Re: how to debug a program when the session freezes

[Rod Butcher <rbutcher hyenainternet com>]

Elijah Newren wrote:
> On 1/8/06, Rod Butcher <rbutcher hyenainternet com> wrote:
>
> > Can somebody tell me how to get a gdb trace when the app I'm debugging
> > freezes my X session ? I'm running gnome-system-monitor from a
> > terminal : gdb gnome-system-monitor which freezes up the X session when
> > I click on the top menu line. The only way I can get out is by by
> > al-ctl-f1 and killing the X session from the linux command line. How do
> > I send the gdb output to a file for later retrieval, and is it possible
> > to get the stack trace some other way, without X ?
>
>
> Yes, you can get a stack trace of an X application outside of X.  On
> occasion I have to do the same for debugging something in a Metacity
> patch I'm writing.  After (re)starting metacity, I switch to a virtual
> terminal and do a 'ps -ef | grep metacity | grep -v grep', take the
> PID number (the value in the second column), and then run 'gdb
> ./src/metacity <PID>'.  As soon as I get the gdb prompt, I type "cont"
> to let metacity continue running, switch back to X, do whatever is
> necessary to cause it to crash, and then switch back to the virtual
> terminal to inspect the stack trace.
Many thanks Elijah and Andrew, I've got my stack trace comining out 
nicely on the virtual terminal. Now how do I save the output from the 
virtual terminal (say the last 500 lines) to a file ? I've always used 
my mouse to cut and paste before under X, but no mouse from the command 
line.
thanks
Rod
> If all I want is a stack trace, though, and I'm on a machine with
> loads of memory (and slow execution of the program isn't an issue)
> then sometimes it's easier to use valgrind as it detects segfaults
> (along with tons of other problems) and adds a stack trace to the log
> file automatically in those cases.
>
> If the program you are using isn't crashing but just locking up or
> using lots of CPU in some kind of infinite loop, then you have to do
> something slightly different.  valgrind isn't much use there.  You can
> still use the gdb instructions above (just switch to the virtual
> terminal when it seems to be in its deadlock state and hit ctrl-c,
> then get your backtrace).  However, for these kinds of things, it's
> often a lot better to run a profiler (e.g. oprofile, sysprof) and get
> the output from that.
>
>
> Hope that helps,
> Elijah

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