In many UNIX systems, it is possible for a program to retain access to the login terminal after the user has logged out. This poses obvious security risks and can also confuse the modem control signals. We solve this for System V by adding a layer of indirection known as the session tty driver. At login time, a session device is linked to the physical terminal. User program have access to the session device only, and may not open the physical line. Upon logout or carrier drop, the link is severed. New login sessions are given new session devices, and are thus insulated from persistent processes.

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