Basically, the TTY driver keeps track of the foreground process group id, but only in a passive way. A user process can affect the behavior of any TTY device by manipulating the corresponding device file under /dev. For example, a user process can send a signal to the TTY driver, which will then suspend the process. TTY driver keeps track of the size of the connected terminal, but this information has to be updated explicitly, by the terminal emulator or even by the user. The TTY driver is a passive object.