2009-04-28 15:10:20 -04:00
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Debugging i3: How To
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==================
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Michael Stapelberg <michael+i3@stapelberg.de>
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April 2009
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This document describes how to debug i3 suitably for sending us useful bug reports, even
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if you have no clue of C programming.
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First of all: Thank you for being interested in debugging i3. It really means something
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to us to get your bug fixed. If you have any questions about the debugging and/or need
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further help, do not hesitate to contact us!
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== Enabling logging
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i3 spits out much information onto stdout. To have a clearly defined place where logfiles
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2009-04-28 16:25:51 -04:00
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will be saved, you should redirect stdout and stderr in xsession. While you’re at it,
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putting each run of i3 in a separate logfile with date/time in it is a good idea to not
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get confused about the different logfiles later on.
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2009-04-28 15:10:20 -04:00
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2009-04-28 16:25:51 -04:00
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--------------------------------------------------------------------
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exec /usr/bin/i3 >/home/michael/i3/i3log-$(date +'%F-%k-%M-%S') 2>&1
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--------------------------------------------------------------------
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2009-04-28 15:10:20 -04:00
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== Enabling coredumps
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When i3 crashes, often you have the chance of getting a coredump (an image of the memory
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of the i3 process which can be loaded into a debugger). To get a core-dump, you have to
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make sure that the user limit for core dump files is set high enough. Many systems ship
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with a default value which even forbids core dumps completely. To disable the limit
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completely and thus enable coredumps, use the following command (in your .xsession, before
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starting i3):
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-------------------
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ulimit -c unlimited
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-------------------
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Furthermore, to easily recognize core dumps and allow multiple of them, you should set
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a custom core dump filename pattern, using a command like the following:
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---------------------------------------------
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sudo sysctl -w kernel.core_pattern=core.%e.%p
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---------------------------------------------
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This will generate files which have the executable’s file name (%e) and the process id
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(%p) in it. You can save this setting across reboots using +/etc/sysctl.conf+.
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== Compiling with debug symbols
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To actually get useful coredumps, you should make sure that your version of i3 is compiled
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with debug symbols, that is, that they are not stripped during the build process. You
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can check whether your executable contains symbols by issuing the following command:
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----------------
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file $(which i3)
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----------------
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You should get an output like this:
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------------------------------------------------------------------------------
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/usr/bin/i3: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically
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linked (uses shared libs), for GNU/Linux 2.6.18, not stripped
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------------------------------------------------------------------------------
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Notice the +not stripped+, which is the important part. If you have a version which is
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stripped, please have a look if your distribution provides debug symbols (package +i3-wm-dbg+
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on Debian for example) or if you can turn off stripping. If nothing helps, please build
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i3 from source.
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== Generating a backtrace
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Once you have made sure that your i3 is compiled with debug symbols and that coredumps
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are enabled, you can start getting some sense out of the coredumps.
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Because the coredump depends on the original executable (and its debug symbols), please
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do this as soon as you encounter the problem. If you re-compile i3, your coredump might
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be useless afterwards.
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Please install +gdb+, a debugger for C. No worries, you don’t need to learn it now.
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Start gdb using the following command (replacing the actual name of the coredump of
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course):
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----------------------------
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gdb $(which i3) core.i3.3849
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----------------------------
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Then, generate a backtrace using:
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---------
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backtrace
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---------
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Also, getting an overview of the local variables might help:
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-----------
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info locals
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-----------
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If your backtrace looks like this:
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---------------------------------------------------------------------------------------------------
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(gdb) backtrace
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#0 0x041b1a01 in vfprintf () from /lib/libc.so.6
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#1 0x041b2f80 in vprintf () from /lib/libc.so.6
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#2 0x080555de in slog (fmt=0x8059ba0 "%s:%s:%d - Name should change to \"%s\"\n") at src/util.c:60
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#3 0x0804fa73 in handle_windowname_change_legacy (data=0x0, conn=0x42da908,
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state=0 '\0', window=8389918, atom=39, prop=0x4303f90) at src/handlers.c:752
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#4 0x0406cace in ?? () from /usr/lib/libxcb-property.so.1
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#5 0x00000000 in ?? ()
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---------------------------------------------------------------------------------------------------
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you need to find the first frame which actually belongs to i3 code. You can easily spot them, as
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their filename starts with src/ and has a line number. In this case, frame 2 would be the correct
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frame, so before getting the local variables, switch to frame 2:
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-----------
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frame 2
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info locals
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-----------
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== Sending bugreports/debugging on IRC
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When sending bugreports, please paste the relevant part of the log (if in doubt, please send us rather
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too much information than too less) and the whole backtrace (if there was a coredump).
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When debugging with us in IRC, be prepared to use a so called nopaste service such as http://nopaste.info
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because pasting large amounts of text in IRC sometimes leads to incomplete lines (servers have line
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length limitations) or flood kicks.
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