Fast user switching is a feature on some modern multi-user operating systems such as Windows XP and newer, Mac OS X, Linux. It allows users to switch between user accounts on a single PC without quitting applications and logging out. Analogous functionality was first developed on consumer level hardware by the Xenix operating system which supported multiple virtual consoles. Linux, BSD, and most other PC Unixes adopted virtual terminals and further developed the user interfaces, including allowing users to optionally run separate graphical X Window System sessions.
Many free software environments, such as GNOME or KDE, support fast user switching with a drop menu, in a way analogous to OS X. For installations with older environments, the functionality must be enabled in the configuration file of the X display manager (for example GDM) then a hot key sequence such as CTRL-ALT-F8 is pressed. A separate login window will now appear and the second user can log in (or even the first user again). Alternatively, in the default install, new X sessions can be started at will by using different display parameters to have them run in different virtual terminals (e.g. "startx -- :1" or "X :1 -query localhost"). Again, hot key sequences allow the user switching to take place.
Fast user switching may potentially introduce various security-related complications, and is handled differently among operating systems, each having its advantages and disadvantages. One possibility, simple and secure, is that only the first user gets ownership of resources. A second option is to grant ownership of resources to each new user. The last one to log in takes ownership. A third is to allow all users access to shared resources. This is easier and more intuitive, but allows (for example) one user to record another user's conversation. In Windows, shared resources, such as sound, are available to all sessions. In Red Hat Linux, the default behavior is to give ownership of "console resources" to the first connected session, but it can share resources among groups of console users or be configured to manage console ownership differently.
Read more about Fast User Switching: Windows Implementation
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