基于libevent实现tcp客户端及服务端

基于libevent库实现TCP客户端和服务端主要是利用其异步事件处理机制，简化网络编程的复杂性。libevent是一个高效的、跨平台的事件驱动框架，特别适合用于实时应用和网络服务器。

**TCP客户端示例**:

#include <event2/event.h>

void on_connect(struct evconnbase *cb, short revents) {
    if (revents & EV_ERROR) {
        perror("Connect failed");
        return;
    }
    struct sockaddr_in server_addr;
    // ...填充服务器地址...
    int fd = accept(cb->ev_base->fd, (struct sockaddr *)&server_addr, NULL);
    if (fd == -1) {
        perror("Accept failed");
        return;
    }
    event_set(&client_event, fd, EV_READ | EV_PERSIST, on_read, cb);
    event_add(&client_event, NULL);
}

void on_read(struct evconnbase *cb, short revents) {
    if (revents & EV_TIMEOUT) {
        fprintf(stderr, "Read timeout\n");
        return;
    }
    char buf[1024];
    ssize_t n = read(cb->fd, buf, sizeof(buf));
    if (n <= 0) {
        if (n == 0)
            printf("Connection closed by peer\n");
        else
            perror("Read error");
        event_del(&client_event);
    } else {
        // 发送响应或数据处理
    }
}

int main() {
    // 初始化libevent
    event_init();

    // 创建连接事件
    struct event_base *base = event_base_new();
    struct evconnbase *client_event = evconnbase_new(base, on_connect);

    // 连接服务器，设置超时等
    // ...实际连接过程...

    // 主循环
    while (1) {
        event_base_dispatch(base);
    }

    event_base_free(base);
    event_system_exit(0);
}

**TCP服务端示例**:

void on_connection(struct evconnbase *cb, short revents) {
    if (revents & EV_ERROR) {
        perror("Connection failed");
        return;
    }
    struct sockaddr_in client_addr;
    socklen_t addr_len = sizeof(client_addr);
    int fd = accept(cb->ev_base->fd, (struct sockaddr *)&client_addr, &addr_len);
    if (fd == -1) {
        perror("Accept failed");
        return;