Fortran Wiki
C preprocessor

The C preprocessor (cpp) is designed for C and C++ source code, and is not a very good choice for other programming languages. However, it has often been used for other languages because it is well-defined and commonly available. In Fortran, problems with cpp include conflicts with comments in expressions, continuation lines, and case-sensitivity. That is why fpp was created. It works like cpp, but understands Fortran syntax.