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Most discussions of free software licenses bore listeners. In fact, licenses are usually of such little interest that 85%of the projects on Github fail to have one.

However, one aspect of licensing never fails to stir partisan responses: the debate over the relative advantages of copyleft licenses such as the GNU General Public License (GPL), and permissive licenses such as the MIT or the Apache 2 licenses.

You only have to follow the links to Occupy GPL! that are making the rounds to see the emotions that this unending debate can still stir. Calling for an end to "GPL purism," and dismissing the GPL as "not a free license," the site calls on readers to use permissive licenses instead, describing them as "truly OSS [Open Source Software] licenses and urging readers to "Join the Fight!"

Occupy GPL! itself is unlikely to have a future. Anonymous calls to actions rarely succeed; people prefer to know who is giving the call to arms before they muster at the barricades. Nor is the site's outdated name and inconsistent diction, nor the high number of exclamation and question marks likely to inspire many readers. Still, the fact that the site exists at all, and the counter-responses in comments on Google+ show that the old debate is still very much alive.

Copyleft vs. Permissive

Both copyleft and permissive licenses license allow users to freely copy, distribute, and change the software that use them. To this extent, both are considered free licenses.

The main difference -- and the source of the debate -- is the conditions under which users of the software can do these things.

Under a copyleft license, users must do these things under the same license as the original software. They cannot, for example, take a GPL-licensed piece of software and release it under a proprietary license. Or, as the third version of the GPL states, "Each time you convey a covered work, the recipient automatically receives a license from the original licensors, to run, modify and propagate that work, subject to this License."

By contrast, permissive licenses do not restrict the licenses under which these acts can be done. For example, the Apache 2.0 license states that you can add "Your own copyright statement to your modifications and may provide additional or different license terms and conditions."

Although both types of licenses can have other conditions, these conditions are the ones that are the source of contention.

Copyleft supporters are concerned with ensuring that their work remains available to everyone. Many, too, prefer not to see someone else profit from their efforts, a few going so far as to refer to proprietary licensing as exploitive and ethically or morally wrong.

This position is roughly equivalent to that of free speech advocates who argue for restrictions on bigoted speech on the grounds that it tends to destroy free speech for others. Historically, it has been extremely successful in encouraging the spread of free software, with the GPL and related licenses remaining by far the most popular free software licenses.

Advocates of permissive licenses, however, take a position analogous to that of social libertarians. To them, the licensing restrictions mean that the GPL is not a free license at all. While the name "copyleft" is meant to suggest an alternative to conventional copyright, supporters of permissive licenses argue that copyleft licenses simply replace one set of restrictions with another, even though the copyleft restrictions are far milder than those of copyright.

Furthermore, supporters of permissive licenses argue that their alternative encourages the use of free software. For instance, a company that changes the license on software from permissive to proprietary has no need to run license audits to check that they are in compliance with copyleft licenses.

Historically, such arguments seem to have been especially persuasive to writers of server software, and, if generally less popular than the GPL, permissive licenses have undoubtedly done their part to promote free software in general.

Both sides argue passionately and as a matter of principle. However, on a personal level perhaps the difference can be summarized in these terms: Copyleft supporters are concerned with retaining some control over what they create, while permissive supporters simply want to see their software used as widely as possible.

Is a Choice Necessary?

My own preference is for copyleft licenses. As a writer myself, I would prefer to use a license with more flexibility than conventional copyright, but to retain some control over it. For instance, I am currently finishing a book which I plan to release under the Creative Commons Attribution - Sharealike license, which allows the text to be freely distributed and modified, so long as my name remains on it, and any modifications use the same license. I consider this license a copyleft license for artistic works.

I also find the arguments against copyleft licenses unconvincing. Perhaps permissive licenses are less restrictive than copyleft ones, but they are not completely free, either. If nothing else, many permissive licenses insist that the copyright notice be preserved.

More importantly, so long as both categories of licenses exist, software with a permissive license is generally unable to borrow code that uses a copyleft license, but software with a copyleft license can freely borrow code that uses a permissive license. This is the situation that handicaps Apache OpenOffice in its rivalry with LibreOffice, and it appears a distinct disadvantage.

However, despite my individual preferences, I think it a mistake to be doctrinaire about licenses. Even the Free Software Foundation (FSF), although the major advocate of copyleft licenses, suggests that, in some circumstances, a strong copyleft license may not be the best choice.

For example, for software libraries, the FSF suggests using the Lesser GPL, which is often described as a weak copyleft license, because it allows interaction with proprietary software. However, for libraries that compete against well-established proprietary standards, as Ogg Vorbis does against MP3, the FSF suggests using the permissive Apache License 2.0.

Another possibility is to dual license software, although the FSF does not encourage the practice.

The main problem with sites like Occupy GPL! is that they fail to show a similar flexibility. Individuals may prefer either copyleft or permissive licenses, yet the free software movement may have prospered not because of either alone, but because the availability of both allowed developers to adapt as necessary to different situations.

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