libfwrapper.c is a C file that wraps a few functions of the LAMMPS library API in src/library.h so that it can be called from Fortran. See the couple/simple/simple_f77.f90 program for an example of a Fortran code that does this. See the README file in that dir for instructions on how to build a Fortran code that uses this wrapper and links to the LAMMPS library. This legacy wrapper is deprecated and will be removed in a future release of LAMMPS. Please use the Fortran 90 module instead.