StackTraces

A stack trace is one of the most important pieces of information you can provide to help others debug an application crash. This page details the importance of stack traces and outlines several methods for obtaining stack traces.

If you are experiencing a crash, the basic steps to generating a useful stack trace for common Gnome desktop applications are:

• Install the appropriate -debuginfo RPM(s) before the crash (see instructions below).
• Wait for the crash to happen or perform whatever steps reproduce it.
• ABRT (Automatic Bug Reporting Tool) in Fedora will automatically detect the crash and obtain a stack trace.
• Include the stack trace in your bug report (see Bugs and feature requests for full instructions).

If ABRT does not start automatically, you will need to start the program in a special way, using a debugger (such as gdb). See gdb instructions below.

Special instructions apply for Java programs and Firefox.

Starting with Fedora 12, ABRT is replacing Bug Buddy. See abrt for more info.

What is a stack trace (backtrace)?

A stack trace (sometimes also called a backtrace) is a list of function calls that leads to some point in the program. Debugging tools like gdb or bug-buddy can get stack traces from crashed applications so that developers can figure out what went wrong.

What does a stack trace look like?

A typical stack trace looks similar to the following:

[New Thread 8192 (LWP 15167)] 

0x420ae169 in wait4 () from /lib/i686/libc.so.6
.
.
.

A better backtrace, with debuginfo symbols (see below) looks like that:

0x000000350a6c577f in *__GI___poll (fds=0xe27460, nfds=9, timeout=-1) at ../sysdeps/unix/sysv/linux/poll.c:83
83          return INLINE_SYSCALL (poll, 3, CHECK_N (fds, nfds), nfds, timeout);

.
.
.

Notice the filenames and line numbers of where the functions are called appearing.

What are debugging symbols, and why are they important?

When a program is compiled with special switches to generate debugging symbols (the -g compiler switch) extra information is stored in the program file. This information can be used to generate a stack trace that contains much more information, such as the exact line number of the source file where things went wrong. Without this information it is very hard to figure out what went wrong by looking at the stack trace.

What are debuginfo rpms, and how do I get them?

Fedora includes a special type of rpms called debuginfo rpms. These automatically created rpms contains the debugging information from the program files, but moved into an external file. All the tools that handle debugging information knows how to automatically look in these files. This lets you easily install debugging information when you need it. You must install the exact matching version and architecture of debuginfo as the application you are trying to debug.

Example: Verifying Matching Version and Arch

[warren@computer ~] $ rpm -q --qf '%{name}-%{version}-%{release}.%{arch}\n' gaim gaim-debuginfo
gaim-2.0.0-0.26.beta5.i386
gaim-debuginfo-2.0.0-0.26.beta5.i386

Each package with binaries in the distribution has a corresponding debuginfo package.

Installing debuginfo RPMs using yum

debuginfo-install is a handy utility, part of yum-utils package that automatically enable the debuginfo repositories and download all the debuginfo packages needed. You can do:

$ debuginfo-install foo

to install all the debuginfo packages needed for package foo.

To install only the minimal set of debuginfo packages use the output of debuginfo-install (without actually installing anything) to get the names of debuginfo packages and their respective repositories. Then construct the install command according to the following example:

$ yum --enablerepo fedora-debuginfo,updates-debuginfo install foo-debuginfo

This is useful when you don't want to install debuginfo for all the dependencies of the debugged package as their debuginfo is often not required.

Installing debuginfo RPMs manually

These packages can be downloaded from the normal fedora mirrors in the "debug" subdirectory of the architecture directory. For example, the debuginfo packages for the latest development version is available from the official fedora ftp at ftp://download.fedora.redhat.com/pub/fedora/linux/core/development/i386/debug/ . Please use the mirror closest to you when downloading.

Packagers

If you are a packager looking information about debuginfo rpms, see Packaging/Debuginfo .

How do I generate a backtrace?

First make sure that you have installed the debuginfo packages for the application you are debugging and all related libraries. A developer often tells you to install specific debuginfo packages because he can tell from a stack trace which libraries are involved in the crash. See below for recommended packages for some common type of applications.

There are several ways to get a stack trace:

• Obtaining a stack trace using just GDB.
• Obtaining a stack trace from a core dump with GDB.
• Obtaining a stack trace from a application using Automatic Bug Reporting Tool.

What debuginfo packages should I install?

At the very least you will need to install the debuginfo package for the application that is crashing. You can find out what package this application is in by typing rpm -qf path-of-program.

For certain types of programs it is also very useful to install a couple of default packages that are usefull for almost all stack traces:

• Gnome applications and applications using Gtk+: glib2-debuginfo, pango-debuginfo, gtk2-debuginfo
• KDE applications: qt-debuginfo, kdelibs-debuginfo

Obtaining a stack trace using just GDB

NOTE: If you are running a Java program such as Eclipse or Tomcat, the situation is a bit more complicated - see JavaStackTraces for details.

First, run the following command to start gdb:

gdb name-of-program

Where name-of-program is the name of the program that crashed (for example: /usr/bin/gnome-panel).

Then, at the gdb prompt, type:

run

If you need to give any arguments to the program, give them after the run command, like:

run --argument

Once the program is running, reproduce the crash and go back to the terminal where you ran gdb. The gdb prompt should be shown - if not, hit Control+C to break into the debugger. At the gdb debugger prompt, type:

thread apply all bt full

If that does not work (meaning that you don't get any output--this may be the case in programs which are not multi-threaded), type bt instead. If you still do not have any output, read this note about obtaining a stack trace under special circumstances. The output is the stack trace. Cut and paste all of it into a text file.

You can quit gdb by typing quit.

Obtaining a stack trace from a core dump

If the program that crashes leaves a core dump, you can use GDB to obtain a stack trace. Core dumps are saved in a file on your hard disk and are usually named something along the lines of "core" or "core.3124". To obtain a stack trace from one of these files, run the following command:

gdb name-of-program core-file-name

Where name-of-program is the name of the program that crashed (for example: /usr/bin/gnome-panel), and core-file-name is the name of the core file that contains the core dump (for example: core.7812).

Then, at the gdb prompt, type:

thread apply all bt full

If that does not work (meaning that you don't get any output--this may be the case in programs which are not multi-threaded), type bt instead. If you still do not have any output, read this note about obtaining a stack trace under special circumstances. The output is the stack trace. Cut and paste all of it into a text file.

You can quit gdb by typing quit.

Note that creation of core files is disabled in Fedora Core by default (in /etc/profile). To enable them for a shell session, type at the shell prompt:

ulimit -c unlimited

How to install ABRT

If you installed Fedora via a LiveCD image, ABRT should already be installed. You should be able to start it in Applications -> System Tools -> Automatic Bug Reporting Tool. If ABRT is not installed, for whatever reason, you can install it manually by doing the following on a command line:

 su -c 'yum install abrt'

or go to System -> Administration' -> Add/Remove Software in Gnome, and type abrt in the search box and select Find. Select the abrt package and apply the changes.

Configuring ABRT for Bugzilla

Go to Application -> System Tools -> Automated Bug Reporting Tool and select it to start it manually. Once the gui window appears, choose Edit -> Plugins and from the Settings window, scroll down, highlight Bugzilla and choose "Configure Plugin". The Bugzilla URL should be https://bugzilla.redhat.com and enter your own Bugzilla login and password in the proper boxes. (NOTE: if you do not yet have a Bugzilla account, now is the time to get one, just go to the URL as displayed on that page and create a new account.) https://bugzilla.redhat.com/createaccount.cgi

Next time you have a program crash, and ABRT triggers, when you hit Report, ABRT will be able to automatically log in to Bugzilla and submit a Bug Report for you.

Using ABRT

(The following assumes Gnome as the desktop ... someone else will have to update for KDE/Others)

If ABRT detects a crashed program, you will get an ABRT alert. This will be visually indicated by a flashing red light in the system tray. Left click on the alert light, and the Automatic Bug Reporting Tool should start, displaying all of the crashes it has registered. To report the bug, right click on it and choose report. ABRT will gather the logs it needs to submit with the bug and will then let you know it is going to submit a bug on your behalf. If you have configured ABRT as in the previous section, it will then ask you to verify whether or not to include the various logs, then will automatically go out to Bugzilla and open a bug, attaching the logs into the bug. It will then show you the bug number so that you can track the bug as it is worked on.

Configuring ABRT when missing Debuginfos

When you right click on a bug in ABRT and choose Report, ABRT will attempt to go out and retrieve the logs it will need to send as part of the bug report. The developers have added code to detect whether or not symbolic traces are included within the backtrace, and if it detects that there are none, ABRT will alert you of this, and will show you the command to run. This is the same as what is shown in the debuginfo section.

Programs running as another user

If you do not get any output from gdb after typing thread apply all bt or bt, it may be because the program is run as root or as another user. In GNOME for example, this is the case when running gnome-games. In such cases, you will need to be root in order to capture a trace. So, quit gdb, login as root, and then repeat the steps to obtain a stack trace.

Firefox

• Install Firefox and Xulrunner debug info packages - run "debuginfo-install firefox xulrunner" as root on the command line.
• Run "firefox -g" on the command line. That will start firefox running inside of gdb debugger.
• In gdb, issue the command "run -safe-mode". A dialog window will pop up, disable all add-ons here and run firefox in safe mode.
• Do whatever is necessary to make firefox crash and follow the instructions above for gdb usage.

Thunderbird

It's almost the same as for Firefox, just the debug info packages are different. Install them by "debuginfo-install thunderbird" command as root on the console.

Java programs

See JavaStackTraces for info on getting stack traces from programs running in Java.

Daemons and their spawn

You will need to gather the backtrace from the core file.

Make sure your daemon's initscript isn't forbidding dumping core files to the disk. Add the line DAEMON_COREFILE_LIMIT=unlimited to its configuration file in /etc/sysconfig. For example, the Bluetooth daemon (hcid) uses /etc/sysconfig/bluetooth.

Then setup the kernel so that the core dump is written to a known location such as /tmp/. As root, run:

echo /tmp/core > /proc/sys/kernel/core_pattern

To make this change permanent, add that line to /etc/sysctl.conf:

kernel.core_pattern = /tmp/core

And run sysctl -p to apply it straight away.

A full list of possible patterns for the core file are available at in the sysctl/kernel.txt kernel Documentation .

Finally, after reproducing your problem, you can double-check which binary created the core file with file /tmp/core.1234. Then run gdb on the file to create a post-mortem stack trace:

gdb /path/to/binary/file /tmp/core.1234

and follow the instructions above for gdb usage.

Note: you can test whether dumping a core file would work by running kill -SEGV 1234 where 1234 is the PID of the program you're testing.

Other tools

Valgrind

The brilliant tool valgrind is often able to say more about what is going wrong; it can give a strack trace to the point where things start to go wrong, which might be long time before the program actually crashes. Programs run through valgrind will run an order of magnitude slower and use more memory, but it will be buddyamazingly usable.

With valgrind installed ("yum install valgrind") you can use it on a program:

valgrind name-of-program program-arguments

With debuginfo installed stacktraces will use symbolic names. See valgrind.org for more info and tips and tricks.

strace

strace can track all system calls made by a program, which can also be helpful in debugging, though it cannot produce stack traces. Install with "yum install strace", and see "man strace" for details.

References

• Much of the text from this page comes from the excellent GNOME bugsquad on getting stack traces.
• http://linux.bytesex.org/gdb.html