Multi Threads Programming

[Shakazahn]

Hi,

I've been coding a Game Engine of some sort, and I've decided to do this in C++.
I've read about multi threading for better perfomance, read a few tutorials about it, but I haven't understood quite yet the idea of thread safe program.

I got the concept, but I'm having trouble figuring the practical use.

In my example, I have something like:

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
  GameWindow gameWindow; //The window screen
  GameInput gameInput; //The mouse/keyboard input
  GameProcessing gameProcessing; //The actual processing, colisions and such
  GameGraphics gameGraphics; //The drawing of the processed objects
  GameSound gameSound; //The sound

  gameWindow.Start(); //Start it's own thread, will keep running until called
  gameInput.Start();
  gameProcessing.Start();
  gameGraphics.Start();
  gameSound.Start();

  //Msg Code below
  //etc
}

In this example, I need the Windows Messages to be sent to the gameInput, where I have variables processing each keyboard stroke, each mouse action, etc
But then in the gameProcessing, I need to acess gameInput's variables and move objects accordingly.

I've read that if an object is just reading from another thread variables (like this example), it shouldn't be a problem, is it right?

And if I wanted to change gameInput's variables from another thread, say, gameProcessing, how should I do it thread safe?