reboot — reboot or enable/disable Ctrl-Alt-Del


/* For libc4 and libc5 the library call and the system call
   are identical, and since kernel version 2.1.30 there are
   symbolic names LINUX_REBOOT_* for the constants and a
   fourth argument to the call: */

#include <unistd.h>
#include <linux/reboot.h>
int reboot( int magic,
  int magic2,
  int cmd,
  void *arg);

/* Under glibc some of the constants involved have gotten
   symbolic names RB_*, and the library call is a 1-argument
   wrapper around the 3-argument system call: */

#include <unistd.h>
#include <sys/reboot.h>
int reboot( int cmd);


The reboot() call reboots the system, or enables/disables the reboot keystroke (abbreviated CAD, since the default is Ctrl-Alt-Delete; it can be changed using loadkeys(1)).

This system call will fail (with EINVAL) unless magic equals LINUX_REBOOT_MAGIC1 (that is, 0xfee1dead) and magic2 equals LINUX_REBOOT_MAGIC2 (that is, 672274793). However, since 2.1.17 also LINUX_REBOOT_MAGIC2A (that is, 85072278) and since 2.1.97 also LINUX_REBOOT_MAGIC2B (that is, 369367448) and since 2.5.71 also LINUX_REBOOT_MAGIC2C (that is, 537993216) are permitted as value for magic2. (The hexadecimal values of these constants are meaningful.) The cmd argument can have the following values:


(RB_DISABLE_CAD, 0). CAD is disabled. This means that the CAD keystroke will cause a SIGINT signal to be sent to init (process 1), whereupon this process may decide upon a proper action (maybe: kill all processes, sync, reboot).


(RB_ENABLE_CAD, 0x89abcdef). CAD is enabled. This means that the CAD keystroke will immediately cause the action associated with LINUX_REBOOT_CMD_RESTART.


(RB_HALT_SYSTEM, 0xcdef0123; since 1.1.76). The message "System halted." is printed, and the system is halted. Control is given to the ROM monitor, if there is one. If not preceded by a sync(2), data will be lost.

LINUX_REBOOT_CMD_KEXEC (since Linux 2.6.13)

Execute a kernel that has been loaded earlier with kexec_load(2). This option is available only if the kernel was configured with CONFIG_KEXEC.


(0x4321fedc; since 2.1.30). The message "Power down." is printed, the system is stopped, and all power is removed from the system, if possible. If not preceded by a sync(2), data will be lost.


(RB_AUTOBOOT, 0x1234567). The message "Restarting system." is printed, and a default restart is performed immediately. If not preceded by a sync(2), data will be lost.


(0xa1b2c3d4; since 2.1.30). The message "Restarting system with command '%s'" is printed, and a restart (using the command string given in arg) is performed immediately. If not preceded by a sync(2), data will be lost.

Only the superuser may call reboot().

The precise effect of the above actions depends on the architecture. For the i386 architecture, the additional argument does not do anything at present (2.1.122), but the type of reboot can be determined by kernel command-line arguments ("reboot=...") to be either warm or cold, and either hard or through the BIOS.


For the values of cmd that stop or restart the system, a successful call to reboot() does not return. For the other cmd values, zero is returned on success. In all cases, −1 is returned on failure, and errno is set appropriately.



Problem with getting user-space data under LINUX_REBOOT_CMD_RESTART2.


Bad magic numbers or cmd.


The calling process has insufficient privilege to call reboot(); the CAP_SYS_BOOT capability is required.


reboot() is Linux-specific, and should not be used in programs intended to be portable.


sync(2), bootparam(7), capabilities(7), ctrlaltdel(8), halt(8), reboot(8)


This page is part of release 3.52 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at−pages/.

  Copyright (c) 1998 Andries Brouwer (, 24 September 1998

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
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Modified, 27 May 2004, Michael Kerrisk <>
    Added notes on capability requirements