GPL Licensing – Must Source Code Be Human-Readable?

The GNU General Public License v2 reads:

[1] You may copy and distribute verbatim copies of the Program's source code as you receive it, in any medium, provided that you conspicuously and appropriately publish on each copy an appropriate copyright notice and disclaimer of warranty; keep intact all the notices that refer to this License and to the absence of any warranty; and give any other recipients of the Program a copy of this License along with the Program.

[2] You may modify your copy or copies of the Program or any portion of it, thus forming a work based on the Program, and copy and distribute such modifications or work under the terms of Section 1 above, provided that you also meet all of these conditions:

a) You must cause the modified files to carry prominent notices stating that you changed the files and the date of any change.

b) You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License.

That means that when your project is based on GPLv2 software, it also has to be released under GPLv2.

For that reason the GPL is often criticized for being a "viral" license - using GPL code in a project "infects" it with the GPL and prohibits you from releasing it under a different license.

To do so you have to:

• Make sure the GPL in text form is included in the distribution of the program and make clear that these license conditions apply to your program
• Leave any copyright or warranty notices of the other authors intact
• Publish the sourcecode
• Add a change date as a comment to each source file you edit

This situation is covered in the GNU GPL FAQ, in the section "I'd like to incorporate GPL-covered software in my proprietary system. Can I do this?"

in many cases you can distribute the GPL-covered software alongside your proprietary system. To do this validly, you must make sure that the free and non-free programs communicate at arms length, that they are not combined in a way that would make them effectively a single program.

The difference between this and “incorporating” the GPL-covered software is partly a matter of substance and partly form. The substantive part is this: if the two programs are combined so that they become effectively two parts of one program, then you can't treat them as two separate programs. So the GPL has to cover the whole thing.

If the two programs remain well separated, like the compiler and the kernel, or like an editor and a shell, then you can treat them as two separate programs—but you have to do it properly. The issue is simply one of form: how you describe what you are doing. Why do we care about this? Because we want to make sure the users clearly understand the free status of the GPL-covered software in the collection.