Microsoft's offer to license parts of its Windows source code would be a poison pill for open-source projects attempting to improve interoperability with Windows, critics say.

Last week Microsoft announced it would offer third parties access to the parts of the Windows source code that govern key aspects of interoperability. The move is part of the ongoing struggle between Microsoft and the European Commission over the company's compliance with a 2004 antitrust decision.

"The move is less appealing than it seems up front," said Carlo Piana, an attorney representing the Free Software Foundation Europe (FSFE) in the anti-trust case. "We only need the compatibility information, nothing more, nothing less, and the ability to legally use it."

Microsoft presented the offer as a way of ensuring that third parties have everything they need to ensure interoperability with Windows. "The Windows source code is the ultimate documentation of Windows Server technologies," said Brad Smith, Microsoft senior vice president and general counsel, at a press conference in Belgium last week.

"We are putting our most valuable intellectual property on the table so we can put technical compliance issues to rest and move forward with a serious discussion about the substance of this case," Smith said.

But source code is the last thing an open-source project would want access to, according to Piana. Open-source licences require that the software doesn't include any material with other legal attachments, such as patents or copyrights. If a contributor to an open-source project had viewed proprietary source code, the owner of the code could allege that the contributor had copied that code, Piana said.

"We want to develop our already-existing alternative product on our own code-base, making it fully compatible with Microsoft's product, not taking a ride on Microsoft's code," he said, referring to Samba, the networking software used by Linux and other operating systems to connect to Windows networks.

The source code offer is intended to address criticisms of the documentation provided by Microsoft. But doesn't address a more fundamental problem with Microsoft's anti-rust compliance, said Piana - the communications protocols involved are covered by a licence that is incompatible with Samba and other open-source software. "Samba must remain Free Software, not poisoned by proprietary licence conditions," he said.

As with the licensing conditions, the offer of source code seems particularly designed to lock out open-source software, Piana said. "Once again, it is clear that the only ones that Microsoft really fears are the fellows of Free Software, the only ones really uninterested in source code," he said.

EU competition commissioner Neelie Kroes told Reuters that the source code offer was "a surprise" and said it may not even meet the needed documentation requirements. "Normally speaking, the source code is not the ultimate documentation of anything, which is precisely the reason why programmers are required to provide comprehensive documentation to go along with their source code," she said.

In 2004 the Commission found that Microsoft had abused its dominant market position in PC desktop operating systems to extend its influence in server software. Microsoft was fined a record 497 million euros and was forced to make several key changes to its business practices.

One of these changes, the requirement to license desktop communications protocols to rivals, has proven to be a sticking point between Microsoft and the Commission. Microsoft's open-source competitors have argued that the software giant's proposal on the matter effectively locked them out.

Microsoft argues that to go any further would mean revealing sensitive trade secrets, while open-source organizations, such as the FSFE and the Samba project, say the information is only valuable because it prevents full competition.