I wanted to try a server/client connection kind of thing, and so I modified the code from a YouTube tutorial (New to C ). So far everything works just fine, except when I try to print the message that was sent by the client.

The code used for sending and receiving data is the following:

char buf[4096];

while (true)
{
    ZeroMemory(buf, 4096);

    int bytesReceived = recv(clientSocket, buf, 4096, 0);
    if (bytesReceived == SOCKET_ERROR)
    {
        cerr << "Error in recv()! Quitting" << endl;
        break;
    }

    if (bytesReceived == 0)
    {
        cout << "Client disconnected!" << endl;
        break;
    }

    cout << buf << endl;
    send(clientSocket, buf, bytesReceived   1, 0);
}

Sending it back works fine, as well a printing it to the console. However, in the console it prints a crazy amount of new lines after the initial output. I guess it has something to do with the size of the buf, which is 4096, but I don't know how to fix it.

CodePudding user response：

recv() tells you how many bytes were actually put into your buf, but you are ignoring that value when printing the contents of buf. The operator<< you are using expects a null-terminated string, but recv() does not guarantee your buf is null-terminated (even if the client sends a null terminator, it may not have arrived yet when recv() exits).

Use write() instead of operator<< when printing buf to std::cout, so that you can specify bytesReceived as the size to print.

You are also sending 1 more byte back to the client then you actually received. You should not be adding 1 to bytesReceived when calling send().

You also need to take into account that send() may accept fewer bytes for transmission than you ask for, so it needs to be called in a loop, just like with recv().

Try this instead:

char buf[4096], *ptr;
int bytesReceived, bytesSent, errCode;

while (true)
{
    bytesReceived = recv(clientSocket, buf, sizeof(buf), 0);
    if (bytesReceived == SOCKET_ERROR)
    {
        errCode = WSAGetLastError();
        cerr << "Error " << errCode << " in recv()! Quitting" << endl;
        break;
    }

    if (bytesReceived == 0)
    {
        cout << "Client disconnected!" << endl;
        break;
    }

    cout.write(buf, bytesReceived);
    //cout << endl;

    ptr = buf;
    while (bytesReceived > 0)
    {
        bytesSent = send(clientSocket, ptr, bytesReceived, 0);
        if (bytesSent == SOCKET_ERROR)
        {
            errCode = WSAGetLastError();
            cerr << "Error " << errCode << " in send()! Quitting" << endl;
            break;
        }
        ptr  = byteSent;
        bytesReceived -= byteSent;
   }

   if (bytesReceived > 0)
       break;
}