Have time in a pure manner in Fortran?

Question

I am looking for a pure way to have access to time information. I thought about intrinsic functions and subroutines of standards compiler (date_and_time,cpu_time, system_clock, ltime, ctime, ...), the format do not really matter to me. I also thought about MPI function, but it is the same as intrinsic functions, there are all impure functions.

Here is a minimal example:

elemental subroutine add(message, text)
  ! function add
  IMPLICIT NONE
  character(len=:),allocatable,intent(inout)   :: message
  character(len=*), intent(in)                 :: text
  character(len=1), parameter                  :: nl=char(10)
  character(10) :: time
  ! character(8)  :: date

  ! time= 'hhmmss.sss'
  call DATE_AND_TIME(time) 
  Message= Message//time////text//nl

end subroutine add

and I get a logical error :

Error: Subroutine call to intrinsic ‘date_and_time’ at (1) is not PURE  

Thus, I am wandering if a pure way to have a time information exists, or if it is impossible to have it purely (maybe because it has to use cpu information which, for a reason unknown to me, could be thread-unsafe).

Maybe, a subquestion, could be is there a solution to force the compiler to consider date_and_time pure (or any other function of that kind)?

Answer 1

The answer about a pure manner to get time is no. A function that returns the current time or date, is impure because at different times it will yield different results—it refers to some global state.

There are certainly tricks to persuade the compiler that a subroutine is pure. One is to flat out lie in an interface block.

But there are consequences from lying to the compiler. It can do optimizations which are unsafe to do and the results will be undefined (most often correct anyway, but...).

module m
contains

  elemental subroutine add(text)
    IMPLICIT NONE
    character(len=*), intent(in)                 :: text
    character(len=1), parameter                  :: nl=char(10)
    character(10) :: time
          intrinsic date_and_time

    interface
      pure subroutine my_date_and_time(time)
        character(10), intent(out) :: time
      end subroutine
    end interface

    call MY_DATE_AND_TIME(time) 
  end subroutine add

end module

program test
  use m

  call add("test")
end program

subroutine my_date_and_time(time)
  character(10), intent(out) :: time

  call date_and_time(time)

end subroutine

Notice I had to delete your message because that was absolutely incompatible with elemental.