who call tty_open() in linux kernel? - c

Question

For mouse, keyboard and joystick drivers, it may X call the 'open' then using these drivers.
( when I check CentOS 7 (text mode only) in VirtualBox, I didn't see X after executed lsof /dev/input/event2 ( keyboard ), but my keyboard still working )
How about tty_open() ?
According to a call trace, we can roughly know the sequence:
( at least, we know someone called 'sys_open' )
https://bugzilla.redhat.com/show_bug.cgi?id=630464
However, I still don't know who calls the 'open'?

Answer 1

When I check CentOS 7, text mode only, in VirtualBox, I didn't see X after executed lsof /dev/input/event2 (keyboard), but my keyboard still works
That is because you use the virtual console then, one of the /dev/ttyN devices. These are directly wired to the Linux input event subsystem inside the kernel itself (by the vt module); essentially all keyboard-like devices act as inputs to the currently active virtual terminal.
X itself uses a virtual console, just so that the kernel can switch between it and any text-based virtual consoles.
How about tty_open()?
If you run sudo lsof /dev/tty[0-9], you can see which processes are accessing a virtual console.
The processes with getty in their name are the ones that provide login terminals. (The ones that are used with serial terminals are very similar; in fact, many getty programs can handle both virtual consoles and serial terminals just fine.)
When you log in, that getty launches login (program!), which in turn starts the default shell defined for that user (see getent passwd username; it is the last field, and it must be one that is listed in /etc/shells to be allowed).
The kernel itself supports switching to a different virtual console via Ctrl+Alt+Fn (F1 for tty1, F2 for tty2, and so on). If you use suitable graphics drivers, you can even switch between Xorg and virtual consoles. (Usually, Xorg is run on tty6 or tty7, but that varies from distribution to distribution.) The Ctrl+Alt+← and Ctrl+Alt+→ can also usually be used to switch to the previous or next virtual console.

Answer 2

Thanks for Nominal Animal for the information.
I am tracing linux 2.0 ( https://mirrors.edge.kernel.org/pub/linux/kernel/v2.0/ ), thing maybe a little different.
Sorry, I can't find an online Linux 2.0 source to link reference.
For keyboard, in text mode, I think no one calling 'open' in kernel,
In 'int tty_init(void)' ( tty_io.c ), it will call 'kbd_init();'
then we can see:
request_irq(KEYBOARD_IRQ, keyboard_interrupt, 0, "keyboard", NULL);
therefore, keyboard HW can work with this IRQ & callback ('keyboard_interrupt').
For tty_open(), I finally see it in main.c
open("/dev/tty1",O_RDWR,0);
For the newer version of kernel, I think this part migrated to getty application, that's why I can't find it in kernel source.

Answer 3

tty_* routines are UNIX hardware independent code to implement the terminal interface in the kernel. They are responsible of the tty disciplines, the job control stuff and the linemode processing of characters (including the generation of signals on Ctrl-C for example) They also implement the interface to tty/pty pairs and the like.
This is common code to include all the common functionality of the terminal drivers into any kind of hardware you can have attached for (console keyboard/display is just one example, but you have, at least, uart RS-232C interfaces, usb serial dongles, PCI serial line multiplexors, etc)
Terminal functionality dates from the ancient v7 unix, and has many improvements coming from BSD unix (like job control)
By the way, linux implementation of serial interface is not very good, you should look at the BSD implementation for a good one.