORITIES表示pri的上限。调用这个函数时为pri给出更大的值是错误的。

注意:

必须在任何事件激活之前调用这个函数，最好在创建event_base后立刻调用。

关于示例，请看event_priority_set的文档。

默认情况下，与event_base相关联的事件将被初始化为具有优先级pri/ 2。event_base_priority_init（）函数定义在中，从libevent 1.0版就可用了。

07. fork之后重新初始化base

不是所有事件后端都在调用fork（）之后可以正确工作。所以，如果在使用fork（）或者其他相关系统调用启动新进程之后，希望在新进程中继续使用event_base，就需要进行重新初始化。

event_reinit函数

/**
  Reinitialize the event base after a fork

  Some event mechanisms do not survive across fork.   The event base needs
  to be reinitialized with the event_reinit() function.

  @param base the event base that needs to be re-initialized
  @return 0 if successful, or -1 if some events could not be re-added.
  @see event_base_new()
*/
int event_reinit(struct event_base *base);
功能:
	重新初始化base
参数:
	base event_base对象
返回值:
	成功 0
	失败 -1

官方参考示例

struct event_base *base = event_base_new();
 
/* ... add some events to the event_base ... */
 
if (fork()) {
    /* In parent */
    continue_running_parent(base); /*...*/
} else {
    /* In child */
    event_reinit(base);
    continue_running_child(base); /*...*/
}

event_reinit（）定义在*中，在libevent 1.4.3-alpha*版中首次可用。

08. 参考

相关书籍： http://www.wangafu.net/~nickm/libevent-book/Ref2_eventbase.html

官方参考网站： https://www.monkey.org/~provos/libevent/doxygen-2.0.1/index.html