How to allocate an array inside fortran routine “called” from C

Question

I think title says what I need. I know we can use \"asd\" function to do this, but for some reasons I need to do the allocation in Fortran (i.e. in subroutine \"asd_\"). Her

Answer 1

If you need a thread safe solution and/or the possibility to deallocate the space from C again, the example below would do the job:

#include 

void test_mem_alloc(float ** array, void **wrapper);
void free_wrapper(void **wrapper);

int main()
{

  float *array;
  void *wrapper;

  /* Allocates space in Fortran. */
  test_mem_alloc(&array, &wrapper);
  printf( "Values are: %f %f\n", array [0], array [1]);
  /* Deallocates space allocated in Fortran */
  free_wrapper(&wrapper);

  return 0;
}

On the Fortran side, you have a general wrapper type CWrapper, which can carry any type of derived type. Latter contains the data you would like to pass around. The CWrapper type accept arbitrary payload, and you would always invoke the free_wrapper() routine from C to release the memory.

module memalloc
  use, intrinsic :: iso_c_binding
  implicit none

  type :: CWrapper
    class(*), allocatable :: data
  end type CWrapper

  type :: CfloatArray
    real(c_float), allocatable :: array(:)
  end type CfloatArray

contains

  subroutine test_mem_alloc(c_array_ptr, wrapper_ptr)&
      & bind(C, name="test_mem_alloc")
    type (c_ptr), intent (out) :: c_array_ptr
    type(c_ptr), intent(out) :: wrapper_ptr

    type(CWrapper), pointer :: wrapper

    allocate(wrapper)
    allocate(CfloatArray :: wrapper%data)
    select type (data => wrapper%data)
    type is (CfloatArray)
      allocate(data%array(2))
      data%array(:) = [2.5_c_float, 4.4_c_float]
      c_array_ptr = c_loc(data%array)
    end select
    wrapper_ptr = c_loc(wrapper)

  end subroutine test_mem_alloc


  subroutine free_cwrapper(wrapper_ptr) bind(C, name='free_wrapper')
    type(c_ptr), intent(inout) :: wrapper_ptr

    type(CWrapper), pointer :: wrapper

    call c_f_pointer(wrapper_ptr, wrapper)
    deallocate(wrapper)

  end subroutine free_cwrapper

end module memalloc