learning freebsd kernel

gahn ipfreak at yahoo.com
Fri Apr 6 20:02:09 UTC 2012

thanks devin for the great tip. yeah, now i got core dumps...:)

but where is my "gdb" under that "debug.kdb.available"?



----- Original Message -----
From: Devin Teske <devin.teske at fisglobal.com>
To: 'gahn' <ipfreak at yahoo.com>; 'freebsd general questions' <freebsd-questions at freebsd.org>
Sent: Friday, April 6, 2012 2:13 PM
Subject: RE: learning freebsd kernel

> -----Original Message-----
> From: owner-freebsd-questions at freebsd.org [mailto:owner-freebsd-
> questions at freebsd.org] On Behalf Of gahn
> Sent: Friday, April 06, 2012 11:05 AM
> To: freebsd general questions
> Subject: learning freebsd kernel
> hi gurus:
> how could i create the core dumps on freebsd kernel? i am trying to create a
> kernel core dump on 8.1 but it didn't happen:
> # sysctl -w debug.kdb.panic=1
> well the system went panic, entered the mode db>. i did "reboot' but there was
> no core/kernel dumps under /var/crash.

Did you set the "dumpdev" directive in /etc/rc.conf?

% grep dumpdev /etc/defaults/rc.conf
dumpdev="NO"            # Device name to crashdump to (or NO).
savecore_flags=""       # Used if dumpdev is enabled above, and present.

You should set the value of dumpdev to your swap device.

You can get this value by executing the following:

awk '$3~/swap/{print $1}' /etc/fstab

Should produce something like "/dev/mfid0s1b" or "da0s1b" or "ada0s1b" or
"ad0s1b" etc. depending on your RELEASE and hardware.

If the output is, for example, "/dev/mfid0s1b", you should add the following to



> the customized kernel has those information enabled:
> # Debugging for use in -current
> options         KDB                     # Enable kernel debugger support.
> options         DDB                     # Support DDB.
> options         GDB                     # Support remote GDB.
> user at host:~:$ sysctl -a | grep debug.kdb
> debug.kdb.stop_cpus: 1
> debug.kdb.trap_code: 0
> debug.kdb.trap: 0
> debug.kdb.panic: 0
> debug.kdb.enter: 0
> debug.kdb.current: ddb
> debug.kdb.available: ddb
> by the way, where is my gdb? on one of my company's machisne, it looks like
> this:
> debug.kdb.available: ddb gdb
> thank you all
> /gahn
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