test_caller_called.f90

program main

  implicit none

  interface
    subroutine to_be_called(arg, nx, ny, arr)
      integer :: arg, nx, ny
      integer, allocatable, intent(out) :: arr(:, :)
    end subroutine to_be_called
  end interface

  print *, "Hello"

  call caller(5, to_be_called)

end program main

subroutine to_be_called(iarg, nx, ny, arr)
  integer :: iarg, nx, ny
  integer, allocatable, intent(out) :: arr(:, :)

  allocate (arr(nx, ny))
  arr = iarg
end subroutine to_be_called

subroutine caller(arg, called)
  integer :: arg
  interface
  subroutine called(arg, nx, ny, arr)
    integer :: arg, nx, ny
    integer, allocatable, intent(out) :: arr(:, :)
  end subroutine called
  end interface

  integer :: nx, ny
  integer, allocatable :: arr(:, :)

  nx = 5;
  ny = 5;

  call called(arg, nx, ny, arr)

  print *, size(arr)
  print *, arr


end subroutine caller