libevent笔记2：Hello_World

本篇通过libevent提供的Hello_World demo简单介绍基于libevent的TCP服务器的实现

listener

listener是libevent提供的一种监听本地端口的数据结构，在有客户端的连接到来时调用给定的回调函数。

bufferevent

上一篇中的event是不带缓存区的，读写直接在文件描述符所指向的对象（上一节中是有名管道）上进行。bufferent则是带缓冲区的event，对bufferevnet的读写操作不会直接作用在I/O上，而是对输入或输出缓存区操作。对bufferevent的读操作会从文件描述符相应的输入缓存区读数据；而写操作会将数据写进文件描述符相应的输出缓存区。

Hello_World

以下是官网提供的demo

#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

    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);
    //create a listrener
    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;
    }

    signal_event = evsignal_new(base, SIGINT, signal_cb, (void *)base);

    if (!signal_event || event_add(signal_event, NULL)<0) {
        fprintf(stderr, "Could not create/add a signal event!\n");
        return 1;
    }

    event_base_dispatch(base);

    evconnlistener_free(listener);
    event_free(signal_event);
    event_base_free(base);

    printf("done\n");
    return 0;
}
//invoke when a connection is listened by listener created in main
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;
    //create a new bufferevent over a existing socket
    
    bev = bufferevent_socket_new(base, fd, BEV_OPT_CLOSE_ON_FREE);
    if (!bev) {
        fprintf(stderr, "Error constructing bufferevent!");
        event_base_loopbreak(base);
        return;
    }
    bufferevent_setcb(bev, NULL, conn_writecb, conn_eventcb, NULL);
    bufferevent_enable(bev, EV_WRITE);//enable write to bev
    bufferevent_disable(bev, EV_READ);//disable read from bev
    
    bufferevent_write(bev, MESSAGE, strlen(MESSAGE));
}

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)
{
    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*/
    }
    /* 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);
}

编译并运行文件：

sunminming@sunminming:~/libevent/helloworld$ gcc helloworld.c -o main -levent
sunminming@sunminming:~/libevent/helloworld$ ./main

之后另开一个终端：

sunminming@sunminming:~$ nc 127.0.0.1 9995
Hello, World!

在第一个终端键入Ctrl+C：

sunminming@sunminming:~/libevent/helloworld$ gcc helloworld.c -o main -levent
sunminming@sunminming:~/libevent/helloworld$ ./main
flushed answer
^CCaught an interrupt signal; exiting cleanly in two seconds.
done
sunminming@sunminming:~/libevent/helloworld$

对于这个TCP服务器，整体的逻辑如下：

新建一个event_base，然后在其上绑定一个listener来监听特定端口(9995);
新建一个处理信号的事件signal_event，并与上一步中的event_base绑定；
调用event_base_dispatch来监控两个事件，包括指定的TCP连接及SIGINT信号：
- 当监听到一个连接时，libevent会触发listen_callback，即为上面的listener_cb函数，该函数首先基于底层的socket建立一个bufferevent，并设置为可写不可读，最后调用bufferevent_write函数向其中缓存区写。
  - bufferevent_write函数写后回触发写回调函数conn_writecb，由于该例子中不需要其他操作，所以conn_writecb函数直接释放这个连接。（写回调函数的机制比较复杂，具体可以看这篇博客）
- 当中断信号(Ctrl+C)出现时，libevent会触发signal_event的回调函数signal_cb，该函数会在2秒后停止event_base的监听，并退回到主函数；
主函数依次释放listener，signal_event和event_base的资源，结束。

libevent笔记2：Hello_World

listener

bufferevent

Hello_World

相关函数