I've read here that Eigen has closed source users thanks to LGPL library. But how is it possible to compile Eigen as a shared library (which LGPL requires you to do if you close your sources)?

The problem is that Eigen doesn't really have .c/.cpp which would be compiled into shared library, since all of the code is in .h files. Am I missing something here?

Yes, you refer to LGPLv2/v2.1
Eigen is distributed under version 3.

If using v2.1, a header-only library would be equivalent to GPL. With v3 you can write proprietary software using a header only lib.

Seb wrote:If using v2.1, a header-only library would be equivalent to GPL. With v3 you can write proprietary software using a header only lib.

Thanks for the explanation, Seb!

So, Basically if a library contains only .h files LGPL 3 allows you to compile it into your final executable statically. Did I get that right?

Wow, kudos to Eigen for this!

Eigen is a pure template lib (only headers), there used to be a problem with this in the LGPL 2.1, that's why we specifically require LGPL3+ which fixes this issue, see
http://eigen.tuxfamily.org/index.php?ti ... #Licensing