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.\" Copyright (c) 1980, 1991, 1993
.\" The Regents of the University of California. All rights reserved.
.\"
.\" 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.
.\" 3. Neither the name of the University nor the names of its contributors
.\" may be used to endorse or promote products derived from this software
.\" without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS 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 REGENTS 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.
.\"
.\" @(#)dmesg.8 8.1 (Berkeley) 6/5/93
.\" $FreeBSD$
.\"
.Dd May 7, 2022
.Dt DMESG 8
.Os
.Sh NAME
.Nm dmesg
.Nd "display the system message buffer"
.Sh SYNOPSIS
.Nm
.Op Fl ac
.Op Fl M Ar core Op Fl N Ar system
.Sh DESCRIPTION
The
.Nm
utility displays the contents of the system message buffer.
If the
.Fl M
option is not specified, the buffer is read from the currently running kernel
via the
.Xr sysctl 3
interface.
Otherwise, the buffer is read from the specified core file,
using the name list from the specified kernel image (or from
the default image).
.Pp
The options are as follows:
.Bl -tag -width indent
.It Fl a
Show all data in the message buffer.
This includes any syslog records and
.Pa /dev/console
output.
.It Fl c
Clear the kernel buffer after printing.
.It Fl M
Extract values associated with the name list from the specified core.
.It Fl N
If
.Fl M
is also specified,
extract the name list from the specified system instead of the default,
which is the kernel image the system has booted from.
.El
.Sh SYSCTL VARIABLES
The following
.Xr sysctl 8
variables control how the kernel timestamps entries in the message buffer:
The default value is shown next to each variable.
.Bl -tag -width indent
.It kern.msgbuf_show_timestamp : No 0
If set to 0, no timetamps are added.
If set to 1, then a 1-second granularity timestamp will be added to most lines
in the message buffer.
If set to 2, then a microsecond granularity timestamp will be added.
This may also be set as a boot
.Xr loader 8
tunable.
The timestamps are placed at the start of most lines that the kernel generates.
Some multi-line messages will have only the first line tagged with a timestamp.
.El
.Sh FILES
.Bl -tag -width ".Pa /var/run/dmesg.boot" -compact
.It Pa /var/run/dmesg.boot
usually a snapshot of the buffer contents
taken soon after file systems are mounted
at startup time
.El
.Sh SEE ALSO
.Xr sysctl 3 ,
.Xr syslogd 8
.Sh HISTORY
The
.Nm
utility appeared in
.Bx 3 .
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