|REBOOT(2)||Linux Programmer's Manual||REBOOT(2)|
/* Since kernel version 2.1.30 there are symbolic names LINUX_REBOOT_* for the constants and a fourth argument to the call: */
#include <linux/reboot.h> /* Definition of LINUX_REBOOT_* constants */ #include <sys/syscall.h> /* Definition of SYS_* constants */ #include <unistd.h>
int syscall(SYS_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 <sys/reboot.h> /* Definition of RB_* constants */ #include <unistd.h>
int reboot(int cmd);
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.
The values that can be supplied in cmd when calling reboot() in this case are as follows:
- LINUX_REBOOT_CMD_RESTART, LINUX_REBOOT_CMD_RESTART2
- The "init" process is terminated, and wait(2) in the parent process reports that the child was killed with a SIGHUP signal.
- LINUX_REBOOT_CMD_POWER_OFF, LINUX_REBOOT_CMD_HALT
- 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.
- 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.