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 ioctl() requests supported by the device that
tty refers to, the ioctl() request TIOCNOTTY is supported.
TIOCNOTTY Detach the current process from its controlling terminal.
If the process is the session leader,
then SIGHUP and SIGCONT signals are sent to the foreground process group
and all processes in the current session lose their controlling tty.
This ioctl() call only works on file descriptors connected
to /dev/tty. 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.