libevent 创建tcp客户端

libevent是一个事件驱动的网络库，可以用来创建高效的网络应用程序。下面是使用libevent库创建TCP客户端的步骤：

1. 引入libevent库头文件：

#include <event2/event.h>
#include <event2/bufferevent.h>
#include <event2/buffer.h>
#include <event2/util.h>

2. 创建event_base对象：

struct event_base* base = event_base_new();

3. 创建连接：

struct bufferevent* bev = bufferevent_socket_new(base, -1, BEV_OPT_CLOSE_ON_FREE);
bufferevent_setcb(bev, readcb, NULL, eventcb, NULL);
bufferevent_enable(bev, EV_READ|EV_WRITE);
bufferevent_socket_connect(bev, (struct sockaddr*)&serveraddr, sizeof(serveraddr));

其中，readcb是读取数据回调函数，eventcb是事件回调函数，serveraddr是服务器地址。

4. 发送数据：

const char* data = "hello, world!";
bufferevent_write(bev, data, strlen(data));

5. 接收数据：

void readcb(struct bufferevent* bev, void* ctx)
{
    struct evbuffer* input = bufferevent_get_input(bev);
    size_t len = evbuffer_get_length(input);
    char* data = malloc(len+1);
    evbuffer_remove(input, data, len);
    data[len] = '\0';
    printf("received data: %s\n", data);
    free(data);
}

6. 事件循环：

event_base_dispatch(base);

完整代码示例：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <event2/event.h>
#include <event2/bufferevent.h>
#include <event2/buffer.h>
#include <event2/util.h>

void readcb(struct bufferevent* bev, void* ctx)
{
    struct evbuffer* input = bufferevent_get_input(bev);
    size_t len = evbuffer_get_length(input);
    char* data = malloc(len+1);
    evbuffer_remove(input, data, len);
    data[len] = '\0';
    printf("received data: %s\n", data);
    free(data);
}

void eventcb(struct bufferevent* bev, short events, void* ctx)
{
    if (events & BEV_EVENT_CONNECTED) {
        printf("connected\n");
    } else if (events & BEV_EVENT_ERROR) {
        printf("error\n");
    }
}

int main(int argc, char** argv)
{
    struct event_base* base = event_base_new();
    struct sockaddr_in serveraddr;
    memset(&serveraddr, 0, sizeof(serveraddr));
    serveraddr.sin_family = AF_INET;
    serveraddr.sin_port = htons(12345);
    inet_pton(AF_INET, "127.0.0.1", &serveraddr.sin_addr);

    struct bufferevent* bev = bufferevent_socket_new(base, -1, BEV_OPT_CLOSE_ON_FREE);
    bufferevent_setcb(bev, readcb, NULL, eventcb, NULL);
    bufferevent_enable(bev, EV_READ|EV_WRITE);
    bufferevent_socket_connect(bev, (struct sockaddr*)&serveraddr, sizeof(serveraddr));

    const char* data = "hello, world!";
    bufferevent_write(bev, data, strlen(data));

    event_base_dispatch(base);

    bufferevent_free(bev);
    event_base_free(base);
    return 0;
}