Fortran File Extensions
Part of the confusion about modern Fortran is what file extension to use. This confusion has its roots in the Fortran 90 standard (ISO/IEC 1539:1991) and the introduction of "free-form" source (the older FORTRAN 77 punched-card style was renamed "fixed-form" at this time). Unfortunately, compiler vendors began to use the extension .f90 for free-form files (common fixed-form extensions were .f or .for). Of course, the Fortran standard says nothing about file extensions. This is just one of those things the Lords of Fortran pretend are not important, like the ability to access the command line (not standardized until Fortran 2008) or the file system (still waiting for this). So, that left the compiler vendors (and users) free to do what they wanted, and all of them used .f90 for the new file type.
This was fine until the Fortran 95 standard came along (ISO/IEC 1539-1:1997). Fortran 95 was a minor update to the language (mostly cleaning up some of the half-baked bits of Fortran 90, but also adding some new stuff). This is when things started to go wrong. Since Fortran 90 used the .f90 extension, Fortran 95 should use .f95, right? And then Fortran 2003 came along, and then Fortran 2008, etc. The end result is a confusion of file extensions for free-form source such as .f90, .f95, .f03, .f08, and soon .f15. However, it doesn't make any sense to use the file extension to indicate the Fortran standard. Fortran has a high degree of backward compatibility (Python users take note). So, for example, almost all of Fortran 95 code is valid Fortran 90 code. Even a good deal of FORTRAN 77 code is still perfectly valid Fortran 2008 code. In fact, fixed-form source is still valid even in the Fortran 2008 standard (although only a lunatic would write object-oriented code in fixed-form format).
To sum up: .f90 means free-form source, not Fortran 90, and it's the only extension you should to be using. Stop the madness, or we're really going to be in trouble 75 years from now.
See also
- Steve Lionel, Source Form Just Wants to be Free [Intel.com] January 11, 2013.