glutDialsFunc sets the dial & button box dials callback for the current window. The dial & button box dials callback for a window is called when the window has dial & button box input focus (normally, when the mouse is in the window) and the user generates dial & button box dial changes. The dial parameter will be the dial number (starting at one). The number of available dial & button box dials can be determined with glutDeviceGet(GLUT_NUM_DIALS). The value measures the absolute rotation in degrees. Dial values do not "roll over" with each complete rotation but continue to accumulate degrees (until the int dial value overflows).
Registering a dial & button box dials callback when a dial & button box device is not available is ineffectual and not an error. In this case, no dial & button box dials callbacks will be generated.
Passing NULL to glutDialsFunc disables the generation of dial & button box dials callbacks. When a new window is created, no dial & button box dials callback is initially registered.