|TTY(4)||Linux Programmer's Manual||TTY(4)|
In addition to the ioctl(2) requests supported by the device that tty refers to, the ioctl(2) request TIOCNOTTY is supported.
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(2) call works only 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.