libevent实现echoclient

基于libevent实现了一个简单的echoclient。之前在网上看到的都是echoserver。这里演示一下使用libevent进行客户端编程的基本步骤。

先看代码：

#include "stdafx.h"
#include "event2/event.h"
#include "event2/util.h"

#define ECHO_PORT   8888
#define ECHO_SERVER "127.0.0.1"

struct echo_context{
    struct event_base *base;
    struct event *event_write;
    struct event *event_read;
    const char * echo_contents;
    int echo_contents_len;
    int recved;
};

void write_cb(evutil_socket_t sock, short flags, void * args)
{
    struct echo_context *ec = (struct echo_context *)args; 

    int ret = send(sock, ec->echo_contents, ec->echo_contents_len, 0);
    printf("connected, write to echo server: %d\n", ret);
    event_add(ec->event_read, 0);
}

void read_cb(evutil_socket_t sock, short flags, void * args)
{
    struct echo_context *ec = (struct echo_context *)args; 
    char buf[128];
    int ret = recv(sock, buf, 128, 0);
    
    printf("read_cb, read %d bytes\n", ret);
    if(ret > 0)
    {
        ec->recved += ret;
        buf[ret] = 0;
        printf("recv:%s\n", buf);
    }
    else if(ret == 0)
    {
        printf("read_cb connection closed\n");
        event_base_loopexit(ec->base, NULL);
        return;
    }
    if(ec->recved < ec->echo_contents_len)
    {
        event_add(ec->event_read, 0);
    }
}

static evutil_socket_t make_tcp_socket()
{
    int on = 1;
    evutil_socket_t sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);

    evutil_make_socket_nonblocking(sock);
#ifdef WIN32
    setsockopt(sock, SOL_SOCKET, SO_KEEPALIVE, (const char *)&on, sizeof(on));
#else
    setsockopt(sock, SOL_SOCKET, SO_KEEPALIVE, (void *)&on, sizeof(on));
#endif

    return sock;
}

static void echo_client(struct event_base *base)
{
    evutil_socket_t sock = make_tcp_socket();
    struct sockaddr_in serverAddr;
    struct event * ev_write = 0;
    struct event * ev_read = 0;
    struct timeval tv={10, 0};
    struct echo_context *ec = (struct echo_context*)calloc(1, sizeof(struct echo_context));
    
    serverAddr.sin_family = AF_INET;
    serverAddr.sin_port = htons(ECHO_PORT);
#ifdef WIN32
    serverAddr.sin_addr.S_un.S_addr = inet_addr(ECHO_SERVER);
#else
    serverAddr.sin_addr.s_addr = inet_addr(ECHO_SERVER);
#endif
    memset(serverAddr.sin_zero, 0x00, 8);

    connect(sock, (struct sockaddr*)&serverAddr, sizeof(serverAddr));

    ev_write = event_new(base, sock, EV_WRITE, write_cb, (void*)ec);
    ev_read = event_new(base, sock, EV_READ , read_cb, (void*)ec);

    ec->event_write = ev_write;
    ec->event_read = ev_read;
    ec->base = base;
    ec->echo_contents = strdup("echo client tneilc ohce\n");
    ec->echo_contents_len = strlen(ec->echo_contents);
    ec->recved = 0;

    event_add(ev_write, &tv);
}

int _tmain(int argc, _TCHAR* argv[])
{
    struct event_base * base = 0;
#ifdef WIN32
    WORD wVersionRequested;
    WSADATA wsaData;

    wVersionRequested = MAKEWORD(2, 2);

    (void) WSAStartup(wVersionRequested, &wsaData);
#endif

    base = event_base_new();
    echo_client(base);
    event_base_dispatch(base);
    event_base_free(base);

	return 0;
}

代码是在windows上实现的。

使用libevent进行客户端编程的基本步骤，在《libevent实现http client》中已经提过，基本如下：

1. 初始化event_base（后续要运行事件循环）
2. 创建socket，设置为异步，连接server
3. 创建写读写事件，先将写事件加入事件循环
4. 在写事件回调中向server端发送请求并将读事件加入事件循环
5. 在读事件回调中处理数据，并根据数据是否读取完毕决定是否继续添加读事件

结合本文的示例代码，通过event_base_new()创建event_base，在初始化客户端socket（echo_client）之后，调用event_base_dispatch运行事件循环。

echo_client函数初始化了socket，设置为非阻塞模式，调用connect连接到echoserver，然后添加一个写事件到事件循环中。当连接成功后，会触发写事件。这里将write_cb作为写事件的回调，在调用event_new()时传入。

write_cb()写入一个字符串，然后添加一个读事件，在读事件中读取echoserver的回应。

基本的过程就是这样子了。这是个简单的例子，更复杂的例子，基本流程也是一样的。