.\"     $NetBSD: panic.9,v 1.22 2018/07/15 05:16:41 maxv Exp $
.\"
.\" Copyright (c) 1996 The NetBSD Foundation, Inc.
.\" All rights reserved.
.\"
.\" This code is derived from software contributed to The NetBSD Foundation
.\" Michael Graff.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
.\" PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
.\" POSSIBILITY OF SUCH DAMAGE.
.\"
.Dd July 14, 2018
.Dt PANIC 9
.Os
.Sh NAME
.Nm panic
.Nd bring down system on fatal error
.Sh SYNOPSIS
.In sys/types.h
.In sys/systm.h
.Ft void
.Fn vpanic "const char *fmt" "va_list ap"
.Ft void
.Fn panic "const char *fmt" "..."
.Sh DESCRIPTION
The
.Fn panic
and
.Fn vpanic
functions terminate the
.Nx
system.
The message
.Fa fmt
is a
.Xr printf 3
style format string which is printed to the console and saved in the
variable
.Va panicstr
for later retrieval via core dump inspection.
A newline character is added at the end automatically, and is thus
not needed in the format string.
.Pp
If a kernel debugger is installed, control is passed to it after the
message is printed.
If the kernel debugger is
.Xr ddb 4 ,
control may be passed to it, depending on the value of
.Em ddb.onpanic .
See
.Xr options 4
for more details on setting
.Em ddb.onpanic .
If control is not passed through to
.Xr ddb 4 ,
a
.Xr ddb 4 Ns -specific
function is used to print the kernel stack trace, and then control returns
to
.Fn panic .
.Pp
If control remains in
.Fn panic ,
an attempt is made to save an image of system memory on the
configured dump device.
.Pp
If during the process of handling the panic,
.Fn panic
is called again
.Pq from the filesystem synchronization routines, for example ,
the system is rebooted immediately without synchronizing any filesystems.
.Pp
.Fn panic
is meant to be used in situations where something unexpected has happened
and it is difficult to recover the system to a stable state, or in
situations where proceeding might make things worse, leading to data
corruption and/or loss.
It is not meant to be used in scenarios where the system could easily
ignore and/or isolate the condition/subsystem and proceed.
.Pp
In general developers should try to reduce the number of
.Fn panic
calls in the kernel to improve stability.
.Sh RETURN VALUES
The
.Fn panic
function never returns.
.Sh SEE ALSO
.Xr sysctl 3 ,
.Xr ddb 4 ,
.Xr options 4 ,
.Xr savecore 8 ,
.Xr swapctl 8 ,
.Xr sysctl 8