What is the tty subsystem for?

Question

By now I have now spent at least 10 hours trying to get my head around the famous blog post by Linus Akesson, and Im still struggling. So let me ask my doubts about tty/ptty as a series of short questions.

1) Is the tty/ptty in user space or kernel space?

2) What is tty/ptty's connection to devices or drivers or some numbering or something?

3) The tty seems to be linked to something called the controlling terminal of a process, What is the relation and is every process related to a terminal?

4) On the whole I still dont understand where the heck this terminal concept fits in. A process wants to read something from the stdio, cant it simply do it from the required device file. What exactly is the problem that the tty intends to take care of?

5) I read somewhere that there are attempts to move the tty from the userspace to the kernel space. Is the tty simply a historical residue than a strong design feature.??

Answer 1

A clarification (which might answer some of your questions): I think you meant pty (and not ptty) which is pseudo-tty/pseudo-terminal.

1. A tty (/dev/ttyx) - stands for teletype - is the original terminals (used a line printer for output and a keyboard for input!). A terminal is basically just a user interface device that uses text for input and output.

2. A pty (/dev/pty/n) is a pseudo-terminal - it's a software implementation that appears to the attached program like a terminal, but instead of communicating directly with a "real" terminal, it transfers the input and output to another program. It's the end point of telnet/SSH or even the GNOME terminal.

For example, when you ssh into a remote machine and run ls, the ls output is sent to a pseudo-terminal, the other side of which is attached to the SSH daemon.

EDIT: As far as I know, the tty and so pty, are usermode. BUT they represent terminal-driver. What I mean is: the device file /dev/tty1 is the first virtual console. Most code lives in drivers/char, in the files tty_io.c and n_tty.c and vt.c (kernel source). In contrast to character devices in order to open those files tty_open routine is called, and trust me, it's way messier than opening a character device...

Tty/pty stands for terminal drivers mentioned above but they stands for serial ports (the"numbering" you said). I know very little about it so I don't want to say incorrect data... but you can search the net about it (or someone else can continue from here)

EDIT2: You have changed the question so now it seems like I spoke out of context... Anyway, tty has many different roles even nowday. Terminal driver is the way user-kernel can "communicate". There are some techniques such as terminal drivers, character device etc.

If you still have a question please comment and don't change the whole post....