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.\"     @(#)reboot.2	8.1 (Berkeley) 6/4/93
.\" $FreeBSD$
.\"
.Dd July 10, 2018
.Dt REBOOT 2
.Os
.Sh NAME
.Nm reboot
.Nd reboot system or halt processor
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.In sys/reboot.h
.Ft int
.Fn reboot "int howto"
.Sh DESCRIPTION
The
.Fn reboot
system call
reboots the system.
Only the super-user may reboot a machine on demand.
However, a reboot is invoked
automatically in the event of unrecoverable system failures.
.Pp
The
.Fa howto
argument
is a mask of options; the system call interface allows the following
options, defined in the include file
.In sys/reboot.h ,
to be passed
to the new kernel or the new bootstrap and init programs.
.Bl -tag -width RB_POWERCYCLE
.It Dv RB_AUTOBOOT
The default, causing the system to reboot in its usual fashion.
.It Dv RB_ASKNAME
Normally the system only prompts the user if the loader specified root
file system has an error.
This flag forces it to always prompt the user for the root partition.
.It Dv RB_DFLTROOT
Use the compiled in root device.
Normally, the system uses the device from which it was booted
as the root device if possible.
(The default behavior is dependent on the ability of the bootstrap program
to determine the drive from which it was loaded, which is not possible
on all systems.)
.It Dv RB_DUMP
Dump kernel memory before rebooting; see
.Xr savecore 8
for more information.
.It Dv RB_HALT
The processor is simply halted; no reboot takes place.
This option should be used with caution.
.It Dv RB_POWERCYCLE
After halting, the shutdown code will do what it can to turn
off the power and then turn the power back on.
This requires hardware support, usually an auxiliary microprocessor
that can sequence the power supply.
At present only the
.Xr ipmi 4
driver implements this feature.
.It Dv RB_POWEROFF
After halting, the shutdown code will do what it can to turn
off the power.
This requires hardware support.
.It Dv RB_KDB
Load the symbol table and enable a built-in debugger in the system.
This option will have no useful function if the kernel is not configured
for debugging.
Several other options have different meaning if combined
with this option, although their use may not be possible
via the
.Fn reboot
system call.
See
.Xr ddb 4
for more information.
.It Dv RB_NOSYNC
Normally, the disks are sync'd (see
.Xr sync 8 )
before the processor is halted or rebooted.
This option may be useful if file system changes have been made manually
or if the processor is on fire.
.It Dv RB_REROOT
Instead of rebooting, unmount all filesystems except the one containing
currently-running executable, and mount root filesystem using the same
mechanism which is used during normal boot, based on
vfs.root.mountfrom
.Xr kenv 1
variable.
.It Dv RB_RDONLY
Initially mount the root file system read-only.
This is currently the default, and this option has been deprecated.
.It Dv RB_SINGLE
Normally, the reboot procedure involves an automatic disk consistency
check and then multi-user operations.
.Dv RB_SINGLE
prevents this, booting the system with a single-user shell
on the console.
.Dv RB_SINGLE
is actually interpreted by the
.Xr init 8
program in the newly booted system.
.El
.Pp
When no options are given (i.e.,
.Dv RB_AUTOBOOT
is used), the system is
rebooted from file
.Dq kernel
in the root file system of unit 0
of a disk chosen in a processor specific way.
An automatic consistency check of the disks is normally performed
(see
.Xr fsck 8 ) .
.Sh RETURN VALUES
If successful, this call never returns.
Otherwise, a -1 is returned and an error is returned in the global
variable
.Va errno .
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er EPERM
The caller is not the super-user.
.El
.Sh SEE ALSO
.Xr crash 8 ,
.Xr halt 8 ,
.Xr init 8 ,
.Xr reboot 8 ,
.Xr savecore 8
.Sh HISTORY
The
.Fn reboot
system call appeared in
.Bx 4.0 .