The beep(3) and flash(3) routines are used
to alert the terminal user. The routine beep(3) sounds an audible
alarm on the terminal, if possible; otherwise it flashes the screen
(visible bell). The routine flash(3) flashes the screen, and if that is
not possible, sounds the alert. If neither alert is possible,
nothing happens. Nearly all terminals have an audible alert (bell
or beep), but only some can flash the screen.