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debug-symbols.md

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Using debug symbols in the Steam Runtime

If your game runs in the LD_LIBRARY_PATH-based Steam Runtime environment, it is likely to be loading a mixture of libraries from the host system and libraries from the Steam Runtime. In this situation, debugging with tools like gdb benefits from having debug symbols.

Like typical Linux operating system library stacks, the Steam Runtime libraries do not contain debug symbols, to keep their size small; however, they were compiled with debug symbols included, so we can make their corresponding detached debug symbols available for download.

The steps to attach a debugger to a game apply can in fact apply equally to the Steam client itself.

Example scenario

Suppose you are using a SteamOS 2 'brewmaster' host system, and you are having difficulty with the PulseAudio libraries, similar to steam-for-linux#4753.

  • Use a SteamOS 2 'brewmaster' host system
  • Ensure that your Steam client is up to date
  • Do not have libpulse0:i386 or libopenal1:i386 installed, so that the 32-bit libpulse.so.0 and libopenal.so.1 from the Steam Runtime will be used
  • Run the Steam client in "desktop mode", from a terminal
  • Put the Steam client in Big Picture mode, which makes it initialize PulseAudio
  • Run a 32-bit game like Floating Point
  • Alt-tab to a terminal
  • Locate the main Steam process with pgrep steam | xargs ps, or locate the main Floating Point process with pgrep Float | xargs ps. Let's say the process you are interested in is process 12345.
  • Run a command like gdb ~/.steam/root/ubuntu12_32/steam 12345 (for the Steam client) or gdb ~/.steam/steam/steamapps/common/"Floating Point"/"Floating Point.x86" 12345 (for the game).
  • In gdb: set pagination off
  • In gdb: thread apply all bt to see a backtrace of each thread.
  • At the time of writing, the Steam client has two threads that are calling pa_mainloop_run(), while Floating Point has one such thread. Because you don't have debug symbols for libpulse.so.0, these backtraces are quite vague, with no information about the source code file/line or about the function arguments.
  • Exit from gdb so that the Steam client or the game can continue to run.

Getting the debug symbols for the host system

This is the same as it would be without Steam. For a Debian, Ubuntu or SteamOS 2 host, apt install libc6-dbg:i386 is a good start. For non-Debian-derived OSs, use whatever is the OS's usual mechanism to get detached debug symbols.

More OS-specific information:

Getting the debug symbols for the Steam Runtime

Look in ~/.steam/root/ubuntu12_32/steam-runtime/version.txt to see which Steam Runtime you have. These instructions assume you are using at least version 0.20190716.1. At the time of writing, the public stable release is version 0.20191024.0.

Look in https://repo.steampowered.com/steamrt-images-scout/snapshots/ for a corresponding version of the Steam Runtime container builds.

Download com.valvesoftware.SteamRuntime.Sdk-amd64,i386-scout-debug.tar.gz from the matching build. Create a directory, for example /tmp/scout-dbgsym-0.20191024.0, and untar the debug symbols tarball into that directory.

The /tmp/scout-dbgsym-0.20191024.0/files directory is actually the /usr/lib/debug from the SDK container, and has most of the debug symbols that you will need.

Re-running gdb

Run gdb the same as you did before, but this time use the -iex option to tell it to set the new debug symbols directory before loading the executable, for example:

gdb -iex \
'set debug-file-directory /tmp/scout-debug-0.20191024.0/files:/usr/lib/debug' \
~/.steam/root/ubuntu12_32/steam 12345

You will get some warnings about CRC mismatches, because gdb can now see two versions of the debug symbols for some libraries. Those warnings can safely be ignored: gdb does the right thing.

Example scenario revisited

  • Do the setup above
  • Run a command like gdb ~/.steam/root/ubuntu12_32/steam 12345 (for the Steam client) or gdb ~/.steam/steam/steamapps/common/"Floating Point"/"Floating Point.x86" 12345 (for the game).
  • In gdb: set pagination off
  • In gdb: thread apply all bt to see a backtrace of each thread.
  • At the time of writing, the Steam client has two threads that are calling pa_mainloop_run(), while Floating Point has one such thread. Now that you have debug symbols for libpulse.so.0, these backtraces are more specific, with the source file, line number and function arguments for calls into libpulse.so.0, and details of functions that are internal to libpulse.so.0.
  • Similarly, for start_thread() in libc.so.6 (which came from the host system), you should see file and line information from libc6-dbg:i386.
  • If you use info locals or thread apply all bt full, you'll also see that you can even get information about local variables.