简单TCP代码

服务器：

    SOCKET s;
    s = ::socket(AF_INET,SOCK_STREAM,0);
    sockaddr_in addr;
    addr.sin_family = AF_INET;
    addr.sin_port = htons(75);
    addr.sin_addr.S_un.S_addr = INADDR_ANY;
    ::bind(s,(sockaddr*)&addr,sizeof(addr));
    ::listen(s,5);

    SOCKET s2;
    sockaddr_in addr2;
    int n = sizeof(addr2);
    s2  = ::accept(s,(sockaddr*)&addr2,&n);
    char sztext[20] = {0};
    ::recv(s2,sztext,sizeof(sztext),0);
    
    ::closesocket(s2);
    ::closesocket(s);

客户端：

    SOCKET s;
    s = ::socket(AF_INET,SOCK_STREAM,0);
    sockaddr_in addr;
    addr.sin_family = AF_INET;
    addr.sin_port = htons(75);
    addr.sin_addr.S_un.S_addr = inet_addr("127.0.0.1");

    ::connect(s,(sockaddr*)&addr,sizeof(addr));
    char sztext[] = _T("你好");
    ::send(s,sztext,sizeof(sztext),0);
    ::closesocket(s);    

TCP的保活机制：由于TCP一端可能意外而断开连接，这时候另一端如何知道对面是否还在连接呢。

　　1、自己写一个心跳程序，定时去向通信对方发送信息并得到反馈。

       2、使用自带的保活函数。以下是windows的保活函数使用。

#include <mstcpip.h>
BOOL SetTcpKeepAlive(int fd, int start, int interval)  
{  
    // 开启KeepAlive
    BOOL bKeepAlive = TRUE;
    int nRet = ::setsockopt(fd, SOL_SOCKET, SO_KEEPALIVE, (char*)&bKeepAlive, sizeof(bKeepAlive));
    if (nRet == SOCKET_ERROR)
    {
        return FALSE;
    }

    // 设置KeepAlive参数
    tcp_keepalive alive_in                = {0};
    tcp_keepalive alive_out                = {0};
    alive_in.keepalivetime                = start;                // 开始首次KeepAlive探测前的TCP空闭时间
    alive_in.keepaliveinterval        = interval;                // 两次KeepAlive探测间的时间间隔
    alive_in.onoff                                = bKeepAlive;
    unsigned long ulBytesReturn = 0;
    nRet = WSAIoctl(fd, SIO_KEEPALIVE_VALS, &alive_in, sizeof(alive_in),
        &alive_out, sizeof(alive_out), &ulBytesReturn, NULL, NULL);
    if (nRet == SOCKET_ERROR)
    {
        return FALSE;
    }

    return TRUE;  
}