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.\"	from: @(#)crash.8	6.5 (Berkeley) 4/20/91
.\"
.Dd February 23, 2000
.Dt crash 8
.Os
.Sh NAME
.Nm crash
.Nd system failure and diagnosis
.Sh DESCRIPTION
This section explains what happens when the system crashes
and (very briefly) how to analyze crash dumps.
.Pp
When the system crashes voluntarily it prints a message of the form
.Pp
.Bd -literal
        panic: why i gave up the ghost
.Ed
.Pp
on the console, and enters the kernel debugger
.Xr ddb 4
if it is compiled into the kernel.
If
.Xr ddb 4
is not in the kernel, the system takes a dump on a mass storage
peripheral, and then invokes an automatic reboot procedure as
described in
.Xr reboot 8 .
(If auto-reboot is disabled (in a machine dependent way) the system
will simply halt at this point.)
Unless some unexpected inconsistency is encountered in the state
of the file systems due to hardware or software failure, the system
will then resume multi-user operations.
.Pp
The system has a large number of internal consistency checks; if one
of these fails, then it will panic with a very short message indicating
which one failed.
In many instances, this will be the name of the routine which detected
the error, or a two-word description of the inconsistency.
A full understanding of most panic messages requires perusal of the
source code for the system.
.Pp
The most common cause of system failures is hardware failure, which
can reflect itself in different ways.  Here are the messages which
are most likely, with some hints as to causes.
Left unstated in all cases is the possibility that hardware or software
error produced the message in some unexpected way.
.Bl -tag -width indent
.It no init
This panic message indicates filesystem problems, and reboots are likely
to be futile.  Late in the bootstrap procedure, the system was unable to
locate and execute the initialization process,
.Xr init 8 .
The root filesystem is incorrect or has been corrupted, or the mode
or type of /sbin/init forbids execution.
.It timeout table overflow
.ns
This really shouldn't be a panic, but until the data structure
involved is made to be extensible, running out of entries causes a crash.
If this happens, make the timeout table bigger.
.It trap type %d, code=%x, pc=%x
A unexpected trap has occurred within the system; the trap types are
machine dependent and can be found listed in
.Pa /sys/arch/ARCH/include/trap.h .
.Pp
The code is the referenced address, and the pc at the
time of the fault is printed.  These problems tend to be easy to track
down if they are kernel bugs since the processor stops cold, but random
flakiness seems to cause this sometimes.
The kernel debugger
.Xr ddb 4
can be used to locate the instruction and subroutine corresponding
to the PC value.
If that is insufficient to suggest the nature of the problem,
more detailed examination of the system status at the time of the trap
usually can produce an explanation.
.It init died
The system initialization process has exited.  This is bad news, as no new
users will then be able to log in.  Rebooting is the only fix, so the
system just does it right away.
.It out of mbufs: map full
The network has exhausted its private page map for network buffers.
This usually indicates that buffers are being lost, and rather than
allow the system to slowly degrade, it reboots immediately.
The map may be made larger if necessary.
.El
.Pp
That completes the list of panic types you are likely to see.
.Pp
When the system crashes it writes (or at least attempts to write)
an image of memory into the back end of the dump device,
usually the same as the primary swap
area.  After the system is rebooted, the program
.Xr savecore 8
runs and preserves a copy of this core image and the current
system in a specified directory for later perusal.  See
.Xr savecore 8
for details.
.Pp
To analyze a dump you should begin by running
.Xr gdb 1 .
Once gdb starts, use the command
.Pp
.Bd -literal
        target kcore /dev/mem
.Ed
NIKLAS NIKLAS.
.Pp
Then a traceback and other such things can be gotten.
.Sh "SEE ALSO"
.Xr gdb 1 ,
.Xr ddb 4 ,
.Xr reboot 8