GNU Fortran accepts programs written in either fixed form or free form.
Fixed form corresponds to ANSI FORTRAN 77 (plus popular extensions, such as allowing tabs) and Fortran 90's fixed form.
Free form corresponds to
Fortran 90's free form (though possibly not entirely up-to-date, and
without complaining about some things that for which Fortran 90 requires
diagnostics, such as the spaces in the constant in R = 3 . 1).
The way a Fortran compiler views source files depends entirely on the
implementation choices made for the compiler, since those choices
are explicitly left to the implementation by the published Fortran
standards.
GNU Fortran currently tries to be somewhat like a few popular compilers
(f2c, Digital ("DEC") Fortran, and so on).
This section describes how g77 interprets source lines.