libevent源码解析（八）hello, world

太过爱你忘了你带给我的痛 2022-05-13 05:24 187阅读 0赞

在前面一到七篇介绍了一些基本数据结构和相关API的内部源码后，本文主要看看libevent自带的测试代码里最简单的经典：hello, world源码。

完整源码及注释如下：

/* 简单的TCP服务器，监听TCP连接并回复消息 "hello, world" This example program provides a trivial server program that listens for TCP connections on port 9995. When they arrive, it writes a short message to each client connection, and closes each connection once it is flushed. Where possible, it exits cleanly in response to a SIGINT (ctrl-c). */
    
    
    #include <string.h>
    #include <errno.h>
    #include <stdio.h>
    #include <signal.h>
    #ifndef _WIN32
    #include <netinet/in.h>
    # ifdef _XOPEN_SOURCE_EXTENDED
    # include <arpa/inet.h>
    # endif
    #include <sys/socket.h>
    #endif
    
    #include <event2/bufferevent.h>
    #include <event2/buffer.h>
    #include <event2/listener.h>
    #include <event2/util.h>
    #include <event2/event.h>
    
    static const char MESSAGE[] = "Hello, World!\n";
    
    static const int PORT = 9995;
    
    static void listener_cb(struct evconnlistener *, evutil_socket_t,
        struct sockaddr *, int socklen, void *);
    static void conn_writecb(struct bufferevent *, void *);
    static void conn_eventcb(struct bufferevent *, short, void *);
    static void signal_cb(evutil_socket_t, short, void *);
    
    int
    main(int argc, char **argv)
    { 
    	struct event_base *base;
    	struct evconnlistener *listener;
    	struct event *signal_event;
    
    	struct sockaddr_in sin;
    #ifdef _WIN32
    	WSADATA wsa_data;
    	WSAStartup(0x0201, &wsa_data);
    #endif
    
    	/*添加基本事件库*/
    	base = event_base_new();
    	if (!base) { 
    		fprintf(stderr, "Could not initialize libevent!\n");
    		return 1;
    	}
    
    	memset(&sin, 0, sizeof(sin));
    	sin.sin_family = AF_INET;
    	sin.sin_port = htons(PORT);
    
    	/*创建监听套接字*/
    	listener = evconnlistener_new_bind(base, listener_cb, (void *)base,
    	    LEV_OPT_REUSEABLE|LEV_OPT_CLOSE_ON_FREE, -1,
    	    (struct sockaddr*)&sin,
    	    sizeof(sin));
    
    	if (!listener) { 
    		fprintf(stderr, "Could not create a listener!\n");
    		return 1;
    	}
    
    	/*创建信号事件*/
    	signal_event = evsignal_new(base, SIGINT, signal_cb, (void *)base);
    
    	/*将事件添加到事件堆*/
    	if (!signal_event || event_add(signal_event, NULL)<0) { 
    		fprintf(stderr, "Could not create/add a signal event!\n");
    		return 1;
    	}
    
    	/*分发事件，核心是event_base_loop，循环中检测事件是否发生，发生则按照设定好的cb响应*/
    	event_base_dispatch(base);
    
    	/*结束后释放资源*/
    	evconnlistener_free(listener);
    	event_free(signal_event);
    	event_base_free(base);
    
    	printf("done\n");
    	return 0;
    }
    
    /*监听回调函数，负责创建套接字、设置写、连接回调*/
    static void
    listener_cb(struct evconnlistener *listener, evutil_socket_t fd,
        struct sockaddr *sa, int socklen, void *user_data)
    { 
    	struct event_base *base = user_data;
    	struct bufferevent *bev;
    
    	bev = bufferevent_socket_new(base, fd, BEV_OPT_CLOSE_ON_FREE);
    	if (!bev) { 
    		fprintf(stderr, "Error constructing bufferevent!");
    		event_base_loopbreak(base);
    		return;
    	}
    	/*设置写数据和连接事件回调*/
    	bufferevent_setcb(bev, NULL, conn_writecb, conn_eventcb, NULL);
    	bufferevent_enable(bev, EV_WRITE);
    	bufferevent_disable(bev, EV_READ);
    
    	/*写套接字*/
    	bufferevent_write(bev, MESSAGE, strlen(MESSAGE));
    }
    
    /*写事件回调*/
    static void
    conn_writecb(struct bufferevent *bev, void *user_data)
    { 
    	struct evbuffer *output = bufferevent_get_output(bev);
    	if (evbuffer_get_length(output) == 0) { 
    		printf("flushed answer\n");
    		bufferevent_free(bev);
    	}
    }
    
    static void
    conn_eventcb(struct bufferevent *bev, short events, void *user_data)
    { 
    	/*文件结束或者出错则打印信息，并释放资源*/
    	if (events & BEV_EVENT_EOF) { 
    		printf("Connection closed.\n");
    	} else if (events & BEV_EVENT_ERROR) { 
    		printf("Got an error on the connection: %s\n",
    		    strerror(errno));/*XXX win32*/
    	}
    	
    	/* 超时也会触发 * None of the other events can happen here, since we haven't enabled * timeouts */
    	bufferevent_free(bev);
    }
    
    static void
    signal_cb(evutil_socket_t sig, short events, void *user_data)
    { 
    	struct event_base *base = user_data;
    	struct timeval delay = {  2, 0 };
    
    	printf("Caught an interrupt signal; exiting cleanly in two seconds.\n");
    
    	event_base_loopexit(base, &delay);
    }

从例子中，我们总结下最简单的libevent使用套路。

（1）调用event\_base\_new()建立事件库

（2）创建事件并添加至库。在本例中是evconnlistener\_new\_bind添加了监听套接字，然后创建信号事件evsignal\_new，最终调用event\_add。

（3）调用event\_base\_dispatch()分发事件，进入时间循环。

（4）释放资源。在本例中先后释放套接字、信号事件、事件库。