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reboot() - Unix, Linux System Call

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NAME

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

SYNOPSIS

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 flag, 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 flag);

DESCRIPTION

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 flag argument can have the following values:

TagDescription
LINUX_REBOOT_CMD_RESTART
  (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.
LINUX_REBOOT_CMD_HALT
  (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_POWER_OFF
  (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.
LINUX_REBOOT_CMD_RESTART2
  (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.
LINUX_REBOOT_CMD_CAD_ON
  (RB_ENABLE_CAD, 0x89abcdef). CAD is enabled. This means that the CAD keystroke will immediately cause the action associated with LINUX_REBOOT_CMD_RESTART.
LINUX_REBOOT_CMD_CAD_OFF
  (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).
Only the superuser may use this function.

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.

RETURN VALUE

On success, zero is returned. On error, -1 is returned, and errno is set appropriately.

ERRORS

TagDescription
EFAULT Problem with getting userspace data under LINUX_REBOOT_CMD_RESTART2.
EINVAL Bad magic numbers or flag.
EPERM The calling process has insufficient privilege to call reboot(); the CAP_SYS_BOOT capability is required.

CONFORMING TO

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

SEE ALSO


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