Pass arrays from C/C++ to Fortran and return a calculated array

Question

I am trying to pass an array from C/C++ into a Fortran 2003 module and get the calculated values back into C/C++. I\'ve been able to pass and return single values (scalars)

Answer 1

Before francescalus confirms it, I was going to say that from what I know that was a little bit old, the interoperability does not permit what you are trying to do with arrays. In addition, some good habits are always critical when coding. For example using implicit none in fortran to force the declaration of all variables before they are used. The use of named constant when the language permits it, for example the 2 that you are using as array size in fortran.

Below is a modified version of your code that should do something like what you want to achieve.

//Fortran

module ConvertUnitsLib

use :: iso_c_binding ! for C/C++ interop
!real(c_double), bind(c) :: degF, degC
implicit none

public DegCtoF

contains

!
! Convert temperature degrees Celsius Fahrenheit
!
subroutine DegCtoF(degC, degF, n)&
    bind(c, name = "DegCtoF")

    integer, intent(in) :: n
    real(c_double), intent(in), dimension(n) :: degC
    real(c_double), intent(out), dimension(n) :: degF
    integer :: i

    do i = 1, n
        degF(i) = ( degC(i) * 1.8 ) + 32
    end do

end subroutine DegCtoF

// C++

#include 

#ifdef __cplusplus
extern"C" {
    #endif
    double DegCtoF(double [], double [], const int *);
    #ifdef __cplusplus
}
#endif


/**********************************************************************/


int main(int argc, char *argv[])
{
    const int N = 2;
    printf("C/C++ and Fortran together!\n");

    double DegreesC[N] = {32, 64};
    double DegreesF[N];

    DegCtoF(DegreesC, DegreesF, &N);
    for(int i = 0; i