Question

I’m reading OpenGL Superbible 4th ed. In Chapter 2, the example code sets up the callback followed by the clear color as follows:

main()
{
//...
glDisplayFunc(RenderScene);
SetupRC();
//..
}    

void RenderScene(void)
{
  glClear(GL_COLOR_BUFFER_BIT);
  glFlush();
}

void SetupRC(void)
{
  glClearColor(0.0f, 0.0f, 1.0f, 1.0f);
}

Is it posisble that we have a race condition here, where glClear might be executed before glClearColor?

Answer 1

It is not a race condition, because glutMainLoop() runs in the same thread and calling the glDisplayFunc() does not invoke any GL functions (it only saves the pointer to your callback).

From the docs:

glutMainLoop enters the GLUT event processing loop.
This routine should be called at most once in a GLUT program.
Once called, this routine will never return.
It will call as necessary any callbacks that have been registered

OpenGL can only render to the GL contexts created in the same thread. Thus the calls to glClearColor() and RenderScene() will be called in the same thread. Since the call to glutMainLoop() is called later in your main(), the glClearColor() will be called strictly before glClear() in RenderScene().