6.7 External Libraries

As maintainer of an FSF-copyrighted GNU package, how do you use separately-published general-purpose free modules? (We also call them “libraries” because we are using them as libraries; it doesn’t matter whether they are packaged as libraries or not.)

It would be unreasonable to ask their authors to assign copyright to the FSF. They didn’t write those modules as contributions to GNU. We just happen to want to use them, as any developer might. It would be rude to ask developers, out of the blue, to give the FSF their copyright. Please don’t ask for that in cases like these.

The proper way to use those modules is to link your package with them and say they are not part of your package. See below for the mechanics of this.

To avoid present or future legal trouble, you must make sure the license of the module is compatible with current and future GPL versions. “GNU GPL version 3 or later” is good, and so is anything which includes permission for use under those GPL versions (including “GNU GPL version 2 or later”, “LGPL version n or later”, “LGPL version 2.1”, “GNU Affero GPL version 3 or later”). Lax permissive licenses are ok too, since they are compatible with all GPL versions.

“GPL version 2 only” is obviously unacceptable because it is incompatible with GPL version 3. “GPL version 3 only” and “GPL version 2 or 3 only” have a subtler problem: they would be incompatible with GPL version 4, if we ever make one, so the module would become an obstacle to upgrading your package’s license to “GPL version 4 or later”. Don’t use such modules.

One library you need to avoid is goffice, since it allows only GPL versions 2 and 3.

So, here are the mechanics of how to arrange your package to use the unrelated free module.

1. Assume the module is already installed on the system, and link with it when linking your program. This is only reasonable if the module really has the form of a library.
2. Include the module in your package’s distribution, putting the source in a separate subdirectory whose README file says, “This is not part of the GNU FOO program, but is used with GNU FOO.” Then set up your makefiles to build this module and link it into the executable.

   With this method, it is not necessary to treat the module as a library and make a ‘.a’ file from it. You can link directly with the ‘.o’ files in the usual manner.

Both of these methods create an irregularity, and our lawyers have told us to minimize the amount of such irregularity. So use these methods only for general-purpose modules that were not written for your package. For anything that was written as a contribution to your package, please get papers signed.