@Ashok, no need translate SISL from C. See my Fortran-C interop interfaces that I developed in response to the original OP in this thread.
Also, I actually started developing my own NURBS package in native Fortran several years ago (almost 12 years ago now) to support IGA but put it aside due to lack of time and funding. Its about 50% complete. I’m planning on retireing sometime in the next 8 months or so and will probably revisit it then.
Also several, Python based IGA codes had supporting Fortran implementations of the basic NURBS capability etc you need for IGA. See
also