I am trying to figure out how a tty works1 (the work-flow and responsibilities of each elements). I have read several interesting articles about it, but there are still some blurry areas.
This is what I understand so far:
- The emulated terminal makes different system calls to
/dev/ptmx
, the master part of the pseudo-terminal. - The master part of the pseudo terminal allocates a file in
/dev/pts/[0-N]
, corresponding to the obsolete serial port, and "attaches" a slave pseudo terminal to it. - The slave pseudo terminal keeps information such as session Id, foreground job, screen size.
Here are my questions:
- Has ptmx any purpose besides allocating the slave part? Does it provide some kind of "intelligence", or the emulated terminal
(xterm for instance) has all the intelligence of behaving like a
terminal? - Why does xterm has to interact with the master part, as it only forwards the stdout and stdin of the slave part? Why can't it
directly write and read from the pts file? - Is a session ID always attached to one pts file and vice versa?
Could I type a ps command and found 2 sessionId for the same
/dev/pts/X? - What other information does the
pts
store? Does Xterm update all
fields by himself, or does theptm
add some "intelligence" on it?
1. I base my understanding on the TTY demystified by Linus Akesson, and the Linux Kernel by Andries Brouwer posts, as on several other questions on these sites
Best Answer
Terminal emulators
The master side replaces the line (the pair of TX/RX wires) that goes to the terminal.
The terminal displays the characters that it receives on one of the wires (some of those are control characters and make it do things like move the cursor, change colour...) and sends on another wire the characters corresponding to the keys you type.
Terminal emulators like xterm are not different except that instead of sending and receiving characters on wires, they read and write characters on their file descriptor to the master side. Once they've spawned the slave terminal, and started your shell on that, they no longer touch that. In addition to emulating the pair of wire, xterm can also change some of the line discipline properties via that file descriptor to the master side. For instance, they can update the size attributes so a SIGWINCH be sent to the applications that interact with the slave pty to notify them of a changed size.
Other than that, there is little intelligence in the terminal/terminal emulator.
What you write to a terminal device (like the pty slave) is what you mean to be displayed there, what you read from it is what you have typed there, so it does not make sense for the terminal emulator to read or write to that. They are the ones at the other end.
The tty line discipline
A lot of the intelligence is in the tty line discipline. The line discipline is a software module (residing in the driver, in the kernel) pushed on top of a serial/pty device that sits between that device and the line/wire (the master side for a pty).
A serial line can have a terminal at the other end, but also a mouse or another computer for networking. You can attach a SLIP line discipline for instance to get a network interface on top of a serial device (or pty device), or you can have a tty line discipline. The tty line discipline is the default line discipline at least on Linux for serial and pty devices. On Linux, you can change the line discipline with
ldattach
.You can see the effect of disabling the tty line discipline by issuing
stty raw -echo
(note that the bash prompt or other interactive applications likevi
set the terminal in the exact mode they need, so you want to use a dumb application likecat
to experience with that). Then, everything that is written to the slave terminal device makes it immediately to the master side for xterm to read, and every character written by xterm to the master side is immediately available for reading from the slave device.The line discipline is where the terminal device internal line editor is implemented. For instance with
stty icanon echo
(as is the default), when you typea
, xterm writesa
to the master, then the line discipline echoes it back (makes aa
available for reading byxterm
for display), but does not make anything available for reading on the slave side. Then if you type backspace, xterm sends a^?
or^H
character, the line discipline (as that^?
or^H
corresponds to theerase
line discipline setting) sends back on the master a^H
,space
and^H
forxterm
to erase thea
you've just typed on its screen and still doesn't send anything to the application reading from the slave side, it just updates its internal line editor buffer to remove thata
you've typed before.Then when you press Enter, xterm sends
^M
(CR), which the line discipline converts on input to a ^J (LF), and sends what you've entered so far for reading on the slave side (an application reading on/dev/pts/x
will receive what you've typed including the LF, but not thea
since you've deleted it), while on the master side, it sends a CR and LF to move the cursor to the next line and the start of the screen.The line discipline is also responsible for sending the
SIGINT
signal to the foreground process group of the terminal when it receives a^C
character on the master side etc.Many interactive terminal applications disable most of the features of that line discipline to implement it themselves. But in any case, beware that the terminal (
xterm
) has little involvement in that (except displaying what it's told to display).And there can be only one session per process and per terminal device. A session can have a controlling terminal attached to it but does not have to (all sessions start without a terminal until they open one).
xterm
, in the process that it forks to execute your shell will typically create a new session (and therefore detach from the terminal where you launchedxterm
from if any), open the new/dev/pts/x
it has spawned, by that attaching that terminal device to the new session. It will then execute your shell in that process, so your shell will become the session leader. Your shell or any interactive shell in that session will typically juggle with process groups andtcsetpgrp()
, to set the foreground and background jobs for that terminal.As to what informations are stored by a terminal device with a tty discipline (serial or pty), that's typically what the
stty
command displays and modifies. All the discipline configuration: terminal screen size, local, input output flags, settings for special characters (like ^C, ^Z...), input and output speed (not relevant for ptys). That corresponds to thetcgetattr()
/tcsetattr()
functions which on Linux map to theTCGETS
/TCSETS
ioctls, andTIOCGWINSZ
/TIOCSWINSZ
for the screen size. You may argue that the current foreground process group is another information stored in the terminal device (tcsetpgrp()
/tcgetpgrp()
,TIOC{G,S}PGRP
ioctls), or the current input or output buffer.Note that that screen size information stored in the terminal device may not reflect the reality. The terminal emulator will typically set it (via the same ioctl on the master size) when its window is resized, but it can get out of sync if an application calls the ioctl on the slave side or when the resize is not transmitted (in case of an ssh connection which implies another pty spawned by
sshd
ifssh
ignores theSIGWINCH
for instance). Some terminals can also be queried their size via escape sequences, so an application can query it that way, and update the line discipline with that information.For more details, you can have a look at the
termios
andtty_ioctl
man pages on Debian for instance.To play with other line disciplines:
Emulate a mouse with a pseudo-terminal:
Above, the master side of the pty is terminated by socat onto a named pipe (
fifo
). We connect that fifo to a process (the shell) that writes 0x87 0x0a 0x00 which in the mouse systems protocol meansno button pressed, delta(x,y) = (10,0)
. Here, we (the shell) are not emulating a terminal, but a mouse, the 3 bytes we send are not to be read (potentially transformed) by an application from the terminal device (mouse
above which is a symlink made bysocat
to some/dev/pts/x
device), but are to be interpreted as a mouse input event.Create a SLIP interface:
Above, the serial wire is emulated by
socat
as a TCP socket in-between hostA and hostB. The SLIP line discipline interprets those bytes exchanged over that virtual line as SLIP encapsulated IP packets for delivery on thesl0
interface.