Linux commonly connotes with open-source zealots and a small PC market share, not blockbuster video games. However, the arrival of Steam on the platform might change the outlook quite dramatically, and Linux support may soon become a must-have feature for your game. Setting the open-source ideology aside, this lecture is an overview of the technical challenges a game developer may face while porting their game to this platform, along with solutions.

1. Linux as a gaming platform
Ideology aside
Digital Dragons · 19-04-2013
Leszek Godlewski
Programmer
leszek.godlewski@thefarm51.com
www.thefarm51.com

2. 2
What is this talk about?
www.thefarm51.com
It's NOT about:
● Free software
● Ideology
● Business
It is about:
● Linux 101
● Development
● Deployment
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3. 3
Who is this guy?
www.thefarm51.com
Cross-platform work
(→ de facto – Linux)
● github.com/inequation
● Toy projects, personal utilities,
university stuff
● Games as well!
● AC-130
● Crystal Space
● idTech 3-based games
● Painkiller Hell & Damnation
● Linux as preferred OS for 7+ years
● Former open-source evangelist
● Not anymore – lost interest
3

4. 4
Who is this guy?
www.thefarm51.com4
WHAT IF I TOLD YOU
WE PREACHED FREE SOFTWARE

5. 5
Who is this guy?
www.thefarm51.com5

6. 6
www.thefarm51.com
Linux 101
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OMG LINUX!!11
www.thefarm51.com7

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OMG LINUX!!11
www.thefarm51.com
Richard M. Stallman
Source: http://commons.wikimedia.org/wiki/File:RMS_iGNUcius_techfest_iitb.JPG
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9. 9
OMG LINUX!!11
www.thefarm51.com
“F**k you, NVIDIA!” - Linus Torvals
Source: http://www.youtube.com/watch?v=iYWzMvlj2RQ
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Kernel and the distros
www.thefarm51.com10
Linux distro
(Ubuntu, Debian, Fedora...)
Linux distro
(Ubuntu, Debian, Fedora...)
Linux
kernel
Linux
kernel
Free
software
Free
software
Proprietary
software
Proprietary
software
27+ supported CPU architectures!
http://en.wikipedia.org/wiki/List_of_Linux_supported_architectures
We usually mean i386/amd64

11. 11
Who and why uses Linux?
www.thefarm51.com11
Who:
● Scientists (TOP500)
● IT companies
● Hackers and
power users
Why:
● It's free (well, duh...)
● It's robust
● It's customizable
● Long, intrinsic traditions of free software
● No established channels of proprietary
software distribution
● But Steam can become a gamechanger!

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Standards
www.thefarm51.com
● Linux adheres to ISO/IEC standards, such
as POSIX and Single Unix Specification...
● System API – file system access, threading,
access control, IPC etc.
● Command-line shell utilities
● ...which also happen to be supported by
MacOS X, Android, iOS (yes, really!) and
a handful of other OSes
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Standards
www.thefarm51.com
● Linux has OpenGL (ES) for 3D graphics
rendering
● Linux has OpenAL for 3D audio with DSP
● Linux has BSD sockets for networking
● Linux has lots of other portable libraries
● And all of these things are also available
on other platforms – Android, MacOS X,
iOS, Windows!
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What does that mean?
www.thefarm51.com14
MacOS X
Linux Mobile
*
* not to scale; actual
overlap is greater

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There are issues, too
www.thefarm51.com
● Open source legacy → unstable ABIs
● Why need a stable one when you can just
recompile the source?
● This is why NVIDIA distributes glue code with
their drivers
● Extreme heterogeneity
● Package managers (software distribution
mechanisms)
● Patches applied to upstream code → potential
differences in functionality
● Highly customized system configurations
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There are issues, too
www.thefarm51.com
● Proprietary vs free GPU drivers
● Rule of thumb: free drivers perform more
poorly than proprietary ones
● Intel only has free, but the hardware is bad anyway ☺
● Legacy AMD/ATI GPUs only have free, and it's quite
good actually
● NVIDIA drivers are equally good as the
Windows ones
● AMD/ATI drivers are a bit worse than the
Windows ones ☹
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Solution?
www.thefarm51.com17
Assume Ubuntu.
● For development and testing alike
● Probably the most popular desktop
distro*
● This is what Valve does
- Bonus points: Steam compatiblity!
● Power users will get your game running
on others anyway
* there are claims it's Linux Mint now, but it's still based on Ubuntu

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www.thefarm51.com
Development
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Getting started
www.thefarm51.com
● Virtual machine
● Fair enough for building
● No such luck with testing
(unless you can virtualise your GPU)
● Native installation
● Painless dual boot
● Which disc image to download?
● Get whatever Valve recommends for
Steam
(the Most recent LTS release is a good bet)
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Tools
www.thefarm51.com
● Tons and tons of IDEs, mostly average
- Code::Blocks, Codelite are OK
- There's also Eclipse if you can stand it
● Tried & tested native code toolchains
- GCC, LLVM (clang)
- gold – the new multithreaded linker by Google
- GNU binutils – objcopy, objdump etc.
● VM toolchains
- Mono, OpenJDK/Oracle JDK
● Build tools
- GNU make, Cmake, SCons, GNU autotools
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Debugging
www.thefarm51.com
CPU
● Classic command-
line gdb, gdbserver
● Tons of front-ends
- cgdb (command-line)
- Nemiver, DDD (GUI)
Graphics
● gDEBugger/CodeXL
● NVIDIA Nsight for
Eclipse
● apitrace
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SDL – the cross-platform “DirectX”
www.thefarm51.com
● Provides:
- API in C with mappings to other languages
- Timers and threads
- Input – mouse, keyboard, joysticks/pads (incl.
Unicode characters)
- Window management (incl. GL context)
- Audio (very barebone)
- 2D graphics (hardware blitting)
● Be sure to use 2.0 for new projects!
- Support for OpenGL 3+
- Support for multi-viewport apps
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SDL – the cross-platform “DirectX”
www.thefarm51.com
● SDL hides away most platform-specific code
- That's right, no need to even use POSIX!
23
int main (int argc, char *argv[])
{
// initialize SDL
if (SDL_Init(SDL_INIT_VIDEO
| SDL_INIT_TIMER) < 0)
return 1;
// set window caption
SDL_WM_SetCaption("My SDL Game",
"MySDLGame");
// hide mouse, grab input
SDL_ShowCursor(0);
SDL_WM_GrabInput(SDL_GRAB_ON);
// make SDL clean up on exit
atexit(SDL_Quit);
// main game loop
while (1) {
// dispatch events
while (SDL_PollEvent(&ev)){
switch (event.type) {
// ...
}
}
}
SDL_ShowCursor(1);
SDL_WM_GrabInput(SDL_GRAB_OFF);
return 0;
}

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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?!
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Just DIY – POSIX API
www.thefarm51.com
● Most WinAPI features map to POSIX
- Threading (pthreads)
- Advanced file I/O, incl. memory-mapped
- Advanced memory allocation, incl. paging
- IP sockets
25
#if defined(WIN32) || defined(__WIN32__)
Emitter->InternalPtr = VirtualAlloc(NULL,
Size, MEM_COMMIT | MEM_RESERVE,
PAGE_EXECUTE_READWRITE);
// ...
#else // It's a UNIX system! I know this!
Emitter->InternalPtr = mmap(NULL, Size,
PROT_READ | PROT_WRITE | PROT_EXEC,
MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
// ...
#endif

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POSIX API
www.thefarm51.com
There is an extensive, centralised reference
manual in the manpages-dev package
● Type in a shell to access:
man <section number> <subject>
● Section number cheat sheet
1. General console commands
2. System calls
3. C standard library calls
4. Special files (/dev/*) and drivers
5. File formats and conventions
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X11 client API
www.thefarm51.com
● Windows graphical mode maps to X11
- Xlib is a popular client library for the X(11) Window
System server
- XCB is a more modern replacement (often a back-
end for Xlib)
- Raw event pump, window management
- OpenGL interoperability via the GLX extension
27
// event loop
XEvent Event;
while (XPending(GX11Display)) {
XNextEvent(GX11Display, &Event);
switch (Event.type) {
// ...
}
}

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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
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Porting a D3D renderer to OpenGL
www.thefarm51.com29

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Porting a D3D renderer to OpenGL
www.thefarm51.com30
Porting Source To Linux:
Valve's Lessons Learned
Valve & NVIDIA, GTC 2013
Video: http://goo.gl/jMX9A
Slides: http://goo.gl/FtA5k

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How to deal with heterogeneity?
www.thefarm51.com
● As mentioned earlier, library ABIs tend to
be unstable
- Glibc tags its symbols with version → multiple
versions for backwards compatibility
- Compile with newest version → binary might not
run on older versions
● The ELF format is constant; but even then:
- Dynamic libraries' “sonames” (including version
numbers) are baked in at link time → binary might
not run if system provides a different version
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32. 32
How to deal with heterogeneity?
www.thefarm51.com32
How?
Static linking
+ will work forever
- binary bloat
Distribute with libs
+ will work forever
- redundant to other
games (also a form of
bloat)
- potential licensing
issues

33. 33
How to deal with heterogeneity?
www.thefarm51.com33
Or use the Steam Linux Runtime
https://github.com/ValveSoftware/steam-runtime
● Collection of essential packages “ripped”
from Ubuntu repos + patches
- freetype, glew, gtk+2.0, libgsm, libogg, libopenal1, libsdl1.2,
libsdl2, libvorbis, libtheora, libx11, libxcb, pulseaudio...
- Release and debug versions of all libs
● Ready-to-use GCC-based toolchains for
i386 and amd64
● Ships with Steam, so every (non-orthodox)
Linux gamer has it

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Steam Runtime for non-Steam games
www.thefarm51.com
● Start-up shell scripts are common practice
- Search for an installed Steam Runtime
instance
- If not found, install your private copy
- Only when the SLR is available, run the
actual binary
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Steam Runtime for non-Steam games
www.thefarm51.com35
#!/bin/sh
if [ -z $STEAM_RUNTIME ]; then
STEAM_RUNTIME=$HOME/.steam/steam/ubuntu12_32/steam-runtime
fi
if [ ! -d $STEAM_RUNTIME ]; then
# install private copy of Steam Runtime
fi
export LD_LIBRARY_PATH="$STEAM_RUNTIME/i386/lib/i386-linux-gnu:"
"$STEAM_RUNTIME/i386/lib:"
"$STEAM_RUNTIME/i386/usr/lib/i386-linux-gnu:"
"$STEAM_RUNTIME/i386/usr/lib:"
"$STEAM_RUNTIME/amd64/lib/x86_64-linux-gnu:"
"$STEAM_RUNTIME/amd64/lib:"
"$STEAM_RUNTIME/amd64/usr/lib/x86_64-linux-gnu:"
"$STEAM_RUNTIME/amd64/usr/lib:"
"$LD_LIBRARY_PATH"
# launch the actual game here

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Locales
www.thefarm51.com
● The C standard defines locales for language
and regional settings (see man setlocale)
● They affect *printf()/*scanf() and more
● Xlib can modify locale categories for GUI
apps (see man XSetLocaleModifiers)
● Example POSIX locales: pl_PL.UTF8,
en_GB.ISO-8859-1, ru_RU.KOI8-R
● The Linux manual might make it seem like
retrieving current locale is as easy as calling
char *locale = setlocale(LC_ALL, NULL);
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Locales
www.thefarm51.com
No such luck!
// backup the original application locale
char *Locale = setlocale(LC_ALL, NULL);
char *OriginalLocale = NULL;
if (Locale && *Locale)
OriginalLocale = strdup(Locale);
// reset locale to system (user) default
setlocale(LC_ALL, "");
// retrieve the default locale
Locale = setlocale(LC_ALL, NULL);
// process locale here
// restore original locale
if (OriginalLocale)
{
setlocale(LC_ALL, OrigLocale);
free(OriginalLocale);
}
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Extracting debugging symbols
www.thefarm51.com
● We often want to keep symbols for
shipping binaries
● On Windows – keep the .pdb files
● On Linux
objcopy --only-keep-debug
"${tostripfile}"
"${debugdir}/${debugfile}"
strip --strip-debug --strip-unneeded
"${tostripfile}"
objcopy --add-gnu-debuglink=
"${debugdir}/${debugfile}"
"${tostripfile}"
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Crash handler
www.thefarm51.com
● On POSIX this is called a signal handler
● Signals are a primitive IPC mechanism used
not only for crashes
- Debugger traps, floating point exceptions, terminal
hangup, exit requests... See the output of kill -l
● You can take a shortcut by using:
- libSegFault – part of glibc, try:
$ LD_PRELOAD=/lib/libSegFault.so
/lib/libSegFault.so
- Andrew Tridgell's segv_handler
http://www.samba.org/junkcode/#segv_handler
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A word on licensing
www.thefarm51.com
Whenever you use a library, check its
licensing terms!
● GPL is restrictive
- Requires derivative works to use GPL
→ sharing the source code
● LGPL is restrictive with linking exception
- Allows linking to an LGPL library without
sharing the source code
● MIT, BSD and Zlib licenses are generally
permissive, some might require advertising
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www.thefarm51.com
Deployment
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Where do I put the files?
www.thefarm51.com
● 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
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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/
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Creating installation bundles
www.thefarm51.com
● Not relevant for Steam games (duh)
● Simple .tar.gz archives
$ tar czvf <archive name>
<input files...>
● Distro packages (.rpm, .deb...)
● Self-extracting command-line installers as
shell scripts
https://coderwall.com/p/y3upqw
● MojoSetup
http://icculus.org/mojosetup/
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www.thefarm51.com
Summary
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Summary
www.thefarm51.com
● Engineering costs of a Linux port for an
already multiplatfrom game engine are low
- “It's just an API”™
● SDL covers a lot of boilerplate for you
● Overcoming heterogeneity is essential
- Steam Linux Runtime is helping to fix it
● Watch out for those licenses!
● When in doubt:
- Use Ubuntu
- Do whatever Valve does
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www.thefarm51.com
Questions?
leszek.godlewski@thefarm51.com
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Special thanks
www.thefarm51.com
Ryan “Icculus” Gordon
Inspiration & software contributions
Michał Wielgus
Critical review & free software comrade
Reinhard Pollice
Getting stuff done for me on the PKHD port
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Thank you!
Like us on Facebook!
http://www.facebook.com/farm51
http://www.facebook.com/deadfalladventures
http://www.facebook.com/PainkillerGame
leszek.godlewski@thefarm51.com
www.thefarm51.com