If you press Ctrl-Alt-Del (or whatever key was assigned the keysym Boot by loadkeys) then either the machine reboots immediately (without sync), or init is sent a SIGINT. The former behaviour is the default. The default can be changed by root, using the system call reboot(), see ctrlaltdel(8). Some init's change the default. What happens when init gets SIGINT depends on the version of init used - often it will be determined by the pf entry in /etc/inittab (which means that you can run an arbitrary program in this case). In the current kernel Ctrl-AltGr-Del is no longer by default assigned to Boot.

Name            Default binding
-------------------------------
Show_Memory     Shift-Scrollock
Show_Registers  AltGr-ScrollLock
Show_State      Ctrl-ScrollLock
Console_n       Alt-Fn and Ctrl-Alt-Fn  (1 <= n <= 12)
Console_{n+12}  AltGr-Fn                (1 <= n <= 12)
Incr_Console    Alt-RightArrow
Decr_Console    Alt-LeftArrow
Last_Console    Alt[Gr]-PrintScreen
Scroll_Backward Shift-PageUp
Scroll_Forward  Shift-PageDown
Caps_On                                 (CapsLock is a toggle; this key sets)
Compose         Ctrl-.

Ctrl-Alt-Fn     Switch to VT n
Ctrl-Alt-KP+    Next mode
Ctrl-Alt-KP-    Previous mode
Ctrl-Alt-Backspace      Kill X

Perry F Nguyen (pfnguyen@netcom22.netcom.com) writes: AMI BIOS has a feature that locks up the keyboard and flashes the LED's if the Ctrl-Alt-Backspace combination is pressed while a BIOS password is enabled, until the CMOS/BIOS password is typed in.

On some SiS based motherboards the combination Ctrl-Alt-Backspace will cause a power off, or puts the machine in power save mode. (Reported for SiS 630 and for SiS645DX.)

Ctrl-Alt-Fn     Switch to VT n (from version 0.50; earlier Alt-Fn)
Ctrl-Alt-PgDn   Kill dosemu (when in RAW keyboard mode)
(and many other combinations - see the dosemu documentation)

One symbol may be constructed using several keystrokes.

• LeftAlt-press, followed by a decimal number typed on the keypad, followed by LeftAlt-release, yields the symbol with code given by this number. (In Unicode mode this same mechanism, but then with 4 hexadecimal digits, may be used to define a Unicode symbol.)
• A dead diacritic followed by a symbol, yields that symbol adorned with that diacritic. If the combination is undefined, both keys are taken separately. Which keys are dead diacritics is user-settable; none is by default. Five (since 2.0.25 six) dead diacritics can be defined (using loadkeys(1)): dead_grave, dead_acute, dead_circumflex, dead_tilde, dead_diaeresis (and dead_cedilla). Precisely what this adorning means is also user-settable: dead-diacritic, symbol is equivalent to Compose + diacritic + symbol.
• Compose followed by two symbols yields a combination symbol. These combinations are user-settable. Today there are 68 combinations defined by default; you can see them by saying "dumpkeys | grep compose".
• Then there are `Sticky' modifier keys (since 1.3.33). For example, one can type Ctrl-C as SControl, C and Ctrl-Alt-BackSpace as SControl, SAlt, BackSpace.

Note that there are at least four such composition mechanisms:

1. The Linux keyboard driver mechanism, used in conjunction with loadkeys.
2. The X mechanism - see X386keybd(1), later XFree86kbd(1). Under X11R6: edit /usr/X11R6/lib/X11/locale/iso8859-1/Compose.

   See also Andrew D. Balsa's comments at http://wauug.erols.com/~balsa/linux/deadkeys/index.html.

3. The emacs mechanism obtained by loading "iso-insert.el" or calling `iso-accents-mode'.
4. The vim mechanism: insert a composed symbol by pressing Ctrl-K followed by two symbols. A list of the possible combinations is obtained by the command :digraphs.

In case your kernel was compiled with CONFIG_MAGIC_SYSRQ enabled (a feature that is present since Linux 2.1.43) there is a single key (defined in <linux/keyboard.h>) to which special system functions are attached, regardless of the current keyboard mode. For the PC architecture this special key is, naturally, the Alt+SysRq key, and any of the two Alt keys will work. (Note that if CONFIG_MAGIC_SYSRQ was not enabled, the default action of this key is to return to the previous console.)

If you press this key, do not release it, and hit another key, a corresponding action is performed. The action is performed whether anybody is logged in or not, is root or not. For the details, see drivers/char/sysrq.c. Since this feature is meant only for kernel hackers, that should suffice. Still, let me add a few remarks.

For the key r the keyboard mode is reset to K_XLATE. For the key k a SAK and console reset is done. For the key b the machine is rebooted immediately. (See, not something you want to have enabled on a production machine.) For the key o the power is turned off (when the machine is capable of that). For the key s an emergency sync is scheduled. For the key u an emergency read-only remount is scheduled. For the keys p,t,m various information is shown (namely the same information also shown for RAlt,RCtrl,RShift+ScrollLock). For the keys e,i,l all processes get a SIG_TERM or SIG_KILL, respectively; for l even the init process is killed. Digits set the log level. Anything else prints a short summary: SysRq: unRaw saK Boot Off Sync Unmount showPc showTasks showMem loglevel0-8 tErm kIll killalL.

Note: These are very dangerous actions! And they do not use your keymap - indeed, are meant for emergency cases where the state of your keymap, or even of the entire kernel, is uncertain. If you use a dvorak keyboard - bad luck! Most other people will be able to survive: the dangerous letters A,M,Q,W,Y,Z that are differently placed on English, French and German keyboards, are not used for actions. (But if your finger slips and you hit L instead of K - bye bye to your system.)

In Linux 2.3.13 the possibility to enable/disable SysRq was added.

        echo 0 > /proc/sys/kernel/sysrq

A good keyboard accurately reports multiple key presses. Especially for people using a keyboard as Braille input device, where they have to be able to press up to six keys simultaneously, this is important. Many modern keyboards fail here due to sloppy design and testing - they misreport or fail in other ways when four or more keys are pressed simultaneously, sometimes already when two non-modifier keys are pressed simultaneously. This affects Linux users a bit: the SysRq key is not a modifier key, and people report problems using Alt-SysRq-X for various letters or digits X.

Long ago a similar problem (`ghosting') was caused by the design of the scan matrix: when three keys were pressed a fourth was also seen. That problem was solved by the use of diodes. Today it is the interface logic that is deficient.