why gdb shows ?? in bt

meenakshi.khurana · 01-03-2012, 05:33 AM

While debugging issues, gdb sometimes do not show full stack trace causing difficulty in debugging issues. Here is one sample example:

Code:

(gdb) bt 
#0  0x08058ec1 in encode_dlg (dlg_ref_id=6777, encode_dlg=0x1610040) at encode_struct.c:489
#1  0x0805afd3 in log_dlg (dlg_ref_id=0) at log_dlg.c:8
#2  0x00000000 in ?? ()

Can anyone help me with this?? Why frame 2 is not showing me the caller of log_dlg function.

What's the reason gdb shows only ?? in bt in some cases. Whats missing in this case.

NevemTeve · 01-03-2012, 10:39 PM

The stack might be corrupted. Example:

Code:

void bar (int *p)
{
    memset (p, 0, 100*sizeof (int));
}

void (foo (void)
{
    int a[10];

    bar (a);
}

meenakshi.khurana · 01-03-2012, 11:04 PM

Then how can i debug the code with corrupted stack?

NevemTeve · 01-04-2012, 04:02 AM

Set a breakpoint at log_dlg.c:8, and backtrace (bt) when the program reaches that point.

meenakshi.khurana · 01-04-2012, 04:46 AM

Sorry... I forgot to mention... The problem occurs once in a while but not for every request. Say once in months. I want to understand what goes wrong at that time.

I just have core with me now. Can that help me?

NevemTeve · 01-04-2012, 06:05 AM

Debugging isn't an exact science, it's more like an art; it requires knowledge, experience, and luck... For first try 'bt full' and examine every shown variable carefully.

meenakshi.khurana · 01-05-2012, 12:02 AM

I know that.... and i have already tried all options like bt, frames and all. Now i am moving towards analyzing registers... I just thought if anybody can help me with some useful link or doc for further information on that.

Anyways Thanks for your help.

aruna.km · 02-27-2022, 08:48 AM

This happens for me also. seg fault happens only at sometime not all time, also gdb trace at ??
Any information would be helpful

TB0ne · 02-27-2022, 09:57 AM

Quote:

Originally Posted by aruna.km

This happens for me also. seg fault happens only at sometime not all time, also gdb trace at ?? Any information would be helpful

Then you probably should open your own question, since this thread had been closed for *TEN YEARS* before you reopened it. Also, you're not providing any details at all, such as version/distro of Linux, desktop GUI you're using, etc.

NevemTeve · 02-27-2022, 11:46 AM

@aruna.km
As a start, run your program with `valgrind`, and fix every problem it shows.

Also make sure to compile with options `-Wall -W -pedantic -Werror`. If your compiler is gcc, add option `-fanalyzer`.

There are other programs that might help finding problems, they are called `static analyzers` or `linters`.

astrogeek · 02-27-2022, 01:27 PM

Quote:

Originally Posted by aruna.km

This happens for me also. seg fault happens only at sometime not all time, also gdb trace at ??
Any information would be helpful

You have posted to a ten year old thread so your post may not attract current participants able to offer help.

Also, though your problem may seem similar when looking at a single error message, your code is different and others cannot offer any specific advice without knowing what the code is doing, or is supposed to be doing.

Please consider posting your own thread with enough detail so that others may understand your problem and offer more specific suggestions to fix it.

Please review the Site FAQ for guidance in asking well formed questions. Especially visit the link from that page, How to Ask Questions the Smart Way for discussion of things to consider when asking others for help.

Welcome to the LQ Programming forum, and good luck!