Pointers in Co-array Fortran

module pointers_example
  implicit none

contains
  ! We'll show how pointers work in contrast to values with
  ! 2 subroutines: `zeroval` and `zeroptr`. `zeroval` has an
  ! integer parameter, so arguments will be passed to it by
  ! value. `zeroval` will get a copy of `ival` distinct
  ! from the one in the calling program.
  subroutine zeroval(ival)
    integer, intent(inout) :: ival
    ival = 0
  end subroutine zeroval

  ! `zeroptr` in contrast has an integer pointer parameter.
  ! The `iptr` in the subroutine body can be used to change
  ! the value at the referenced address.
  subroutine zeroptr(iptr)
    integer, pointer, intent(inout) :: iptr
    iptr = 0
  end subroutine zeroptr

  subroutine main()
    integer :: i
    integer, pointer :: p
    
    i = 1
    print *, "initial:", i

    call zeroval(i)
    print *, "zeroval:", i

    ! The `p => i` syntax associates the pointer `p` with the target `i`.
    p => i
    call zeroptr(p)
    print *, "zeroptr:", i

    ! In Fortran, we can print the address of a variable using the loc() function
    print *, "pointer:", loc(i)
  end subroutine main
end module pointers_example

program run_example
  use pointers_example
  call main()
end program run_example

To compile and run this Co-array Fortran program:

$ gfortran -coarray=single pointers_example.f90 -o pointers_example
$ ./pointers_example
initial:            1
zeroval:            1
zeroptr:            0
pointer:    140732920744208

zeroval doesn’t change the i in main, but zeroptr does because it has a reference to the memory address for that variable.

In Co-array Fortran, pointers work differently from some other languages. They are more like references and must be associated with a target before use. The => operator is used for pointer assignment.

The loc() function is used to get the memory address of a variable, which is similar to the & operator in some other languages.

Note that Co-array Fortran is an extension of Fortran and may require specific compiler support. The -coarray=single flag is used here for simplicity, but in a true co-array program, you might use multiple images for parallel execution.

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Pointers in Co-array Fortran