Fortran with MPI error

Question

I am working on Fortran code with MPI and introducing the following MPI command

    call MPI_Gather(nlocal,1,MPI_INTEGER,counts,1,MPI_INTEGER,0&
         &,comm_cart,ierror)

with in a particular subroutine gives following error:

This name does not have a type, and must have an explicit type.   [MPI_INTEGER]

I understand that the compiler does not recognize the MPI part of this code. However, all other related variables such as nlocal, counts and comm_cart are recognized except the Fortran MPI data type MPI_INTEGER. Can someone throw light, where I am doing wrong?

Note: The compiler is Intel compiler

Answer 1

You must tell the compiler about the MPI stuff (mainly variables).

The modern way is introducing

use mpi

in every scope.

In the old days it was also done using

include "mpif.h"

but that has several disadvantages. Namely, because it is compatible with FORTRAN 77, it does not introduce explicit interfaces for any MPI subroutines and therefore the error checking is less thorough than with use mpi. The modern method will help you more in keeping your code correct.

On the other hand, if you use use mpi the module mpi must be compiled with the same compiler (sometimes even with the same version) which you use to compile your program.