Experimental Physics and
Industrial Control System

"Johnson, Andrew N." <[email protected]> · Wed, 22 Oct 2014 21:45:39 +0000

Disclaimer: IANAL, TINLA.

On Oct 22, 2014, at 8:20 PM, J. Lewis Muir <[email protected]> wrote:

> I think the issue you're talking about with the GPL is that
> any program that *links with* a GPL program (library) must itself then
> be GPL (or GPL-compatible).  This is because the GPL considers your
> program to be part of the GPL program if you've linked with it.  The
> LGPL specifically allows you to link your program with an LGPL program
> (library) without this requirement.  But I'm pretty sure the LGPL still
> has the rules about making changes to a program (library) licensed under
> it: if you make a change to an LGPL program (library), you have to make
> those changes publicly available.

Actually in many cases you don’t have to make your changes *publicly* available, you only have to make your exact modified source available to anyone to whom you give a copy of the program binary, so that they have the same rights and ability that you do to study and further modify it according to their needs. The public source requirement is only specifically required if you put a copy of a binary on a public network server; see http://www.gnu.org/licenses/gpl-faq.html#AnonFTPAndSendSources and section 6(d) of GPLv3 for details.

Of course if you never give anyone a copy of your modified version of a program, you never have to give away the source code either since the GPL only covers distribution, not use; this applies *within* a company as well.

> Note that if you don't link with a GPL program (library), your program
> doesn't have to be GPL.  So, for example, if you use fork and exec to
> invoke a GPL program, the GPL considers them to be separate programs,
> and so your program would not have to be GPL.  This could be simply
> running some program, or it could be running a "plug-in" program.

I would be careful with making assumptions about using “plug-in” programs, the GPLv3 definition for Corresponding Source "includes interface definition files associated with source files for the work, and the source code for shared libraries and dynamically linked subprograms that the work is specifically designed to require, such as by intimate data communication or control flow between those subprograms and other parts of the work.” This means that “intimate” inter-program communication could also require your program to be released under the GPL in some cases.

Before trying to get around the GPL restrictions you should first talk very carefully to a lawyer with experience in it; at least one of the lawyers who drafted the GPL is a highly experienced programmer (and a law professor).

- Andrew

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System