Debugging i3: How To

i3 spits out much information onto stdout, if told so. To have a clearly defined place where log files will be saved, you should redirect stdout and stderr in xsession. While you’re at it, putting each run of i3 in a separate log file with date/time in it is a good idea to not get confused about the different log files later on.

When i3 crashes, often you have the chance of getting a core dump (an image of the memory of the i3 process which can be loaded into a debugger). To get a core dump, you have to make sure that the user limit for core dump files is set high enough. Many systems ship with a default value which even forbids core dumps completely. To disable the limit completely and thus enable core dumps, use the following command (in your .xsession, before starting i3):

Furthermore, to easily recognize core dumps and allow multiple of them, you should set a custom core dump filename pattern, using a command like the following:

This will generate files which have the executable’s file name (%e) and the process id (%p) in it. You can save this setting across reboots using /etc/sysctl.conf.

To actually get useful core dumps, you should make sure that your version of i3 is compiled with debug symbols, that is, that they are not stripped during the build process. You can check whether your executable contains symbols by issuing the following command:

You should get an output like this:

Notice the not stripped, which is the important part. If you have a version which is stripped, please have a look if 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 that core dumps are enabled, you can start making sense out of the core dumps.

Because the core dump depends on the original executable (and its debug symbols), please do this as soon as you encounter the problem. If you re-compile i3, your core dump might be useless afterwards.

Please install gdb, a debugger for C. No worries, you don’t need to learn it now. Start gdb using the following command (replacing the actual name of the core dump of course):

Then, generate a backtrace using:

When sending bug reports, please paste the relevant part of the log (if in doubt, please send us rather too much information than too less) and the whole backtrace (if there was a core dump).

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