The GNU-GPL (General Public License) - Important facts at a glance
What is the GPL license?
If you obtain software that is licensed under the GPL, you may run and analyze the software. You may also use the software in a new program and distribute copies of the program.
The first version of the GPL was drawn up in 1989 by Richard Stallmann (Free Software Foundation). The GPL is intended to ensure that the software remains "free" for all users. "Free" here does not mean "free of charge", but refers to the freedom to modify and redistribute the software.
The Linux kernel, the GNU Compiler Collection, MySQL and WordPress are examples of software with a GPL license.
What does the copyleft principle cover?
In practice, the protection of the aforementioned freedoms is guaranteed by the copyleft principle. The copyleft principle states that the license of the original work must not change when it is passed on (after modification of the work). In short: Software that has been licensed under the GPL must also be licensed under the GPL when it is redistributed.
What am I allowed to do with GPL-licensed software and what not?
If the software is used within the company and is not subsequently passed on, the software does not have to be licensed with GPL even after modification.
If a developer links his source code to GPL-licensed software, the copyleft principle applies at the time of distribution to third parties. The entire code must be placed under GPL license again when it is passed on. In practice, this means that the source code may only be passed on together with the license text. In addition, modifications to the code must be well documented.
Do I have to make my code publicly available due to the GPL license?
The GPL stipulates that licensees must be able to analyze the code. The consequence of this provision is that the program code can be viewed. However, the duty of disclosure within the GPL does not apply to the general public!
According to the GPL license, only licensees must be able to view the code. Accordingly, the licensor must determine who is the licensee. The licensor can hand over the software together with the source code to a selected group of licensees. However, he can also decide that this should be done on a publicly accessible web server. The latter is often the case with open source software.
May I distribute parts of my program commercially under the GPL?
It is possible to distribute GPL-licensed software for a fee. The decision lies with the licensor. At this point, it must be weighed up whether the undertaking is worthwhile: the person who obtains the software for a fee ultimately also obtains the right under the GPL license to distribute copies of the software (with or without payment) and could therefore make it available to the general public free of charge.
What is GPLv2 and GPLv3 all about?
Two years after the GPL license was published, it was extended (GPLv2). The most important aspect of this extension is the Liberty or Death clause. This emphasizes once again that the software may no longer be distributed if it is not possible to comply with the principles of the GPL for any reason.
A further adaptation (GPLv3) was made in 2007. Among other things, GPLv3 contains clauses that make it easier to use the GPL globally: Previously, it was strongly oriented towards the US legal system, which is why it reached its limits internationally in individual cases.
Summary
After understanding the GPL and the copyleft principle, it is clear why it is found in the context of open source projects. Since open source software is usually developed as part of a large community, it is helpful if the license supports this collaborative form of organization by granting all contributors a range of freedoms.
Despite the freedoms, it is also important to know the associated obligations: Failure to comply with the copyleft principle can be very expensive in case of doubt.
Outlook
The GPL is just one of many licenses. In the following blog article, we would like to take a closer look at these and differentiate them from the GPL license.
