PetscErrorCode PetscMPIFortranDatatypeToC(MPI_Fint unit,MPI_Datatype *dtype)Not Collective
Developer Notes: The MPI documentation in multiple places says that one can never us Fortran MPI_Datatypes in C (or vis-versa) but this is problematic since users could never call C routines from Fortran that have MPI_Datatype arguments. Jed states that the Fortran MPI_Datatypes will always be available in C if the MPI was built to support Fortran. This function relys on this.