What are the main requirements to port a game to Linux [closed]

Question

Steam for Linux is out today and I was surprised to see many games already being sold, which made me wonder what are going to be the difficulties for all those games to be ported to Linux. I broke this though into 3 questions:

1. Is it a matter of basically make the game run in OpenGL?
2. Are there any engines that build to Linux?
3. Did Valve have any hand on the development of some framework/toolset/OpenGL to make porting easier?

Answer 1

Generally speaking, it depends on how the game is programmed.

In the case of Source-engine games, they used a multi-tier software architecture, and moved everything platform-specific into /tier0 in their code. When they need to port something, at least by the original engine design, they just swap out /tier0 files for platform-specific ones, and reimplement the Source engine executable for, say, Linux, re-binding things like audio (ALSA/etc. vs. DirectSound), graphics (OpenGL vs. Direct3D), video playback (who knows), window generation (X/GLUT/whatever vs. Win32/64), and I/O (XInput/whatever vs. DirectInput) to their system-specific counterparts. It was designed like that from the very beginning, though a lot of system-specific stuff has crept into higher tiers of the engine due to bad maintenance over the years. Most of what they did to "port" the Source titles to Linux was just refactoring out the Windows-specific stuff, and reimplementing the core executable.

In terms of "Are there engines that build to Linux?": Of course. With the changes made for this big release, the Source engine now runs on Linux. There are literally hundreds of others, of varying quality or availability.

As for your third question, porting happens by the developers. If you don't design your software with platform-nonspecificity in mind, then it just makes it harder. Nothing is truly impossible to port, disregarding hardware capabilities and computing speed. There's no such comprehensive "tool" for porting games to other platforms, though, since they all work differently under the hood.

Answer 2

I'll share our experience of porting a certain game to Linux (and Mac).

• System-specific APIs - window creation, rendering, audio, networking, input, non-trivial filesystem access etc. The other answers go into details. I'll just say that portable libraries (SFML, SDL, but also things like boost) help a lot.
• Driver bugs - happen on Linux a bit more often than Windows. Or maybe that's a thing of the past, I'm not up to date.
• Storing user data - generally in $HOME on Linux. Perhaps not an issue if you're using Steam?
• Paths to resources - UTF characters, slashes/backslashes, spaces and non-alphanumeric characters can all cause problems. You'll need to check all your paths, especially those generated programmatically.
• System fonts - your game might be using some default resources provided by the operating system; in particular, fonts could be pitfall.
• Quirks of "portable" code interoperations - I think it's pretty obvious that even though every single function you use is portable, it doesn't mean that your whole game will be using those functions in a portable way. For instance, on Mac our SFML code crashed because we tried to preload a font before creating the game window.
• Compiler differences - you might stumble into some if you were using Visual Studio for Windows. g++ and Clang can be used on both platforms, and behave similarly. Applies in a limited fashion to Java and C# code as well.
• Build-system - actually building a proper Linux binary, depending on appropriate shared libraries and providing your own where needed, can actually be a bit tiresome. Again, Steam might help you a lot here.

One more thing - if you were to develop future games which you would intend to port, I would suggest developing them on two platforms at once. Setting up a continuous integration server which run compilation + smoke tests on Linux builds every time our source code was updated (most of us worked on Windows) saved us a ton of time while working on our game.

Answer 3

It's not just graphics (though that might be the hardest part). Consider also audio, input devices and other OS APIs (no Windows registry, etc.)

Additionally, if you are using any third-party library, the library must also be available for Linux.

Different Linux distributions may use different versions of the same library (or provide different libraries for the same purpose). Note that, officially, Steam only runs on Ubuntu so this issue is avoided.

There are multiple engines that can run on Linux. Among the open-source ones are Irrlicht and Ogre. Other compatible engines include Unity 3D 4.0 and (now) the Source engine.

Apparently, Valve used a layer to translate Direct3D calls to OpenGL. However, it required more effort than that. I can't find any information saying what they're using for Audio, Input and I/O.

Answer 4

To answer your 2nd sub-question: A great engine that not only builds on Linux, Windows, OS X and other platforms, but also abstracts away from all the OS-specific build tasks is Unity 3D.

It gives you SDK which allows you to build your project for any of those platforms.