August 9, 2005

The GNU Free Documentation License

Author: Bruce Byfield

"Free software needs free documentation." With this preamble, the Free Software Foundation released the GNU Free Documentation License (FDL) in March 2000. Even with a revision in November 2002, the license has had mixed reviews in the free and open source communities, especially when compared to the widespread enthusiasm for the GNU General Public License (GPL). Some accept the FDL with a few reservations, while others reject it as not being free enough. Both attitudes seem likely to persist at least until the license's next revision.

Just as a software license defines how you can use source code and under what circumstances, so the FDL defines how you can use a document. The main difference is that, in order to accommodate the realities of publishing, the FDL is concerned with many different types of copying and modification, including anthologies, mass production, and translation. To deal with these concerns, the FDL makes several distinctions that are unnecessary for software:

  • Copies can be either transparent or opaque. A transparent copy is one made using
    free, machine-readable formats, such as HTML for text or PNG for graphics. Opaque
    copies are ones in a proprietary format, such as Microsoft's .doc for text or the Adobe Illustrator (.ai) format for graphics. Using transparent formats is easier, but the
    license acknowledges that opaque copies may also exist in professional publishing.
  • Sections of a document can be normal or invariant. An invariant section
    is one that must be reproduced without alteration. To prevent someone from
    undermining the intent of the license, invariant sections must contain only
    secondary material, such as acknowledgements or publishing history, and
    not any information about the document's subject. Cover text, which consists
    of a few words about the document on the front or back covers, can also be
    invariant.
  • If more than 100 opaque copies copies are made, additional requirements apply, such as the need to include a transparent copy with each opaque copy.

Such distinctions make the FDL harder to use than most software licenses. For this reason, the license concludes with an appendix about how to publish a document with it.

Positive reactions

The FDL receives qualified approval from book publishers, one of its main intended audiences. Sara Winge of O'Reilly and Associates says that her company has published four or five books under the FDL, and "all did fine in the marketplace." However, Winge says O'Reilly is unable to measure the impact of the FDL "because no book has an exact counterpart published under a different license." In general, O'Reilly's philosophy is "to let the author choose the license." Winge says that she can remember no instance in which O'Reilly rejected a book because an author wanted an open license.

William Pollock of No Starch Press takes a similar attitude. On the whole, he believes that free licenses help a book's exposure and sales. Yet, he notes that the market for books is much smaller than the market for software. "One difference between free software and free books is that if you give your software away, you can still profit from supporting it or selling services that work with it. Once you give a book away, that's it. If the reader isn't interested in buying a print copy, there isn't much option to recover your investment."

The largest endorsement of the FDL comes from the Wikipedia Foundation, the organization behind the free online encyclopedia. Wikipedia's founder, Jimmy Wales, says that the Foundation considered writing its own license for Wikipedia entries. Instead, the Wikipedia Foundation chose the FDL because the license met its needs and because "GNU licenses mean something to people." Wikipedia currently uses the license without any invariant sections.

Wales admits that some people find it "complicated to reuse our contents" under the FDL, and says he might have considered the Creative Commons Attribution license, had it existed when the organization chose a license. On the whole, Wales describes the FDL as "an excellent license."

The Debian critique of the FDL

Since November 2001, one of the most detailed criticisms of the FDL has come from the Debian project. After lengthy and sometimes vitriolic public debates, the official Debian position is that the FDL does not conform to the Debian Free Software Guidelines (DFSG), the criteria used by the project to define the acceptability of various licenses.

Although the name indicates that the DFSG were intended for software, Debian now applies it to everything shipped with the distribution. Version 3.1, the current Debian release, includes documents licensed under the FDL, but according to Debian Project Leader Branden Robinson, current plans are to remove all documentation released under the FDL in the next release. This documentation includes many of the manuals released by the Free Software Foundation, including the one for the GNU Compiler Collection.

Debian has three main objections to the FDL:

  • Section 2 states that no copy of a document released under the FDL can be subject to "technical measures to obstruct or control." According to Debian, this clause does not permit users to store an encrypted version of an FDL document. It might even prohibit restrictive permissions on the file or the drive on which it is stored.
  • Section 3 requires that opaque copies be accompanied by a transparent version. Debian's position is that transparent copies should only need to be publicly available, not shipped with opaque copies.
  • Invariant sections cause numerous problems. They cannot be corrected or
    updated, only added to, which could cause documents to bulge with unneeded
    material. Moreover, because invariant sections consist of secondary material, by definition they cannot be used as the source of new documents. Some Debian members also express concern that invariant sections will be misused to circumvent the purpose of a free license, although the definition of
    what belong in an invariant section might seem to prevent such abuse.

The Debian objections mainly focus upon the existence of invariant sections. The DFSG states that "the license must allow modifications and derived works." The Debian position is that invariant sections undermine this principle.

These objections have lead some to believe that Debian and the FSF are feuding over the license. Leaders of both projects are quick to reject this interpretation. Although Richard Stallman believes that Debian members "have taken their criteria too literally," he declines to respond to specific criticisms of the FDL. Similarly, Branden Robinson says, "It's like a fight with a sibling. You don't enjoy the extra level of tension in the relationship." He stresses that Debian and FSF representatives have been consulting regularly in an effort to resolve the Debian objections in the next revision of the FDL, adding, "I couldn't be happier with the level of collaboration."

Future plans for the FDL

According to Stallman, the next revision of the FDL won't occur until the release of the third version of the GPL. Since Stallman and Eben Moglen, the authors of earlier versions of the GPL, are still consulting stakeholders about issues the third version needs to address, the next version of the FDL isn't expected for at least a year.

Meanwhile, Stallman suggests several possible future directions for the FDL. A modified version without provisions for invariant sections or cover text is one possibility. Stallman indicates that a clear definition of fair use may be needed, so reviewers and cataloguers can use samples without worrying about possible license violations.

Perhaps in response to the Debian objections, Stallman also wants to explore the possibility of a dual GPL/FDL license. Citing the dual licensing of Qt as a precedent, Robinson suggests that a dual license may bypass all Debian objections to the FDL. However, Stallman notes that avoiding potential conflicts between the two licenses and closing any loopholes created by the dual licensing would be a major undertaking.

For now, Stallman judges the FDL "a partial success." It's partially successful because it was the first to address the need for free documentation and make more documentation available, but as he rightly points out, "We still don't have as much documentation as we need."

Bruce Byfield is a course designer and instructor, and a computer journalist who writes regularly for NewsForge and the Linux Journal Web site.

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