> or using GPL code as a library
No. The copyleft nature still applies to libraries. That's why the LGPL exists. And the exception in the license for gcc for programs compiled by gcc.
No. The copyleft nature still applies to libraries. That's why the LGPL exists. And the exception in the license for gcc for programs compiled by gcc.
This is not limited to GPL, but applies to proprietary libraries as well. It's OK to require a proprietary library at runtime and you don't need a licence to do that. As long as you do not distribute some intellectual property, copyright law and its limitations are not applicable at all.
This sounds quite assertive, so compulsory "IANAL, this is just my interpretation".
The author originally created his own non-GPL library with the same interface as ReadLine and distributed that, noting that the user could (at their own option) link CLISP with GNU ReadLine instead if they wanted that functionality. Stallman argued that wasn't sufficient.
In the end, CLISP ended up being relicensed to GPL. Note though that no judge ever looked at it, so things might have turned out differently if it had gone to court.
The Linux kernel has opinions about this: symbols marked with EXPORT_SYMBOL are considered symbols that every operating system would have, so using them doesn't mean you are writing a derivative work. Symbols market with EXPORT_SYMBOL_GPL are considered implementation details so specific to Linux that if you use them, you can't say that your module isn't derivative of Linux.