Because Fortran 90 is (for all practical purposes) a superset of Fortran 77, Fortran 90 (and future) programs can use the original Fortran interface. The following additional requirements are added:
 
To make  mpif.h compatible with both fixed- and free-source  
forms, to allow automatic inclusion by preprocessors, and to allow  
extended fixed-form line length, it is recommended that requirement  
two be met by constructing  mpif.h without any continuation lines.  This  
should be possible because  mpif.h contains only declarations,  
and because common block declarations can be split among several  
lines. To support Fortran 77 as well as Fortran 90, it may be  
necessary to eliminate all comments from  mpif.h.  
 ( End of advice to implementors.)