C ++套接字，发送和接收不同步

Question

I'm currently working on a multiplayer game using sockets and I encountered some problems at the log-in.

Here's the server function - thread that deals with incoming messages from a user:

void Server::ClientThread(SOCKET Connection)
{
char *buffer = new char[256];

while (true)
{
    ZeroMemory(buffer,256);
    recv(Connection, buffer, 256, 0);
    cout << buffer << endl;
    if (strcmp(buffer, "StartLogIn"))
    {
        char* UserName = new char[256];
        ZeroMemory(UserName, 256);
        recv(Connection, UserName, 256, 0);

        char* Password = new char[256];
        ZeroMemory(Password, 256);
        recv(Connection, Password, 256, 0);

        cout << UserName << "-" << Password << " + "<<  endl;
        if (memcmp(UserName, "taigi100", sizeof(UserName)))
        {
            cout << "SMB Logged in";
        }
        else
            cout << "Wrong UserName";
    }

    int error = send(Connection, "0", 1, 0);
//  error = WSAGetLastError();
    if (error == SOCKET_ERROR)
    {
        cout << "SMB D/Ced";
        ExitThread(0);
    }
}
}

And here is the function that sends the data from the client to the server:

if (LogInButton->isPressed())
{
    send(Srv->getsConnect(), "StartLogIn", 256, 0);
    const wchar_t* Usern = UserName->getText();
    const wchar_t* Passn = Password->getText();
    stringc aux = "";
    aux += Usern;
    char* User = (char*)aux.c_str();

    stringc aux2 = "";
    aux2 += Passn;
    char* Pass = (char*)aux2.c_str();

    if (strlen(User) > 0 && strlen(Pass) > 0)
    {
        send(Srv->getsConnect(), User, 256, 0);
        send(Srv->getsConnect(), Pass, 256, 0);
    }
}

I'm going to try to explain this as easy as possible. The first recv function from the while(true) in the Server-side function receives at first "StartLogIn" but does not enter the if only until the next loop of the while. Because it loops again it changes to "taigi100" ( a username I use ) and then it enters the if even tho it shouldn't.

A way to fix this would be to make a send-recv system in order to not send anything else until it got some feedback.

I want to know if there are any other fast ways of solving this problem and why such weird behaviour happens.

Answer 1

Well it's full of bugs.

• Your overuse of new[]. Ok not a bug but you are not deleting any of these, and you could use either local stack buffer space or vector< char >

• You need to always check the result of any call to recv as you are not guaranteed to receive the number of bytes you are expecting. The number you specify is the size of the buffer, not the number of bytes you are expecting to get.

• strcmp returns 0 if the strings match, non-zero if they do not (actually 1 or -1 depending whether they compare less or greater). But it appears you are using non-zero to mean equal.

• Not sure what stringc is. Some kind of conversion from wide string to string? In any case, I think send is const-correct so there is no need to cast the constness away.

• 3rd parameter of send is the number of bytes you are sending, not the capacity of your buffer. The user name and password are probably not 256 bytes. You need to send them as a "packet" though so the receiver knows what they are getting and will know when they have received a full packet. eg send a string like "User=vandamon\\0". (And you need to check its return value too)

Answer 2

Because send() and recv() calls may not match up, two very good habits to get into are (1) preceed all variable length data by a fixed size length, and (2) only send the bare minimum needed.

So your initial send() call would be written as follows:

char const * const StartLogin = "StartLogIn";
short const StartLoginLength = static_cast<short>(strlen(StartLogin));
send(Srv->getsConnect(), reinterpret_cast<char *>(&StartLoginLength), sizeof(short), 0);
send(Srv->getsConnect(), StartLogin, StartLoginLength, 0);

The corresponding receive code would then have to read two bytes and guarantee that it got them by checking the return value from recv() and retrying if not enough was received. Then it would loop a second time reading exactly that many bytes into a buffer.

int guaranteedRecv(SOCKET s, char *buffer, int expected)
{
    int totalReceived = 0;
    int received;
    while (totalReceived < expected)
    {
        received = recv(s, &buffer[totalReceived], expected - totalReceived, 0);
        if (received <= 0)
        {
            // Handle errors
            return -1;
        }
        totalReceived += received;
    }
    return totalReceived;
}

Note that this assumes a blocking socket. Non-blocking will return zero if no data is available and errno / WSAGetLastError() will say *WOULDBLOCK. If you want to go this route you'll have to handle this case specifically and find some way to block till data is available. Either that or busy-wait waiting for data, by repeatedly calling recv() . UGH.

Anyway, you call this first with the address of a short reinterpret_cast<char *> and expected == sizeof(short) . Then you new[] enough space, and call a second time to get the payload. Beware of the lack of trailing NUL characters, unless you explicitly send them, which my code doesn't.