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.\"     @(#)dmesg.8	8.1 (Berkeley) 6/5/93
.\" $FreeBSD: src/sbin/dmesg/dmesg.8,v 1.5.2.5 2002/08/21 18:58:17 trhodes Exp $
.\"
.Dd September 29, 2016
.Dt DMESG 8
.Os
.Sh NAME
.Nm dmesg
.Nd "display the system message buffer"
.Sh SYNOPSIS
.Nm
.Op Fl a
.Op Fl c
.Op Fl f[f]
.Op Fl M Ar core
.Op Fl N Ar system
.Op Fl n Ar dumpnr
.Sh DESCRIPTION
The
.Nm
utility displays the contents of the system message buffer.
If neither the
.Fl N
nor the
.Fl M
option is 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 (or from the
default one), 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 f[f]
After the initial message buffer dump
.Nm
monitors the kernel for additional data and displays it as it arrives.
.Nm
will not terminate until killed in this mode.
If this option is specified twice,
.Nm
will live loop (using the CPU heavily) to obtain updates as quickly as
possible.
This second mode should only be used when trying to debug a kernel crashing
situation.
.It Fl M Ar core
Extract values associated with the name list from the specified
.Ar core
instead of the default
.Pa /dev/kmem .
.It Fl N Ar system
Extract the name list from the specified
.Ar system
instead of the default, which is the kernel image the system has booted from.
.It Fl n Ar dumpnr
Use the kernel core dump file numbered
.Ar dumpnr
for debugging.
If not found in the current directory,
.Pa /var/crash
is checked.
.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 3.0 .