OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Managing a Background Process

You can specify a function to be executed if no other events are pending—for example, when the event loop would otherwise be idle—with glutIdleFunc(). This is particularly useful for continuous animation or other background processing.

void glutIdleFunc(void (*func)(void));

Specifies the function, func, to be executed if no other events are pending. If NULL (zero) is passed in, execution of func is disabled.