Correct way to set tcp socket keepalive on Win, Mac and Linux. Verified with Wireshark.

keepalive

void setTcpKeepalive(SOCKET &sockfd)
{
    const uint32_t keepaliveIntervalSec = 10;

#ifdef _WIN32
    tcp_keepalive keepaliveParams;
    DWORD ret = 0;
    keepaliveParams.onoff = 1;
    keepaliveParams.keepaliveinterval = keepaliveParams.keepalivetime = keepaliveIntervalSec * 1000;
    WSAIoctl(sockfd, SIO_KEEPALIVE_VALS, &keepaliveParams, sizeof(keepaliveParams), NULL, 0, &ret, NULL, NULL);

#elif __APPLE__
    int on = 1, secs = keepaliveIntervalSec;
    setsockopt(sockfd, SOL_SOCKET,  SO_KEEPALIVE, &on, sizeof(on));
    setsockopt(sockfd, IPPROTO_TCP, TCP_KEEPALIVE, &secs, sizeof(secs));

#elif __linux
    int32_t optval;
    socklen_t optlen = sizeof(optval);
    optval = 1;
    setsockopt(sockfd, SOL_SOCKET, SO_KEEPALIVE, &optval, optlen);
    const uint32_t keepaliveProbeCount = 20;
    optval = keepaliveIntervalSec;
    setsockopt(sockfd, SOL_TCP, TCP_KEEPIDLE, &optval, optlen);
    setsockopt(sockfd, SOL_TCP, TCP_KEEPINTVL, &optval, optlen);
    optval = keepaliveProbeCount;
    setsockopt(sockfd, SOL_TCP, TCP_KEEPCNT, &optval, optlen);
#endif
}