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Author Topic: Non-blocking SocketUDP behavior?  (Read 1771 times)

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charliesnike

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Non-blocking SocketUDP behavior?
« on: November 24, 2014, 02:11:46 pm »
I'm currently making a P2P two-person co-op game.  This is my first experience with network programming.

I started with a simple example where two remote clients each move a square around onscreen, and are updated on the other's movements.

My question pertains to this code in my game loop:

Code: [Select]

Quote
// GAME LOOP
{
...
        // Has the player moved?
        bool moved = false;

        // Handle controls

        // Time since last frame
        float deltaTime = window.GetFrameTime();

        if (window.GetInput().IsKeyDown(sf::Key::Left))
        {
                mySquare.Move(-MOVE_SPEED * deltaTime, 0.f);
                moved = true;
        }
        else if (window.GetInput().IsKeyDown(sf::Key::Right))
        {
                mySquare.Move(MOVE_SPEED * deltaTime, 0.f);
                moved = true;
        }
        if (window.GetInput().IsKeyDown(sf::Key::Up))
        {
                mySquare.Move(0.f, -MOVE_SPEED * deltaTime);
                moved = true;
        }
        else if (window.GetInput().IsKeyDown(sf::Key::Down))
        {
                mySquare.Move(0.f, MOVE_SPEED * deltaTime);
                moved = true;
        }

        // SEND INFORMATION

        sf::Packet outPacket;

        outPacket << mySquare.GetPosition().x << mySquare.GetPosition().y;

        // Only send packets to the other player if I've updated my square's position.
        // NOTE: if I set this to "if (true)", the program works fine.
        if (moved)
        {
                std::stringstream ss;
                ss << "Moved";
                writeError(ss);

                if (socket.Send(outPacket, theirIP, port) != sf::Socket::Done)
                {
                        std::stringstream ss;
                        ss << "Couldn't send packet";
                        writeError(ss);
                }
        }

        // GET INFORMATION

        sf::Packet inPacket;

        if (socket.Receive(inPacket, theirIP, port) != sf::Socket::Done)
        {
                std::stringstream ss;
                ss << "Couldn't send packet";
                writeError(ss);
        }
        else
        {
                // Received a packet
                std::stringstream ss;
                ss << "Received a packet";
                writeError(ss);
        }

        float theirX = theirSquare.GetPosition().x;
        float theirY = theirSquare.GetPosition().y;

        if (!(inPacket >> theirX >> theirY))
        {
                std::stringstream ss;
                ss << "Invalid packet read";
                writeError(ss);
        }
        else
        {
                // Update the remote player's position
                theirSquare.SetPosition(theirX, theirY);
        }
...
}


If I set moved to always be "true" (i.e., I'm sending a packet at each iteration of the game loop regardless of whether or not there's a need to) the program works fine.  Otherwise, the other player's square never moves at all.  I know I'm sending packets, but it's as if the other client is never receiving them.

The single UDP socket I'm using is non-blocking.  My thought is the problem is caused because I'm calling Receive() more often than Send().  How does Receive() behave in this scenario?  And if I do have to match each Receive() call to a Send() call, how can I determine how often the other client is calling Send()?  Would it be better to use a blocking socket in a separate thread instead?

Gambit

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Re: Non-blocking SocketUDP behavior?
« Reply #1 on: November 24, 2014, 04:26:31 pm »
First off, complete and minimal code. Any respectable C++ IDE has the ability to place breakpoints, you should use them. I'm going to guess that anyone you are testing your application with does not have the convenience of an IDE, so you can use other debugging methods such as telling program to output messages when you send and receive packets.

Furthermore, what is window?