Question

The following code is my TCP server program:

    #include
    #include
    #pragma comment (lib,"ws2_32.lib")
    #define PORT 8888
    #define ADDR "127.0.0.1"


int main()
{
    WSADATA wsock;
    SOCKET listensocket,connectsocket;
    SOCKADDR_IN seraddr,cliaddr;
    int cliaddrsize=sizeof(cliaddr);
    int nret=0;
    char buf[100];

    printf("init socket ...\n");
    if(WSAStartup(MAKEWORD(2,2),&wsock)!=0)
    {
        printf("WSAStartup() failed %d\n",WSAGetLastError());
        return 0;
    }
    printf("init successfully\n");
    printf("create TCP socket...\n");

    if((listensocket=socket(AF_INET,SOCK_STREAM,IPPROTO_TCP))==INVALID_SOCKET)
    {
        printf("socket create failed %d\n",WSAGetLastError());
        WSACleanup();
        return 0;
    }
    printf("socket create successfully\n");

    seraddr.sin_family=AF_INET;
    seraddr.sin_addr.s_addr=inet_addr(ADDR);
    seraddr.sin_port=htons(PORT);
    if(bind(listensocket,(SOCKADDR *)&seraddr,sizeof(seraddr))==SOCKET_ERROR)
    {
        printf("bind failed %d\n",WSAGetLastError());
        closesocket(listensocket);
        WSACleanup();
        return 0;
    }
    printf("bind successfully\n");

    if(listen(listensocket,5)==SOCKET_ERROR)
    {
        printf("listen failed %d\n",WSAGetLastError());
        closesocket(listensocket);
        WSACleanup();
        return 0;
    }
    printf("wait for a connection on port %d\n",PORT);

    if(connectsocket=accept(listensocket,(SOCKADDR*)&cliaddr,&cliaddrsize)==INVALID_SOCKET)  //accept
    {
        printf("accept failed %d\n",WSAGetLastError());
        closesocket(listensocket);
        WSACleanup();
        return 0;
    }
    printf("get connection from %s : %d successfully\n",inet_ntoa(cliaddr.sin_addr),ntohs(cliaddr.sin_port));//NB 啊
    closesocket(listensocket);

    printf("wait to receive data...\n");
    memset(buf,0,sizeof(buf));
    while(1)
    {
        if(nret=recv(connectsocket,buf,sizeof(buf),0)==SOCKET_ERROR)      //recv
        {
            printf("recv failed %d\n",WSAGetLastError());
            closesocket(connectsocket);
            WSACleanup();
            return 0;
        }
        printf(buf);
        printf("\n");
        if(strncmp(buf,"exit",sizeof("exit"))==0)
        {
            printf("exit the loop\n");
            break;
        }
        if(nret=send(connectsocket,buf,sizeof(buf),0)==SOCKET_ERROR)
        {
            printf("send failed %d\n",WSAGetLastError());
        }
    }
    closesocket(connectsocket);
    WSACleanup();
    return 0;
}

And I use my network debugging assistent software to run as a TCP client. As follows:

I started my TCP server program, it went as follows:

(...cannot upload images...)

But when I click on my TCP client, the TCP server went wrong :
the dos box shows that : recv failed 10038

My question is why it went wrong ? And how to fix it?

Answer 1

This:

if(connectsocket=accept(listensocket,(SOCKADDR*)&cliaddr,&cliaddrsize)
       ==INVALID_SOCKET)

will result in connectsocket having a value of 0, which does not refer to a valid socket descriptor which the error code 10038 means:

An operation was attempted on something that is not a socket.

because of operator precedence:

// Result of this will be 0 (false) when result of accept()
// is not `INVALID_SOCKET`
accept(listensocket,(SOCKADDR*)&cliaddr,&cliaddrsize) == INVALID_SOCKET

// Which is then assigned to connectsocket
connectsocket = 0

// And the failing if branch is not entered
if (connectsocket)

You need parenthesis around the assignment:

if((connectsocket=accept(listensocket,(SOCKADDR*)&cliaddr,&cliaddrsize))
    ==INVALID_SOCKET)

which you have already done for the earlier call to socket().