January 24, 2012, 3:50 PM — For most of the 2000s, copyleft licenses (in particular the GPLv2) were the most popular choice for new open source projects. In the last few years, developers and companies seem to be trending away from the GPL in favor of permissive licenses for open source projects. What's behind that, does it impact your business and what licenses should you choose for new projects? Let's take a look.
The GPL is in decline, sort of. As Matthew Aslett reported last year, the number of projects using the GPL family has increased in real terms.
However, the usage of the GPL as a percent of all open source projects is in decline. According to Aslett, in 2008 the GPL family was 70% of licenses. As of December of 2011, it was 57%. Clearly, there is a trend at least for now towards permissive licenses.
Defining Permissive Licenses and Copyleft
For those who don't spend a lot of time thinking about open source licensing, let's look at the differences between permissive licenses and copyleft.
Copyleft licenses look to protect developer and user rights. As the Free Software Foundation puts it, the GPL protects the "four freedoms":
- The freedom to run the program, for any purpose.
- The freedom to study how the program works and change it so it does your computing as you wish.
- The freedom to redistribute copies so you can help your neighbor.
- The freedom to distribute copies of your modified versions to others.
You can do all of those things with a permissively licensed project, but what the permissive licenses do not do is carry any requirement that the recipient of the software pass those freedoms on to their users.
For example, companies that work with the Linux kernel and distribute it are obligated to make source available to their users. This is frequently a problem when companies distribute modified GPL'ed software like the Linux kernel or Busybox in embedded devices and neglect to make source available.
Why Companies Like Permissive Licenses
On the other hand, this is not a problem for companies that are distributing software under a permissive license. There are some requirements around copyright notices for some permissive licenses, but they don't require companies to distribute modifications.
In the eyes of some companies and developers, the permissive licenses have several practical advantages.