This document is a series of errata and clarifications from a previous talk on game development for Linux. It addresses shortcomings in SDL for threaded rendering and audio capabilities, and clarifies that OpenGL context sharing is possible in SDL2. It also recommends using SDL2's joystick and game controller APIs rather than low-level kernel interfaces. Placement of game files on Linux systems and XDG standard directories for configuration and user data are explained. The document concludes by thanking readers and suggesting that an upcoming game may also come to Linux.

1. Linux as a gaming platform
Ideology aside
14-07-2013
Leszek Godlewski
Generalist Programmer
leszek.godlewski@thefarm51.com
www.thefarm51.com
ERRATA

2. 2
I stand corrected!
www.thefarm51.com
On April 19, 2013 I gave a talk on game
development for Linux at the Digital
Dragons European Games Festival. In it, I
made a couple of mistakes and omissions
that I hereby would like to correct.
2

3. 3
What if SDL doesn't cut it?
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Despite its awesomeness, SDL has its
shortcomings
● No explicit GLX/WGL context data sharing
and no direct context access
→ no threaded rendering ☹
(hit this corner while porting Painkiller HD)
● No 3D positioning or DSP support in the
stock SDL audio subsystem
- Partially remedied by SDL_mixer
But we need those! Now what?!
Original slide 24

4. 4
What if SDL doesn't cut it?
www.thefarm51.com
Despite its awesomeness, SDL has its
shortcomings
● No explicit GLX/WGL context data sharing
and no direct context access
→ no threaded rendering ☹
(hit this corner while porting Painkiller HD)
● No 3D positioning or DSP support in the
stock SDL audio subsystem
- Partially remedied by SDL_mixer
But we need those! Now what?!
Original slide 24

5. 5
OpenGL context sharing in SDL2
www.thefarm51.com5
It actually is possible!
●
Just undocumented... ☹ Only mention I could find in
Google is the May 2012 commit that introduces the
feature
● Needs an SDL OpenGL attribute set before window
creation:
SDL_GL_SetAttribute(
SDL_GL_SHARE_WITH_CURRENT_CONTEXT, 1);
● Voila - all contexts created from now on with
SDL_GL_CreateContext() will share data with the
currently bound context (buffers, shaders etc.)!

6. 6
OpenGL context sharing in SDL2
www.thefarm51.com
I re-read the Valve slides about porting
their games to Linux, and noticed their
renderer is multithreaded. I knew from
elsewhere they are using SDL2, so I
thought I should check the source code,
not just the SDL2 documentation.
As soon as I found out about my mistake, I
rewrote the Painkiller Hell & Damnation
client code to use SDL2, too. ☺
6

7. 7
Joystick/gamepad API
www.thefarm51.com
● Not part of X11 input event framework
● Kernel block devices
- Created as /dev/input/js*
- Hotplug events via libudev (also in Steam
Linux Runtime)
- Handled via open() and ioctl()
- Detailed documentation in the kernel
http://kernel.org/doc/Documentation/input/joystick-api.txt
Original slide 28
W
ell, yeah,
W
ell, yeah, but...
but...

8. 8
SDL2 joystick and controller APIs
www.thefarm51.com8
Why hurt yourself with the low-level kernel joystick
interface and libudev?

9. 9
SDL2 joystick and controller APIs
www.thefarm51.com9
Joystick API
● Support for axes, buttons, hats and trackballs
● State polling or events
● Hotplugging
● Force feedback via the haptic SDL2 subsystem

10. 1
0 SDL2 joystick and controller APIs
www.thefarm51.com10
Game controller API
● Built on top of the joystick API
● Maps any joystick/gamepad input to a layout
modelled after the Xbox 360 controller
● Built by Valve with Steam Big Picture in mind
● Bindings may be imported/exported via strings
● Popular controllers have built-in mappings
● API very similar to the regular joystick API, but
buttons and axes are identified by enumerations
● SDL_GameControllerAxis
● SDL_GameControllerButton

11. 1
1 Where do I put the files?
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● Windows
- Game data and binaries: C:Program Files
- User data: %APPDATA%, Documents...
- AoS-like organization
● Linux (conventional)
- Game binaries: /usr/bin/, /usr/games/
- Game data: /usr/share/games/
- User data: $HOME/.config/
($XDG_CONFIG_HOME environment variable)
- SoA-like organization
- Filesystem Hierarchy Standard
Original slide 42
W
ell, yeah,
W
ell, yeah, but...
but...

12. 1
2 Where do I put the files?
www.thefarm51.com
● FHS mainly concerns distro packages
● Proprietary software often installs into
/opt/<package name>/
(“Add-on application software packages”)
● Proprietary software also often installs
“wherever”
● Steam games live in
~/.steam/steam/SteamApps/
● Just put game data and binaries in one
place, and save user data to ~/.config/
Original slide 43
W
ell, yeah,
W
ell, yeah, but...
but...

13. 1
3 $XDG_CONFIG_HOME and friends
www.thefarm51.com13
● Actually, $XDG_CONFIG_HOME (a.k.a. $HOME/.config)
is a for textual configuration files
● User data, which is a more accurate description for
savegames, DLC etc., goes into $XDG_DATA_HOME
(defaults to $HOME/.local/share)
● All of this is regulated by a FreeDesktop.org standard:
http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html

14. 1
4 Summary
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That's all, folks, for now. Sorry for not getting this
errata out earlier – at The Farm 51 we're very busy
finishing Deadfall Adventures. What do you think,
should it be coming to Linux as well? ☺
If you spot any more mistakes, feel free to shoot me an
email.
Many thanks to Michael Larabel at Phoronix for
blowing up my Slideshare with all the traffic from their
news story. ☺
Keep on playing, Linux gamers!

15. 1
5
Thank you!
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leszek.godlewski@thefarm51.com
www.thefarm51.com