Re: Need help with a backtrace

[Fernando Nasser <fnasser@redhat.com>]

Matt Schalit wrote:
> 
> Ok.  It looks like it only lists an entry when a function
> calls another function, like main() calling printf()?
> But it doesn't seem to list things like loops or if/case tests
> or variable values?
> 

Yes, because all information it has are the stack frames, which are
created when a function is called.

When you want to debug something in between, you set a breakpoint in a
line of source code and tell your program to run (or continue if you had
stopped somewhere else before). Of course, all this only work before you
program crashes.


> I know how to write C programs.  How would I backtrace or
> step my way through hello-world.c if it exits normally, and
> I just want to see how the trace looks?
> 

Insert a breakpoint at that last function (look at the source file and
line number in the bad backtrace).  Run your program.  When it stops at
the breakpoint use the backtrace command.


> I say that as a possibility, because I read a Solairis Gnome
> Install Howto that mentioned those were needed, and I've noticed,
> when doing a truss on some of these programs that I get an extra '/'
> showing up in paths to applications that are being called, as in:
> 
> xstat(2, "./gnome/config-override//Gnome", 0x08047614) Err#2  ENOENT
> xstat(2, "/usr/local/etc/gnome/config//Gnome", 0x08047614) Err#2  ENOENT
> xstat(2, "./gnome/config//Gnome", 0x08047614)   Err#2  ENOENT
> xstat(2, "/home/matthew/.gnome//Gnome", 0x08047674) = 0
> open("/home/matthew/.gnome//Gnome", O_RDONLY, 0666) = 6
> 
> I guess the last two lines do show a file being found
> using the odd path.
> 

Yes, AFAIK the extra "/" won't hurt.


> Is up and down how I see what variables get set to which values
> while moving through a function?
> 

You only see what the values of these variables where when the next
function was called.  It is a "snapshot".


> And finally, where does
> 
> > > #4  0xbfd95397 in create_label
>         ^^^^^^^^^^
> 

This is the instruction pointer (PC).  That is where the CPU was
executing instructions when another function was called or your program
stopped/crashed.


> that value come from, or what man page should I look at
> for the decoding formula?
> 

With the GDB sources, there is a nice user manual.  It is in the sources
under gdb/doc.   It is in a format called "texinfo" which can produce
several different formats like Postcript for instance.  You just have to
use "make" with the appropriate type on that directory.

I am sure that if you do a Web search you'll find a HTML on line
version.


W.r.t. your original problem, maybe if you post it to the Gnome list
people will recognize that as a known problem or something.  It worth a
try.

Cheers,
Fernando


-- 
Fernando Nasser
Red Hat Canada Ltd.                     E-Mail:  fnasser@redhat.com
2323 Yonge Street, Suite #300
Toronto, Ontario   M4P 2C9