How do I pass a fortran pointer to a subroutine?

Question

Because of some reason, I need to pass a Fortran pointer to a subroutine. The subroutine is inside a module and the main program uses this module to ensure explicit interface.

My question is, what attribute should I specify on the dummy argument of the subroutine in order to receive the passed-in pointer?

I tried the code below.

module aaa

contains

integer*4 function print_ptr_arr_1( ptr )

    implicit none

    integer*4, intent(in), pointer :: ptr(:)

    print *, 'as pointer'
    print *, size(ptr)
    print '(10i3)', ptr
    print *

end function print_ptr_arr_1

integer*4 function print_ptr_arr_2( ptr )

    implicit none

    integer*4, intent(in), target :: ptr(:)

    print *, 'as target'
    print *, size(ptr)
    print '(10i3)', ptr
    print *

end function print_ptr_arr_2

integer*4 function print_ptr_arr_3( ptr )

    implicit none

    integer*4, intent(in) :: ptr(:)

    print *, 'as assumed shape array'
    print *, size(ptr)
    print '(10i3)', ptr
    print *

end function print_ptr_arr_3

end module aaa

and

program main

    use aaa

    implicit none

    integer*4 :: i1, ierr
    integer*4, target  :: arr(10)
    integer*4, pointer :: ptr_arr(:)

    do i1 = 1, 10
        arr(i1) = i1
    end do

    ptr_arr => arr

    ierr = print_ptr_arr_1( ptr_arr )
    ierr = print_ptr_arr_2( ptr_arr )
    ierr = print_ptr_arr_3( ptr_arr )

    nullify( ptr_arr )

end program main

Three subroutines in module aaa show correct outputs as below.

sj2734@sonaram:~/work/practice/fortran_pointer$ ./a.out 
 as pointer
          10
  1  2  3  4  5  6  7  8  9 10

 as target
          10
  1  2  3  4  5  6  7  8  9 10

 as assumed shape array
          10
  1  2  3  4  5  6  7  8  9 10

sj2734@sonaram:~/work/practice/fortran_pointer$

What is correct and what are not? Or, are these all correct?

Exascale Exascale · Accepted Answer · 2016-01-23T08:22:55

It depends on how you're going to use what you're passing to the function. Do you need it to be a pointer? Then give it the pointer attribute. Do you need to point to it with another pointer in the function? Then, give it the target attribute. The dummy argument will take on whatever attributes you give it.

If you declare it as a pointer then you can use it as a pointer and allocate/point to new things within the function.

If you declare it as a target then other pointers within the function could point to it, but you wouldn't be able to point to a target with it since it's not a pointer.

If you just want to treat it as an array then you can just declare it as an assumed-size array and treat it like a regular array, but again you won't be able to use it in the same way as if it had the pointer or target attribute.

So they're all right, in a way. It depends on context. However, unless you really need it to be a pointer or target for a specific reason, and you're just treating it as an array then you're best off declaring it to be an assumed-size-array to avoid potential memory issues.

How do I pass a fortran pointer to a subroutine?

1 Answers