Cleaning up when exiting an OpenGL app

Question

I have an an OSX OpenGL app I'm trying to modify. When I create the app a whole bunch of initialisation functions are called -- including methods where I can specify my own mouse and keyboard handlers etc. For example:

glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DEPTH | GLUT_DOUBLE | GLUT_RGBA);
glutInitWindowPosition(100, 100);
glutInitWindowSize(700, 700);
glutCreateWindow("Map Abstraction");
glutReshapeFunc(resizeWindow);
glutDisplayFunc(renderScene);
glutIdleFunc(renderScene);
glutMouseFunc(mousePressedButton);
glutMotionFunc(mouseMovedButton);
glutKeyboardFunc(keyPressed);

At some point I pass control to glutMainLoop and my application runs. In the process of running I create a whole bunch of objects. I'd like to clean these up. Is there any way I can tell GLUT to call a cleanup method before it quits?

Answer 1

In freeglut if you call this:

glutSetOption(GLUT_ACTION_ON_WINDOW_CLOSE, GLUT_ACTION_CONTINUE_EXECUTION)

Prior to entering the main loop, then when the window closes the main loop function will return and you can do your cleanup.

It's worth noting that at that stage the GL context has already been destroyed so you can't perform any GL operations.

Answer 2

I fell for this once in a while, trying to play with GLUT. I tried everything I could thing of, including IIRC exiting glutMainLoop through an exception catched in the main function but...

When using glutMainLoop

My solution was the following : Create a global Context object, who will be owner of all your resources, and free those resources in the destructor.

This global Context object destructor will be called immediately after exiting the main.

It is important Context is a global variable, and not a variable declared in the main function, because for a reason that still escapes me (I still fail to see the interest of this implementation choice), glutMainLoop won't return.

In my Linux box (Ubuntu), the destructor is called correctly. I guess it should work the same way on Windows and MacOS, too.

Note that this is the C++ version of Francisco Soto's atexit() solution, without the possible limitations.

Using glutMainLoopEvent

Apparently, some implementations have a glutMainLoopEvent which can be used instead of calling glutMainLoop.

http://openglut.sourceforge.net/group__mainloop.html#ga1

glutMainLoopEvent will only resolve the pending events, and then return. Thus, you must provide the event loop (the for(;;) construct) around the call to glutMainLoopEvent, but this way, you can work with a GLUT and still have control on the event loop, and free your resources when needed.

Answer 3

If you are using C/C++ maybe you can use an atexit() call?

Answer 4

I ended up using paercebal's answer above, along with his previous attempt at using a try/catch block around glutMainLoop() as well. Why? Because I wanted to cleanup properly no matter how it was shutdown. The global Context object will get destroyed properly if the app exits cleanly, which is what happens if you close out the app by closing the window (or quitting the app on OS X). But if you hit ctrl-C in the terminal where it was launched (or send it a SIGINT), the cleanup does not happen. To handle this, I added the following to my code:

static bool exitFlag = false;

static void sighandler(int sig) {
  exitFlag = true;
}

static void idleFunc() {
  if(exitFlag) {
    throw NULL;
  }
}

And then in main():

signal(SIGINT, sighandler);
glutIdleFunc(idleFunc);

try {
  glutMainLoop();
} catch(...) {}

This isn't the prettiest bit of code, but it does handle both cases of exiting the program correctly.

One catch (no pun intended) -- any code you place after the catch() block will not be called if you close the window/quit the app normally. You have to place your cleanup code in the global Context object as shown in paercebal's answer. All this code is doing is allow the SIGINT signal to be used to get out of the glutMainLoop().

I think the real lesson here is that for anything really complex, GLUT is just not going to cut it.

Answer 5

Typically you don't need to do this; just exiting the application will tear down any resources you've allocated. Even if you've captured the screen, it should go back to normal.