I am looking for a general purpose library which can solve a 2D or 3D linear elliptic PDE on a rectangular domain with mixed/Robin boundary conditions. I am a C programmer, so I would prefer a C library, but fortran is ok too since its easy to call fortran code from C. So far I have found only 1 option: MUDPACK. This is a fortran library which uses Multigrid and handles 2D and 3D elliptic PDEs.
Just wondering if there are other options out there? Unfortunately for me, many PDE libraries are written in C++ which likely cannot be interfaced with C easily.