How to copy a fortran pointer value to an array after pointer is assigned

Question

The code is an example of the Fortran main program calls the C function with an integer array pointer return. The problem is I want to assign a Fortran pointer to a Fortran array with C array value, so I let the Fortran pointer to pointed to two targets at the same time. I know it is wrong now. But since the Fortran does not have some operation like *p, how could I do to make the Fortran array has the same value as an assigned Fortran pointer?

the fortran main code is here:

 program Test
    use, intrinsic :: iso_c_binding, only : c_ptr,        &
                    c_f_pointer,  &
                    c_int
  USE FTN_C
  type(c_ptr) :: c_p
  integer(c_int), pointer :: f_p(:)
  integer, target :: Solution(10)

 
  c_p = C_LIBRARY_FUNCTION()
  call c_f_pointer(c_p, f_p, [10]) 
  !c_f_pointer assigns the  target, cptr, to the Fortran pointer, 
  fptr, and specifies its shape.
  
  f_p => Solution ! Solution cannot be pointed
  print *, f_p
  print *, Solution

end program Test

C code is here:

int* C_Library_Function(){
  static int  r[10];
  for (int i = 0; i < 10; ++i){
        r[i] = i;
  }
  return r;
}

The result shows:

 ./a.out
      0      1      2       3       4        5        6        7        8         9
 337928192   1   338811792  1    1073741824  0    337933009    1   338567264      1

FYI the ISO_C_Binding Module code is

  MODULE C_Binding
  USE, INTRINSIC :: ISO_C_BINDING
END MODULE C_Binding 

module FTN_C
INTERFACE
    FUNCTION C_LIBRARY_FUNCTION() BIND(C,& 
                     NAME='C_Library_Function')
        USE C_Binding
        IMPLICIT NONE
        type(C_PTR) :: C_LIBRARY_FUNCTION 
    END FUNCTION C_LIBRARY_FUNCTION
END INTERFACE 
end module FTN_C

Welcome, I suggest taking the [tour]. – Vladimir F Героям слава Sep 23 '21 at 18:37 — Vladimir F Героям слава, Sep 23 '21 at 18:37

Vladimir F Героям слава · Answer 1 · 2021-09-24T07:53:21.537

You never initialized Solution. There is no reason why it should contain some particular values. It is undefined. Just set the values to something meaningful.

The fact, that f_p used to point to Solution is irrelevant.

The title of your question is very peculiar. It mentions some allocation, but there is no allocation in your code sample. All data is static. There must be some misunderstanding on your side. The C function does not allocate anything. It just contains a static local array and returns a pointer to this local array.

First, you make the f_p pointer to point to the Solution array local to the main program. But then you change the pointer to point to the r local array in the C function instead. The pointer no longer points to Solution, it points to the r inside the C function. Solution and r are two different independent static arrays.

If you wanted to return an array from the C function, as you can return from a Fortran one - it is not possible. You can do various other things instead Returning an array using C but you then somehow copy the data from that result to your array that you want to set yourself. You would also be responsible for the memory management of any temporary allocated for the result. Instead, it is much better to pass a pointer to the Fortran array to the C function and let it set the values directly.

Sorry, I think I initialized Solution as a pointer target. So it should contain the same value after f_p =>Solution. Could you explain a little bit more? — HSSH, Sep 23 '21 at 18:55
`Solution` is some static array. You make the pointer to point to it. But then you change the pointer to point to the local array in the C function instead. The pointer no longer points to `Solution`, it points to the `r` inside the C function. `Solution` and `r` are two different independent static arrays. — Vladimir F Героям слава, Sep 23 '21 at 18:58

How to copy a fortran pointer value to an array after pointer is assigned

1 Answers1