Determinism in multiplayer simulation with Box2D, and single computer

Question

I wrote a small test car driving multiplayer game with Box2D using TCP server-client communcations.

I ran 1 instance of server.exe and 2 instance of client.exe on the same machine that I code and compile the executables. I type inputs (WASD for a simple car movement) into one of the 2 clients and I can get both clients to update the simulation.

There are 2 cars in the simulation. As long as the cars do not collide, I get the same identical output on both client.exe. I can run the car(s) around for as long as I could they still update the same. However, if I start to collide the cars, very quickly they go out of sync.

My tools: Windows 7, C++, MSVS 2010, Box2D, freeGlut.

My Psuedocode:

// client.exe
void timer(int value)
{
   tcpServer.send(my_inputs);
   foreach(i = player including myself) inputs[i] = tcpServer.receive();
   foreach(i = player including myself) players[i].process(inputs[i]);
   myb2World.step(33, 8, 6); // Box2D world step simulation
   foreach(i = player including myself) renderer.render(player[i]);

   glutTimerFunc(33, timer, 0);
}

// server.exe
void serviceloop
{
    while(all clients alive)
    {
       foreach(c = clients) tcpClients[c].receive(&inputs[c]);

       // send input of each client to all clients
       foreach(source = clients)
       {
          foreach(dest = clients)
          {
              tcpClients[dest].send(inputs[source]);
          }
       }
    }
}

I have read all over the internet and SE the following claims (paraphrased):

1. Box2D is deterministic as long as floating point architecture/implementation is the same.
2. (For any deterministic engine) Determinism is gauranteed if playback of recorded inputs is on the same machine with exe compiled using same compiler and machine.

Additionally my server.exe and client.exe gameloop is single thread with blocking socket calls and fixed time step.

Question: Can anyone explain what I did wrong to get different Box2D output?

Answer 1

Maybe glutTimerFunc(33, timer, 0); produce different results on different machines. Another thing is that your code. takes different amount of time to execute every once you run it (especially on different computers), so that may be the source of desynchronization.

Anyway the right way to do what you want to achieve is to run main physics simulation on server and send results to clients. You can avoid lag by making clients run their own simulation for results to use in time between getting results from the server.

Answer 2

The problem is that the two cars are out of sync due to the cars not syncing, I guess. I would do something like this:

//client
void tick() {
    step(a,b,c)
    for(body in myb2World) {
        if(body.userdata.owner == this) {
            tcpServer.send(body);
        }
    }
}

//server
void serviceloop
{
    while(all clients alive)
    {
       foreach(c = clients) tcpClients[c].receive(&inputs[c]);

       // send input of each client to all clients
       foreach(source = clients)
       {
          foreach(dest = clients)
          {
              tcpClients[dest].send(inputs[source]);
          }
       }
    }
}

and at the client the bodies not of that client are updated with the correct state.