The Controlling Tty

There is no such thing as a "system keyboard" or "screen"
A program interacts through files, and files only
There exists no meaningless "kbhit" function
By convention, there exists standard input, output, error
They can be connected to a keyboard, but they are likely not
Unfortunately, the user needs some control over interaction
Keyboard-kill operations and automatic termination on modem hang-hup
Changes in screen size need to be notified

Thus, each process may have (or not) a controlling tty
The tty is a serial port, with uart-class features
Baud rate, parity, newline conversion, ...
The tty is also a screen, with display-class features
Char-grid size and little more
Other screen-like things travel in the data stream as escape sequences
Special input characters received from a tty make the kernel send signals
The signal is sent only to processes controlled by that tty