how to call a Fortran90 function included in a module in c++ code?

Question

I m including a fortran90 program that is not mine in my C++ project .

In the first stept I try to call the function by their name_() and i get the error "undefined reference to mp_mpi_cartesian_init_ "by dispalying the symbol of the obj file (using nm) i found that the function are called by their module as module_function_ so i add the module name and i Get the same problem but between fortran obj such as "Constants.f90:(.text+0x36): undefined reference to __powi4i4"

here is the c++ code :

 #include <iostream>
 #include <complex>


 using namespace std;

 extern"C" {

        void  mod_save_wave_mp_read_it_psi_(int * it,complex<double>*  psi_E1E2 );
        void  mod_mpi_cartesian_mp_mpi_cartesian_init_( );
        extern int mod_mpl_h_mp_iproc_ ;
 }

 int  main(){
     complex<double>  psi_local[512*24*512*24];
     int it ;
     mod_mpi_cartesian_mp_mpi_cartesian_init_();
     cout << "proc  :" << mod_mpl_h_mp_iproc_ << "avant lecture\n";
     mod_save_wave_mp_read_it_psi_(&it,psi_local);
     cout << "psi ="<< psi_local[0] << "poiur le proc "<<mod_mpl_h_mp_iproc_ <<"\n";
   }

and this is an exemple of a module :

MODULE mod_save_wave

USE mod_constants
USE mod_MPI_CARTESIAN

    USE mod_time_mesure,    ONLY : tempsEcoule
    USE mod_input_data, ONLY : Nt_laserPsansLaser
    USE mod_input_data, ONLY : n_phi, n_rho1_seg, n_rho2_seg
    USE mod_input_data, ONLY : Nt_periode, save_periodique


    !////////////////////////////////////////////////////////////////
    IMPLICIT NONE                           !
    REAL(kind=d_t)      :: prog_start_time, time_max_second !
    character(len=80)   :: IntermedWaveDir
    !================================================================


CONTAINS

SUBROUTINE begin_count_time()
    IMPLICIT NONE

    prog_start_time = tempsEcoule()                 !

END SUBROUTINE begin_count_time


SUBROUTINE READ_IT_PSI( it,  psi_E1E2 )
    IMPLICIT NONE
    !////////////////////////////////////////////////////////////////////////////////
    INTEGER                             :: it       !
    COMPLEX(kind=d_t), DIMENSION(n_phi,n_rho1_seg,n_phi,n_rho2_seg) :: psi_E1E2 !
    !================================================================================

    integer :: c

    do c = 0, c_max-1
        if( mod(iproc,c_max)==c ) then

            !////////////////////////////////////////////////////////////////////////////////
            OPEN( unit=11,file=concat(trim(IntermedWaveDir),concat(concat('BACK/wave_',str_iproc),'_2p2p2')),&
                            status='old', form='unformatted', MODE='READ'       )
                READ(11) it                             !
                READ(11) psi_E1E2                           !
            CLOSE(11)                                   !
            print*,'iproc,readed it=',iproc, it

        endif

        CALL MPI_BARRIER(MPI_COMM_WORLD,infompi)                    !
        !================================================================================
    enddo
    !================================================================================

END SUBROUTINE READ_IT_PSI


SUBROUTINE WRITE_IT_PSI( it, psi_E1E2 )
    IMPLICIT NONE
    !////////////////////////////////////////////////////////////////////////////////
    INTEGER                             :: it       !
    COMPLEX(kind=d_t), DIMENSION(n_phi,n_rho1_seg,n_phi,n_rho2_seg) :: psi_E1E2 !
    !================================================================================

    integer :: c

    do c = 0, c_max-1
        if( mod(iproc,c_max)==c ) then
            !////////////////////////////////////////////////////////////////////////////////
            OPEN( unit=11,file=concat(trim(IntermedWaveDir),concat(concat('wave_',str_iproc),'_2p2p2')),&
                                        form='unformatted') !
                WRITE(11) it+1          !---- recommence a partir de la prochaine iterat!
                write(11) psi_E1E2                          !
            CLOSE(11)                                   !
        endif

        CALL MPI_BARRIER(MPI_COMM_WORLD,infompi)                    !
        !================================================================================
    enddo

END SUBROUTINE WRITE_IT_PSI


END MODULE mod_save_wave

Answer 1

I am assuming you are using the g++, gfortran, mpif90 toolchain. If you have a module

module rocker
contains
subroutine bye_baby
...
end subroutine

The external C declaration for it in C++ is

extern "C"
{
    //          ,-- 2 Leading underscores to start
    //          | ,-- then the module name
    //          | |     ,-- then _MOD_
    //          | |     |    ,-- then the subroutine name
    //          V V     V    V
    extern void __rocker_MOD_bye_baby();
}

You may also need to add attribute((stdcall)) after the extern. By default, C assumes cdecl which stacks the parameters differently.

If you do not want the __rocker_MOD part, then the subroutine/function should not be declared in a module.