What part(s) of the TTY subsystem control the active /dev/ttyN (TTY device driver N)?

Question

The TTY subsystem contains the TTY driver, line discipline(s) and hardware drivers (keyboard driver, display driver). Let's say I use a virtual terminal/console. So, my current session uses ‍/dev/tty1 (TTY device 1). When I press Ctrl+Alt+F2, I open another session which now uses /dev/tty2 (TTY device 2).

So, who controls which /dev/ttyN is the active one? Line discipline?

How about output from inactive /dev/ttyN? Input from process attached to it cannot be output to line discipline because it is an inactive tty. So, input from process will just be dropped?

How do keyboard input and text output work? addresses some of this, but it focuses on X Windows and ssh, and ignores lines disciplines and virtual consoles. What is the exact difference between a 'terminal', a 'shell', a 'tty' and a 'console'? is canonical background reading. — G-Man Says 'Reinstate Monica', May 18 '19 at 03:47
The tty subsystem make use of keyboard/framebuffer driver, which is not part of it. Each running ttyN has bunch of states including its output buffer, input buffer, line discipline states etc, an activated ttyN write to framebuffer and read from keyboard when some process write/read it. Non-actived ttys don't do this but still has working output/input buffers. — 炸鱼薯条德里克, May 19 '19 at 03:28
Also notice that tty drivers also control pesudo-terminals slave side, which is not completely the same as tty, the another side -- master side of pesudo-terminals -- is controlled by userspace, which may not directly read from keyboard device or write directly to framebuffer — 炸鱼薯条德里克, May 19 '19 at 08:48

score 4 · Answer 1 · edited Apr 13 '17 at 12:36

Without OP specifying a particular system, the advice has to be generic.

For a terminal emulator, the most common configuration uses pseudo-terminals. As a rule, those do not use the /dev/ttyXX naming convention:

Before Unix98, the BSD pseudo-terminal naming convention was predominant, using names such as /dev/ttypXX (master) and /dev/ptypXX (slave). You can see some variants in xterm's ptyx.h header.
With Unix98, some of the other variants were consolidated, and library interfaces provided to reduce the need for special device names in programs. The most common form allocates slave devices under /dev/pts.

In contrast, a /dev/ttyXX name usually refers to a (more or less) real console. With Linux, those names are virtual consoles, not related to pseudoterminals. A terminal emulator would not be connecting to those. But you can have active sessions on each of those virtual consoles.

What part(s) of the TTY subsystem control the active /dev/ttyN (TTY device driver N)?

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