Companies like Google that build their business on software such as Linux have a moral imperative to contribute back to the free software community, a prominent open-source advocate said Tuesday.

That's a good thing because it doesn't look like software licenses will require this anytime soon, Eben Moglen, a Columbia university professor and chairman of the Software Freedom Law Center, said Tuesday during a speech at the Open Source Business Conference in San Francisco.

Moglen is putting the finishing touches on what he hopes will be the next software license adopted by developers of the Linux kernel, the GNU General Public License Version 3.

Software that is licensed under the GPL can be freely copied and changed, but anyone who distributes the code must then publicly release their modifications under the same license. This means that companies like Red Hat and Novell must give back all of their Linux code to the community.

But not so for Google or Yahoo, considered two of the largest Linux users on the planet. Although their Web-based software is used by hundreds of millions of people, these companies are service providers, not software distributors. So whatever Linux enhancements they have made can legally remain private.

The organization responsible for the GPL, the Free Software Foundation (FSF), had toyed with the idea of adding language that would have compelled service providers to give back their changes, but that idea was eventually dropped.

On Tuesday, Moglen said that community pressure -- and not software licenses -- would most likely drive Google's continued contributions to GPL projects.

Google is already a contributor to many open source projects, including Linux, but some observers have said that it could be doing more. Still, Moglen said that there are no plans to add provisions to the GPL that would compel companies like Google to give back their code.

"They have ethical and community responsibilities to return at least those modifications that are not critical to their business and that are of general value to the community," Moglen said. "We will see over time whether there are additional measures necessary in order to secure cooperation in the community."

Moglen said he discussed these issues during a talk he gave at Google two months ago, but he did not know whether he had changed any minds there. "I think we all know that Google has a bias toward secrecy," he said in an interview following his talk.

In fact, Google has bumped up its contributions to free software projects over the past two months, said Chris DiBona, Google's open-source program manager, in an e-mail. That's because the FSF and Google recently hammered out a software contributor agreement that was to Google's liking. Since then Google has been able to make contributions to the GNU C Compiler and the company plans to deliver patches to Emacs and other Linux tools as well, he said.