Fortran filename suffix

October 24, 2022

For modern Fortran development, “.f90” is the recommended filename suffix. For legacy Fortran 77 code, “.f” is the recommended filename suffix.

To preprocess Fortran code files (e.g. with #ifdef statements), capitalize the filename suffix e.g. “.F90” Failing to capitalize the suffix of code that requires preprocessing can break build systems such as CMake and Meson. The introspection of Fortran code by CMake or Meson may make an incorrect build graph if the preprocessing is not done. This can show up as random (or deterministic) build failures, whether in serial or parallel build.

We use the first “free-form” syntax Fortran 90 standard “.f90” suffix to indicate the current Fortran standard. This helps avoid too many ambiguous file suffixes.

C and C++ likewise do not indicate their language standard year in the source code file suffix.