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

Vladimir F Vladimir F · Accepted Answer · 2021-09-23T18:35:06

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.

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

1 Answers