RTAI is now a well established and well founded open development adventure, involving both individuals and companies. As such, it is a proof that free software can work. Roughly speaking, the ethos behind it is that basic general tools must be free and open, specific applications are in the hands of its users. They can contribute them back to the community or keep them for themselves. What is absolutely not acceptable is that fixes and improvements to the basic free tools they are exploiting remain just in their hands. That said, it is certain that RTAI developers would get angry if they knew of any violation of such principles, and would use all means available to make any such attempt well known to the public and to stigmatize those responsible.

The best licensing scheme that fits such a standing is LGPL, and RTAI adopted the LGPL from its beginning. In fact, the very first RTAI releases had no licensing scheme at all. They were a kind of informal "do whatever you want but contribute back if you can", with just a sketchy copyright with my name.

So a shift to GPL -- just for the RTAI core, i.e. its schedulers and the very basic interface to Linux -- was decided unanimously by all the RTAI developers, in order to help RTAI users avoid concerns of spreading "FUD" (fear, uncertainty, and doubt) due the (in)famous RTLinux patent.

I personally thought this matter was settled cleanly after the explanation of the GPL with regards to patents in a clear statement by Eben Moglen, made public on the RTAI mailing list on Oct. 22, 2001. Despite that, however, many RTAI users continued fearing that they were threatened by the RTLinux patent.

The shift [in RTAI core license from LGPL to GPL] was made official with the release of RTAI-24.1.8 and was announced also on the Linux kernel list. The announcement reported, once more, Moglen's statement, which can be read in full on the RTAI website. I think that the following excerpt captures the essence of it:

"No application in a running RTLinux or RTAI system does any of the things the patent claims. No applications program is therefore potentially infringing, and no applications program is covered, or needs to be covered, by the license. If an applications program running in a GNU/Linux system as modified by RTLinux or RTAI is constructed so that it does not violate GPL -- which is not the same thing as being licensed under GPL or being free software at all -- its distribution terms are utterly irrelevant."

What is stated above should make it clear the fact that the RTAI core being GPL does not affect application code.

Since all the RTAI developers want RTAI to be available to everyone, including the commercial and industrial sector, the reason the license was changed [from LGPL to GPL] was to free RTAI users from the threat of legal action [due to the RTLinux patent that] they may encounter as a result of using RTAI. One of the important things about the GPL is that it has a clause that prevents patent holders from placing restrictions on code they release under the GPL. By complying with the GPL, RTAI users are protected under this clause.

Moreover, if you look at RTAI you'll see it is not a single block of code; it is a kernel patch and a number of modules. Only the parts that potentially may be claimed to implement the teachings of the [RTLinux] patent are licensed under the GPL, the rest remain under their original LGPL license.

In reality, there is little difference (if any) between GPL and LGPL when it comes to kernel modules, as it has become an accepted fact that using an API implemented in a GPL kernel module (or in the kernel) does not result in contamination that would force your code to be released under GPL (and hence for you to publish your code).

Your obligation is that if you use RTAI in your product, you must make available to your customers the source code for RTAI and any modifications you have made to it. Of course, as already said, the RTAI developers do want you to contribute back your changes to RTAI so others can benefit.

We could thus see RTAI just as a part and extension of the Linux kernel. Any proprietary, binary-only RTAI application module is not at all different from any "normal" binary-only Linux device driver module.

That said, recall that none of the RTAI developers is a lawyer.

• Real-time Linux extension RTAI 24.1.8 released
  
• A developer's perspective on the RTLinux patent
  
• Lineo affirms continued commitment to RTAI
  
• RTLinux patent knocks over first domino?
  
• Yodaiken unveils RTLinux patent license
  
• "The Infamous RTLinux Patent"
  
• DIAPM-RTAI -- a Real-time Linux API
  
• Using GPL software in embedded applications