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.)