= Providing a Stack Trace 
Michael Schwendt; Rahul Sundaram; Ankur Sinha
:revnumber: unspecified
:revdate: 2014
:category: Troubleshooting
:tags: How-to, Bugzilla, Stack Trace

//:page-aliases: debug-wayland-problems.adoc


[IMPORTANT]
====
This page was taken from the previous Fedora Wiki documentation.

It has been cleaned up for publishing here on the Fedora Docs Portal, but it has not yet been reviewed for technical accuracy. 

It is probably

* Containing formatting issues
* Out-of-date
* In need of other love

Reviews for technical accuracy are greatly appreciated. If you want to help, see the https://pagure.io/fedora-docs/quick-docs/blob/master/f/README.md[README file] in the source repository for instructions.

Pull requests accepted at https://pagure.io/fedora-docs/quick-docs

Once you've fixed this page, remove this notice.
====

[abstract]
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 section "<<debuginfo>>" 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 document xref:bugzilla-file-a-bug.adoc["How To File a Bug"] 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 section <<gdb>> below.

Special instructions apply for <<JavaStackTraces,Java programs>> and <<firefox>>.


== 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:

[source,]
----
[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:

[source,]
----
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.

[id=debuginfo]
== What are debuginfo RPMs, and how do I get them?

Fedora includes a special type of RPMs called debuginfo RPMs. These automatically created RPMs contain the debugging information from the program files, but moved into an external file. All the tools that handle debugging information know 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

[source,]
----
[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 DNF 

debuginfo-install is a handy plugin, part of `dnf-plugins-core` package, that automatically enable the debuginfo repositories and download all the debuginfo packages needed. You can do:

[source,]
$ dnf debuginfo-install <pkg-spec>

to install all the debuginfo packages needed for package `<pkg-spec>`.

To install only the minimal set of debuginfo packages use the package listing of the command (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:

[source,]
$ dnf --enablerepo=fedora-debuginfo --enablerepo=updates-debuginfo install <pkg-spec>-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 using yum 

The `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:

[source,]
$ 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:

[source,]
$ 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 https://mirrors.fedoraproject.org/publiclist/Fedora/development/[ http://mirrors.fedoraproject.org/publiclist/Fedora/development/] . Please use the mirror closest to you when downloading.

=== Packagers 
If you are a packager looking for information about debuginfo RPMs, see document xref:packaging-guidelines:Debuginfo.adoc[Debuginfo packages].

//[[Packaging/Debuginfo]]
// Wiki article:
// https://fedoraproject.org/wiki/Packaging:Debuginfo 
// refers to Docs article
// https://docs.fedoraproject.org/en-US/packaging-guidelines/Debuginfo/
// ToDo: check if Wiki can be redirected to Docs article:
// {{#fedoradocs: https://docs.fedoraproject.org/en-US/packaging-guidelines/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 <<gdb>>.
* Obtaining a stack trace from a <<coredump,core dump with GDB>>.
* Obtaining a stack trace from an application using <<abrt,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 useful for almost all stack traces:

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


[id=gdb]
== 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:

[source,]
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:

[source,]
run


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

[source,]
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:

[source,]
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 <code>bt</code> instead. If you still do not have any output, read [[#special|  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`.

Sometimes, the trace can be quite large and it is difficult to copy and paste it. In such situations, storing the trace to a file is convenient:

[source,]
----
gdb
> run
# program crashes
> set logging file backtrace.log
> set logging on
> thread apply all bt full
> set logging off
> quit
----

[id=coredump]
== 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:

[source,]
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:

[source,]
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 <<special,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 by default (in /etc/profile).  To enable them for a shell session, type at the shell prompt:

[source,]
ulimit -c unlimited



[id=abrt]
== 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:

[source,]
$ su -c 'dnf 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[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 https://bugzilla.redhat.com/createaccount.cgi[create a new account].
====

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,debuginfo>> section.

[id=special]
== Special cases

== 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.

[id=firefox]
=== Firefox 

* Install Firefox and Xulrunner debug info packages - run `ebuginfo-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, you should see gdb prompt `(gdb)`. Issue the command `run -safe-mode`. A dialog window will pop up, disable all add-ons here and continue in safe mode.
* Do whatever is necessary to make firefox crash and follow the instructions above for gdb usage.
* When Firefox crashes, <<gdb,obtain the backtrace>> and attach it to bugzilla.

For additional info see xref:troubleshooting-mozilla-products.adoc[Debugging guidelines for Mozilla products].

=== 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.

For additional info see xref:troubleshooting-mozilla-products.adoc[Debugging guidelines for Mozilla products].

[id=JavaStackTraces]
=== Java programs

See document xref:troubleshooting-java-programs[Troubleshooting Java Programs] 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:

[source,]
echo /tmp/core > /proc/sys/kernel/core_pattern

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

[source,]
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 https://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=Documentation/sysctl/kernel.txt;hb=HEAD[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:

[source,]
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 https://valgrind.org/[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 (`dnf install valgrind) you can use it on a program:

[source,]
valgrind name-of-program program-arguments

With debuginfo installed stacktraces will use symbolic names. See [http://valgrind.org/ 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 `dnf install strace`, and see `man strace` for details.

The situation is a bit improved. Now stack trace feature is implemented (`strace -k`). Though it is disabled when building because the implementation is not stable on i386.

== References ==
* Much of the text from this page comes from the excellent link:https://live.gnome.org/GettingTraces[GNOME bugsquad on getting stack traces].
* link:https://linux.bytesex.org/gdb.html[]

