How can I make a custom callback in glut? For example, if I want to watch a log file and update the screen whenever it changes. The only callbacks for input I see are things for processing keyboard, mouse, and other events (ie glutKeyboardFunc, etc). Thanks!

You will have to do that yourself in the idle function. Rather than checking every time though, you might want to do it every say, 5 seconds or whatnot. GLUT is meant to be a graphics-only system, and as such, has no concept of a filesystem, etc.