Passing an array from fortran to a C++ function

Question

I have a main program in Fortran. I am using Intel Visual Fortran XE 2011 on Visual Studio 2010. I would like to use a function which is coded in C++. The function I'm using is getting several arrays (input - set from the main fortran program) and use them to form an output array (to be returned to the main fortran program). I've taken the following steps:

1)I created a Fortran project with the Fortran main program and module and I set it as "startup project".

2)I created a C++ project of type "static library".

3)I added $(IFORT_COMPILERvv)\compiler\lib\ia32 as explained here http://software.intel.com/en-us/articles/configuring-visual-studio-for-mixed-language-applications

The C++ static library is build with no problem. The errors I get is about the declaration of the real(8) variables in the fortran program.

I get the following two errors for all real(8) declarations, i.e. 6 errors in total:

error #5082: Syntax error, found '(' when expecting one of: :: %FILL , TYPE BYTE CHARACTER CLASS DOUBLE DOUBLECOMPLEX DOUBLEPRECISION ...

error #5082: Syntax error, found '::' when expecting one of: ( * , ; [ / = =>

Here is the code I used:

Main Fortran Program:

Program Fort_call_C

use iso_c_binding

implicit none

interface 

   subroutine vec_sum_c(a,b,c) bind (C, name = "vec_sum_c")

      use iso_c_binding

      implicit none

      real(8) (c_double), intent (in), dimension (*) :: a,b
      real(8) (c_double), intent (out), dimension (*) :: c

   end subroutine get_filled_ar

end interface  

integer:: i
integer (c_int)::m
real(8)(c_double),dimension(:):: a, b, c

open(unit=10, file="input_arrays.txt",status="unknown")
read(10,*) m
allocate(a(m),b(m),c(m))

do i=1,m
   read(10,*)a(i),b(i)
end do
close(10)

call vec_sum_c(m,a,b,c)

do i=1,m
   print*, c(i)
end do

pause

end program

And the C++ function is:

extern"C" void vec_sum_c(int *m, double *a, double *b, double *c){
    int mm = *m;
    for(int i=0;i<=m-1;i++){
        c[i]=a[i]+b[i];
     }
}

Could anybody please help me with this issue? And would you please let me know if the idea of sending a whole array from a fortran program to a c++ routine is a safe or problematic (better-to-be-avoided) attempt?

IanH · Accepted Answer · 2013-05-02 05:43:45Z

2

Your Fortran syntax is out. You have the real kind twice. Try

REAL(C_DOUBLE), INTENT(IN), DIMENSION(*) :: a, b

etc.

C_DOUBLE is a named constant. It happens to have the value 8 with that processor.

Also:

you are missing the argument m in the Fortran interface body for the C function.
you change your mind about the name of the subroutine in the Fortran interface body between the opening and closing statement!
Your C++ for loop less than equal compares against m, that should probably be mm.

There are no inherent problem sending whole arrays in this manner.

answered May 2, 2013 at 5:43

IanH

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4 Comments

OmidKarr Over a year ago

Many thanks for your complete answer. Now that I made all these changes, I am getting a new error which reads: "error #8520: An array dummy argument of a BIND(C) procedure must be an explicit shape or assumed size array. [A]" and the same errors for [B] and [C]. Does this mean I can not use allocatable arrays when binding with C? cause I am pretty sure I have seen samples where the same thing has been done. Any clue? Thank you

IanH Over a year ago

You must have changed the argument declarations from what is shown in the code in your question. Using allocatable or deferred shape dummy arguments with BIND(C) functions requires a Fortran 201X compiler, where X is probably >= 5. ifort isn't (yet) that sort of compiler. Perhaps try again in a couple of years time.

OmidKarr Over a year ago

thanks for your answer. I managed to solve the problem. Just another question. If I want to pass a scalar value (a number) which has been obtained in the c++ procedure back to fortran, how do I do that? (for example, in the code above, asume that I want to change the value of m in the c++ function and send it back to fortran.) thank you

Vahid Over a year ago

I am trying to do the same thing, but I have had no luck yet. I was wondering to know how you solved the issue.

Kara · Accepted Answer · 2013-07-26 21:28:26Z

I had only managed to pass the value of a varible from C function to fortran function.

I have here pasted the two source files namely main.c and fortran.f You can use these two files in microsoft visual studio 10. After doing all the settings in visual studio as suggested in page http://software.intel.com/en-us/articles/configuring-visual-studio-for-mixed-language-applications, you need to make another change as;

go to the project property of C/C++ static library ;
go to C/C++
go to Code Generation
set the Runtime Library to Multi-threaded Debug (/MTd)

now you can build the program....

main.c:

#include <stdio.h>
#include <malloc.h>

void testc(double **pa, double **p)
{
 double b;
 double *a, *c;
 int m;

 c = (double*) malloc(sizeof(double));
 *c = 10;

 *p = c;

 a = (double*) malloc(sizeof(double)*5);
 a[0]=1.23;
  a[1]=2.46;
a[2]=3.69;
 a[3]=4.11;
 a[4]=7.21;
 *pa=a;
 for (m=0;m<5;m++)
 {
    b=a[m];
    b=b+1.0;
    a[m]=b;
  }
 }

fortran.f:

 program test
  use iso_c_binding
  implicit none
 interface
  subroutine testc(pa, m) bind(c)
  use iso_c_binding
   type(c_ptr):: m
   type(c_ptr):: pa
  end subroutine testc
  end interface

  type(c_ptr) :: pa
  type(c_ptr) :: m
  real(c_double),pointer::fpa(:)
   real(c_double),pointer::fm(:)

    call testc(pa,m)
     call c_f_pointer(pa, fpa, [5])
     call c_f_pointer(m, fm, [1])
     print *, fm(1)
    print*, fpa(1)
   pause

 end program test

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