How To Get a Meaningful Backtrace
This page will attempt to explain how to get a meaningful debugging backtrace from a reproducible program crash. For this example, we will rebuild & install the "hello" package so that we keep debugging information.
For reference, a "#" at the beginning of a line means that what follows is a command that has to be executed as root. A "$" at the beginning means that what follows is a command that should be executed as your normal user. A "(gdb)" at the beginning means that you should be typing the rest of the command line at the gdb (GNU Debugger) prompt.
Rebuilding the package you're debugging
NOTE: If the package you're trying to debug includes a -dbg package (e.g. for the hello source package, a hello-dbg package exists), you should just be able to install the appropriate -dbg package; skip the rebuilding process in the following instructions and go straight to running gdb.
Install the basic development packages and the build-dependencies for the package we want to rebuild. Note that you can skip the rebuilding part and go straight to running gdb, but it is unlikely that you will get a useful backtrace.
# apt-get install build-essential fakeroot gdb # apt-get build-dep hello
Download & rebuild our package from source, keeping debugging symbols
$ DEB_BUILD_OPTIONS=nostrip,noopt fakeroot apt-get -b source hello
Install our newly built package(s). There may be multiple .deb packages generated, make sure to install the ones you want. In this example, the .deb generated was called hello_2.1.1-4_i386.deb.
# dpkg -i hello_2.1.1-4_i386.deb
Running gdb
Now run your program as follows, replacing "[--args]" with any arguments you want to run the program with:
$ gdb hello ... gdb loads ... (gdb) set pagination 0 (gdb) run [--args] ... hello loads...
- Then try to reproduce your crash. If you're lucky, a crash will occur and you'll be dropped back to the gdb prompt.
- If you are not so lucky to get a crash but instead get a freeze, you can still get gdb prompt by pressing CTRL-C in the terminal running gdb.
At that point, you can run:
(gdb) bt
You'll then get a lot of output, which you can then copy & paste to a bug followup e-mail or other bug reporting tool.
When you're done with gdb, you can just run:
(gdb) quit
If the problem seems to be in a major library such as libc6, xlibs, or libgtk2.0-0, you'll want to install the appropriate -dbg package (e.g. libc6-dbg in the case of libc6) and then run the problematic program again under gdb.
Often, you will see a backtrace where one or more of the top lines is in malloc() or g_malloc(). When this happens, chances are your backtrace isn't very useful. The easiest way to find some useful information is to set the environment variable MALLOC_CHECK_ to a value of 2. You can do this while running gdb by doing this:
$ MALLOC_CHECK_=2 gdb hello
Advanced gdb commands
If the program you're backtracing is multi-threaded, you might want to get a backtrace for all threads:
(gdb) thread apply all bt
Another thing which is quite helpful to report is what parameters came on the stack of function calls:
(gdb) bt full
You might want to report the output of the combination of the preceding options:
(gdb) thread apply all bt full
And if this is too much irrelevant output, you might want to keep only a few calls, such as the top 10:
(gdb) thread apply all bt full 10
If you have a large backtrace, you can log gdb output to a file (the default is gdb.txt):
(gdb) set logging on
Debugging X Errors
If the program has received an X error; i.e. you see a message of the form:
The program 'preview1' received an X Window System error.
then you can try running the program with --sync, and break on the gdk_x_error function in order to obtain a backtrace.