Recently I have to combine my C++ code with the Fortran code from my colleague, and I soon find the complication with the gcc compilers. The Fortran code was written in F77, with a few exceptions when f90 syntax is used. It compiles well with g77, coming with pre-4.0 version of the gcc compilers. gcc 4.0 ships with gfortran rather than g77. gfortran is a Fortran 95 compiler, which is supposed to comile Fortran 77 codes as well, but not for the code I have. Fortunately, I'm able to compile the code with gfortran by some simple changes:
In Fortran 90/95, a "&" at the end of a line, or in the front of a line, is treated as line continuation sign. But in Fortran 77, it has to be a character at the sixth column. In the code I get, most "&"'s comply to the Fortran 77 format, but some are not. So I move all "&"'s to the sixth column, and now the code compile with gfortran.
I also need to compile the code with ifort on a different computer. The only thing that is different from gfortran is one compile option. The fortran code has long lines occasionally (allowed in Fortran 90/95 since its's free format). With g77 or gfortran, the compiler option "-ffixed-line-length-none" does the trick. But this option is not available in the Intel compiler. Fortunately, the long lines in the code is not terribly long, and "-132" option for ifort works. This option assumes the field width to be 132 columns rather than the standard 72 columns.