How do you know when a key is released?

All the tutorials I read show how to detect if a key is pressed. What about when it is released. How may I detect this?

Thanks in advance!

In what API? If you mean Win32 I think there’s a WM_KEYUP event or something. For glut I’m not sure there’s a key up.

With glut :
glutKeyboardUpFunc( keyboardUp );

Thank you!