The GPL and library code

The GNU General Public License (GPL) is an unforgiving beast; if you distribute something derived from GPL-licensed code, the whole derived product must be distributable under the GPL's terms. This provision effectively prevents the use of GPL-licensed in proprietary, closed-source products. That is an inconvenience for proprietary software vendors, but is clearly what the authors of the GPL intended.

In fact, the terms of the GPL work very well for some software vendors as well. Consider this press release from MySQL AB, Sleepycat Software, and Trolltech AS. These companies claim that their 2003 software licensing revenues were up 65% over the previous year. Not bad for companies which make their software available for free.

Each of these companies is a provider of "library" code - tools which can built into an application to give it new capabilities. MySQL and Trolltech make their offerings available under the GPL; Sleepycat has its own license which requires source availability (though in a weaker form than the GPL). In each case, however, there is a twist: for a fee, the company will make the same software available under a license which allows closed-source distribution.

When this model works, it works well. The free software community gets access to high-quality software, and the company gets the benefits of the free development process. At the same time, the company is able to extract money from others who are making money with the code. This model will only work in some situations; the software in question must be attractive as a component of a larger application, and external contributors must be willing to transfer copyrights or otherwise allow their work to be distributed in closed-source form. But, when those conditions apply, the dual-licensing model appears to work well.

There is one interesting problem which occasionally comes up, however; licensing this sort of library code under the GPL can block its use with other software which is available under a free, but GPL-incompatible license. This conflict has been highlighted by the fact that the GPL-incompatible PHP license means that PHP and MySQL 4 cannot be used together (or, more correctly, an application combining the two cannot be redistributed). Since MySQL and PHP are a popular combination, this restriction hurts a lot of people; it also led to a number of distributors sticking with the older MySQL 3 release, which did not have this problem. The GPL-incompatibility of the new XFree86 license is another high-profile example; in that case, the license conflict may be the final straw that signals the end of XFree86 as a viable project.

MySQL AB has now acted to mitigate the problem of free but GPL-incompatible licenses; the company has extended the MySQL client library license with the "MySQL FOSS License Exception." This exception provides a series of licenses which can be applied to parts of derived works involving the MySQL client libraries; it includes the PHP license and several others. With this extension, the PHP license conflict is no more.

The stated intent of the GPL is to ensure that all derived products remain free software. This extension of the license is clearly compatible with that goal; it still does not allow the covered code to be distributed in a non-free manner. If this sort of exception is adopted more widely, it may point toward a need for a new form of the GPL. If the end result is more free software, that would be a good thing.

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