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To: hackers%freebsd.org@inet-gw-1.pa.dec.com
Subject: Re: How do I set a kernel variable? 
Reply-To: gj@FreeBSD.org
Date: Sun, 23 Apr 95 18:40:06 GMT
From: "gj%pcs.dec.com@inet-gw-1.pa.dec.com" <garyj@rks32.pcs.dec.com>
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It turns out that it _is_ possible to set a variable, but there's a trick
to it.

Normally, gdb opens the core-file (e.g. /dev/mem) read-only. In order to get
it to open the core-file for writing, you need to do the following:

root:peedub:bash:69> gdb -k /kernel <--- this...
GDB is free software and you are welcome to distribute copies of it
 under certain conditions; type "show copying" to see the conditions.
There is absolutely no warranty for GDB; type "show warranty" for details.
GDB 4.13 (i386-unknown-freebsd), 
Copyright 1994 Free Software Foundation, Inc...
(kgdb) set write <--- and this...
(kgdb) core-file /dev/mem <--- and this
IdlePTD 19a000
current pcb at 624000
#0  0xf010dcf3 in mi_switch ()
(kgdb) p/x ttydebug
$1 = 0x0
(kgdb) set ttydebug=1
(kgdb) p/x ttydebug
$2 = 0x1
(kgdb) set ttydebug=0
(kgdb) p/x ttydebug
$3 = 0x0
(kgdb) q

this opens the core-file for writing. It has to be done in this order because
the "set write" command sets a variable (write_files) in exec.c which is tested
in kcorelow.c when the core-file is opened.

If you try to just do the following:

gdb -k
set write
exec-file /kernel

you'll get an error, because gdb tries to open /kernel for writing, which
(for some reason) fails miserably.

Maybe we should have a "-w" flag to specify opening the core-file for
writing ? Any opinions ?

Could you add this tip to the kernel-debug.FAQ, J"org ?

Gary J.