Passing allocatable array from fortran to C and malloc it

Question

I am converting some of our Fortran library code into C so that we finally have C functions with Fortran wrappers around it. We are using Fortran 95.

What is the preferred way to handle allocatable arrays?

1. Can I pass an allocatable array to a C function and malloc it inside the C function? (The C function knows the size to be malloc'ed)

2. In the fortran program, can I deallocate something that was malloced in a C function? So finally either the client fortran application deallocates the array or is required to call a C function that frees the memory.

A small example or a link to one would be highly appreciated.

Answer 1

In Fortran 95 you can't "pass" allocatable arrays as an allocatable thing to anything, including Fortran procedures.

In Fortran 2003, the C function can malloc storage for the array, and then return that to the Fortran side as a C_PTR from the ISO_C_BINDING intrinsic module. The storage pointed to by the C_PTR can then be accessed using a Fortran POINTER and the C_F_POINTER procedure from the ISO_C_BINDING module.

To free the storage for the array, the Fortran side would again call into a C procedure, passing the C_PTR, which the C function then uses in a call to free.

#include "stdlib.h"
int *create_storage()
{
   /* Array of four integers. */
   return malloc(sizeof(int) * 4);
}

void destroy_storage(int *ptr)
{
   free(ptr);
}


PROGRAM fortran_side
  USE, INTRINSIC :: ISO_C_BINDING, ONLY: C_PTR, C_F_POINTER, C_INT
  IMPLICIT NONE
  INTERFACE
    FUNCTION create_storage() BIND(C, NAME='create_storage')
      USE, INTRINSIC :: ISO_C_BINDING, ONLY: C_PTR
      IMPLICIT NONE
      TYPE(C_PTR) :: create_storage
    END FUNCTION create_storage
    SUBROUTINE destroy_storage(p) BIND(C, NAME='destroy_storage')
      USE, INTRINSIC :: ISO_C_BINDING, ONLY: C_PTR
      IMPLICIT NONE
      TYPE(C_PTR), INTENT(IN), VALUE :: p
    END SUBROUTINE destroy_storage
  END INTERFACE
  TYPE(C_PTR) :: p
  INTEGER(C_INT), POINTER :: array(:)
  !****
  p = create_storage()
  CALL C_F_POINTER(p, array, [4])   ! 4 is the array size.
  ! Work with array...
  CALL destroy_storage(p)
END PROGRAM fortran_side

In Fortran 201X, C header files and functions may be provided to allow C to work directly with Fortran allocatable variables.