Debugging i3: How To

Michael Stapelberg
January 2014

This document describes how to debug i3 to send us useful bug reports, even if you have no knowledge of C programming.

Thank you for being interested in debugging i3. It really means something to us to get your bug fixed. If you have any questions about the process and/or need further help, do not hesitate to contact us!

1. Verify you are using i3 ≥ 4.22

Only the latest major version of i3 is supported. To verify which version you are running, use:

$ i3 --moreversion 2>&- || i3 --version
Binary i3 version:  4.7 (2013-12-22, branch "tags/4.7")
Running i3 version: 4.7-84-gac74a63 (2014-01-01, branch "next") (pid 1995)

Your version can look like this:

4.7 (release version)

You are using a release version. In many cases, bugs are already fixed in the development version of i3. Even if the bug is not a known fixed one, we will still ask you to reproduce your error with the most recent development version of i3. Therefore, please upgrade to a development version if you can.

4.7-85-g9c15b95 (development version)

Your version is 85 commits newer than 4.7, and the git revision of your version is 9c15b95. Go to and see if the most recent commit starts with the same revision. If so, you are using the latest version.

Development versions of i3 have logging enabled by default and are compiled with debug symbols.

2. Enabling logging

If you are using a development version (see previous section), you don’t need to do anything — skip to section 3.

If you are using a release version with a custom ~/.xsession (or xinitrc) file, execute i3 with a line like this:

# Use 25 MiB of RAM for debug logs
exec i3 --shmlog-size=26214400

If you are NOT using an ~/.xsession file but you just chose "i3" from the list of sessions in your desktop manager (gdm, lxdm, …), edit /usr/share/xsessions/i3.desktop and replace the Exec=i3 line with:

Exec=i3 --shmlog-size=26214400

If you cannot restart i3 for some reason, you can enable debug logging on the fly:

i3-msg 'debuglog on; shmlog on; reload'

3. Reproducing the problem

Before submitting an issue, please make sure to close down on the problem as much as you can yourself. Here are some steps you should consider:

  • Find a deterministic, reliable way to reproduce the problem and provide it with your bug report.

  • Try using the default i3 config to reproduce the problem. If the issue does not appear with the default config, gradually adapt it to track down what change(s) to the config introduce the problem.

  • Reproduce the problem with a minimal setup, i.e., only use as few applications, windows and steps as necessary.

  • In addition, try to stick to applications that are common and, even more importantly, free / open source.

  • Before obtaining the log file, restart i3 in-place, execute the steps to reproduce the problem and then save the logs. This keeps the log file as small as possible and necessary.

Please be aware that we cannot support compatibility issues with closed-source software, as digging into compatibility problems without having access to the source code is too time-consuming. Additionally, experience has shown that often, the software in question is responsible for the issue. Please raise an issue with the software in question, not i3.

4. Obtaining the debug logfile


Logs may contain sensitive information, so please inspect the log before submitting it. Logs may be viewed by anyone, once posted. If you choose to redact the log, make an effort not to discard information which may be relevant to the issue you are reporting.

The best way to avoid submitting such information is to only run the necessary steps to reproduce the behavior when saving the log file. This will also make analyzing the log file easier.

No matter whether i3 misbehaved in some way without crashing or whether it just crashed, the logfile provides all information necessary to debug the problem.

To upload a compressed version of the logfile (for a bugreport), use:

DISPLAY=:0 i3-dump-log | bzip2 -c | curl --data-binary @-

This command does not depend on i3 (it also works while i3 displays the crash dialog), but it requires a working X11 connection.

After running it, you will get a URL to the logfile. Please include that URL in your bug report.

5. On crashes: Obtaining a backtrace

When i3 crashes, it will display a dialog stating “i3 just crashed”, offering you to save a backtrace to a text file.

To actually get useful backtraces, you should make sure that your version of i3 is compiled with debug symbols:

$ file `which i3`
/usr/bin/i3: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically
linked (uses shared libs), for GNU/Linux 2.6.18, not stripped

Notice the not stripped, which is the important part. If you have a version which is stripped, please check whether your distribution provides debug symbols (package i3-wm-dbg on Debian for example) or if you can turn off stripping. If nothing helps, please build i3 from source.

Once you have made sure that your i3 is compiled with debug symbols and the C debugger gdb is installed on your machine, you can let i3 generate a backtrace in the crash dialog.

After pressing "b" in the crash dialog, you will get a file called /tmp/i3-backtrace.%d.%d.txt where the first %d is replaced by i3’s process id (PID) and the second one is incremented each time you generate a backtrace, starting at 0.

6. Sending bug reports/debugging on IRC

When sending bug reports, please attach the whole log file. Even if you think you found the section which clearly highlights the problem, additional information might be necessary to completely diagnose the problem.

When debugging with us in IRC, be prepared to use a so-called nopaste service such as because pasting large amounts of text in IRC sometimes leads to incomplete lines (servers have line length limitations) or flood kicks.

7. Debugging i3bar

To debug i3bar problems, use the --verbose commandline parameter, or add verbose yes to all bar {} blocks in your i3 config, reload your config and then restart all i3bar instances like this:

$ i3 reload
$ killall i3bar
$ for c in $(i3-msg -t get_bar_config | python -c \
      'import json,sys;print("\n".join(json.load(sys.stdin)))'); do \
    (i3bar --bar_id=$c >i3bar.$c.log 2>&1) & \

There will now be i3bar.*.log files in your current directory that you can provide in your bug report.