The file /dev/tty is a character file with major number 5 and
minor number 0, usually of mode 0666 and owner.group root.tty.
It is a synonym for the controlling terminal of a process, if any.
In addition to the
requests supported by the device that
tty refers to, the
request TIOCNOTTY is supported.
Detach the calling process from its controlling terminal.
If the process is the session leader,
signals are sent to the foreground process group
and all processes in the current session lose their controlling tty.
call only works on file descriptors connected
It is used by daemon processes when they are invoked
by a user at a terminal.
The process attempts to open /dev/tty.
If the open succeeds, it
detaches itself from the terminal by using TIOCNOTTY, while if the
open fails, it is obviously not attached to a terminal and does not need
to detach itself.