int val = 1;
setsockopt(socket, SOL_SOCKET, SO_KEEPALIVE, &val, sizeof val)

只启用keepalive。您将获得keepalive探测器的默认计时器，可以使用以下命令查看：

sysctl net.ipv4.tcp_keepalive_time

通常默认值为几个小时。
如果您想更改默认计时器，您可以使用以下命令：

struct KeepConfig cfg = { 60, 5, 5};
set_tcp_keepalive_cfg(fd, &cfg);

使用此处的帮助函数：

struct KeepConfig {
    /** The time (in seconds) the connection needs to remain 
     * idle before TCP starts sending keepalive probes (TCP_KEEPIDLE socket option)
     */
    int keepidle;
    /** The maximum number of keepalive probes TCP should 
     * send before dropping the connection. (TCP_KEEPCNT socket option)
     */
    int keepcnt;

    /** The time (in seconds) between individual keepalive probes.
     *  (TCP_KEEPINTVL socket option)
     */
    int keepintvl;
};

/**
* enable TCP keepalive on the socket
* @param fd file descriptor
* @return 0 on success -1 on failure
*/
int set_tcp_keepalive(int sockfd)
{
    int optval = 1;

    return setsockopt(sockfd, SOL_SOCKET, SO_KEEPALIVE, &optval, sizeof(optval));
}

/** Set the keepalive options on the socket
* This also enables TCP keepalive on the socket
*
* @param fd file descriptor
* @param fd file descriptor
* @return 0 on success -1 on failure
*/
int set_tcp_keepalive_cfg(int sockfd, const struct KeepConfig *cfg)
{
    int rc;

    //first turn on keepalive
    rc = set_tcp_keepalive(sockfd);
    if (rc != 0) {
        return rc;
    }

    //set the keepalive options
    rc = setsockopt(sockfd, IPPROTO_TCP, TCP_KEEPCNT, &cfg->keepcnt, sizeof cfg->keepcnt);
    if (rc != 0) {
        return rc;
    }

    rc = setsockopt(sockfd, IPPROTO_TCP, TCP_KEEPIDLE, &cfg->keepidle, sizeof cfg->keepidle);
    if (rc != 0) {
        return rc;
    }

    rc = setsockopt(sockfd, IPPROTO_TCP, TCP_KEEPINTVL, &cfg->keepintvl, sizeof cfg->keepintvl);
    if (rc != 0) {
        return rc;
    }

    return 0;
}

尽管名字叫keep-alive，但它并不是要保持连接的活动，而是要定期交换数据包，以确保对等点之间存在网络路径，它会在空闲时杀死那些能够在长时间网络中断后幸存下来的连接。
由于此行为，除非有充分的理由，否则不应使用keep-alive，如telnet或SSH连接，在这些连接中，当客户端无法访问时，可以终止会话。
很可能是服务器由于某些连接处理策略而在n小时后关闭连接，而不管keepalive使用情况如何。
最新消息：Dror Harari的评论给这种情况又增加了一条皱纹：NAT中间路由器。一个完全空闲的连接会被它们关闭，而主机并不知道，它们认为它们有连接，但实际上它们没有了。使用某种keepalive（在TCP或应用程序协议级别）是一个有效的解决方案。