glutLeaveMainLoop man page
glutLeaveMainLoop — Breaks out of OpenGLUT's glutMainLoop()
OpenGLUT - mainloop
This function allows you to unilaterally tell OpenGLUT that you are done and wish to exit. This is useful if you have also told OpenGLUT to return to you rather than to call exit() directly.
Could use longjmp(); see oghTakeActionOnWindowClose(). This would let us terminate the entire loop immediately.
Explore man page connections for glutLeaveMainLoop(3).