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


/* 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 and most alternative libc's (including uclibc, dietlibc,
   musl and a few others), some of the constants involved have gotten
   symbolic names RB_*, and the library call is a 1-argument
   wrapper around the 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 fails (with the error 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 values 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 Linux 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.


(RB_KEXEC, 0x45584543, 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.


(RB_POWER_OFF, 0x4321fedc; since Linux 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 Linux 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.


(RB_SW_SUSPEND, 0xd000fce1; since Linux 2.5.18). The system is suspended (hibernated) to disk. This option is available only if the kernel was configured with CONFIG_HIBERNATION.

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.

Behavior inside PID namespaces

Since Linux 3.4, if reboot() is called from a PID namespace other than the initial PID namespace with one of the cmd values listed below, it performs a "reboot" of that namespace: the "init" process of the PID namespace is immediately terminated, with the effects described in pid_namespaces(7).

The values that can be supplied in cmd when calling reboot() in this case are as follows:


The "init" process is terminated, and wait(2) in the parent process reports that the child was killed with a SIGHUP signal.


The "init" process is terminated, and wait(2) in the parent process reports that the child was killed with a SIGINT signal.

For the other cmd values, reboot() returns −1 and errno is set to EINVAL.


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 caller must have the CAP_SYS_BOOT inside its user namespace.


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


systemctl(1), systemd(1), kexec_load(2), sync(2), bootparam(7), capabilities(7), ctrlaltdel(8), halt(8), shutdown(8)


This page is part of release 5.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, 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
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.

Modified, 27 May 2004, Michael Kerrisk <>
    Added notes on capability requirements