Mixing C and Fortran

% cat hello1.c
#include        <stdio.h>
#include        <mpi.h>
extern int printhello (int *, int *) ;
int main(int argc, char *argv[])
{
   int ierror, rank, size;
   char hname[64];
   MPI_Init(&argc, &argv);
   MPI_Comm_rank(MPI_COMM_WORLD, &rank);
   if (rank == 0) printf ("Hello world!\n");
   MPI_Comm_size(MPI_COMM_WORLD, &size);
   (void) printhello (&rank, &size);
   MPI_Finalize();
   return 0;
}

%  cat utils.c
#include        <stdio.h>

int printhello (int *rank, int *size) {
  char hname[64];
  if (gethostname (hname, 32) != 0) hname[0] = 0;
    hname[31] = 0; /* gethostname() may omit trailing 0 if hostname >31 chars */
   printf("I am %d out of %d on %s.\n", *rank, *size, hname);
   return 0;
}

% /usr/local/pkg/gcc/4.7.0/bin/gcc -c -I/usr/lpp/ppe.poe/include hello1.c

% /usr/local/pkg/gcc/4.7.0/bin/gcc -c utils.c

% mpcc utils.o hello.

o 

% cat parhello1.f
 
        PROGRAM HelloWorld
        include 'mpif.h'
         
        call mpi_init(ierr)
        call mpi_comm_size(MPI_COMM_WORLD,npes, ierr)
        call mpi_comm_rank(MPI_COMM_WORLD,irank,ierr)
         
        call printhello (irank, npes)
         
        call mpi_finalize(ierr)
         
        END PROGRAM

% cat utils.f
 
        SUBROUTINE printhello(my_irank,my_npes)
        INTEGER my_irank,my_npes
         
        print*,'Hello World! I am ',my_irank,' of ',my_npes
         
        RETURN
        END

% gfortran -c -I/usr/lpp/ppe.poe/include/thread parhello1.f

% gfortran -c utils.f

% mpxlf utils.o parhello1.o -L/usr/local/pkg/gcc/4.7.0/lib -lgfortran