respectively. Another past use was to kill a frozen graphical program, as the X Window System used to have complete control over graphical mode and input devices. On distributions that do not include a command, the key command may sometimes be able to force a return to a text console. (Use , , , ... , , where is the highest number of text consoles set up by the distribution. would normally be used to reenter GUI mode on a system on which the X server has not crashed.) There are also distributions where + (, ... ,) opens text consoles, reenter GUI mode, reenter GUI mode on the login page, in order to log in another user. Configuration The feature is controlled both by a compile-time option in the kernel configuration, , and a sysctl kernel parameter, . On newer kernels (since 2.6.12), it is possible to have more fine-grained control over how the magic SysRq key can be used. On these machines, the number written to can be 0, 1, or a number greater than 1 which is a bitmask indicating which features to allow. On Ubuntu this is set at boot time to the value defined in . Other ways to invoke Magic SysRq While the magic SysRq key was originally implemented as part of the kernel's keyboard handler for debugging, the functionality has been also exposed via the proc filesystem and is commonly used to provide extended management capabilities to headless and remote systems. From user space programs (such as a command line shell), SysRq may be accessed by writing to (e.g., ). Many embedded systems have no attached keyboard, but instead use a serial console for text input/output to the running system. It is possible to invoke a Magic SysRq feature over a serial console by sending a serial break signal, followed by the desired key. The method of sending a break is dependent on the terminal program or hardware used to connect to the serial console. A sysctl option needs to be set to enable this function. The Linux daemons and provide a method of accessing SysRq features over a TCP connection after authenticating with a plain-text password. The daemon will invoke pre-configured SysRq triggers when system load average exceeds a certain threshold. The Xen hypervisor has functionality to send magic commands to hosted domains via its command. Additionally, a SysRq command can be