Enforce intent(in) declared variables in Fortran as constant also in called subroutines/functions

Question

In a subroutine or function an input variable can be defined with intent(in) and the compiler assures that within the subroutine the variable can not be altered. As soon as

Answer 1

With sufficient compiler options gfortran generates a warning for your example, that an implicit interface is used.

If you make the interface explicit by placing the subroutines into a module, and use intents for all arguments, gfortran will catch the problem:

module mysubs

contains

subroutine test(i)
    integer, intent(in) :: i
    call doSomethingNasty(i)
end subroutine

subroutine doSomethingNasty(i)
    integer, intent (inout) :: i
    i = 42 ! set the full truth ;-)
end subroutine


end module mysubs


program Test_intent_in

use mysubs

    integer i
    i = 21 ! half the truth
    call test(i)
    write (*,*) "21 expected, but is 42: ", i

end program Test_intent_in

gfortran gives error message:

call doSomethingNasty(i)
                          1
Error: Procedure argument at (1) is INTENT(IN) while interface specifies INTENT(INOUT)

When pass the argument "(i)" you are passing an expression rather than a variable. The expression is not definable and thus should not be used as an actual argument for an "out" or "inout" dummy argument.

Another approach for argument "safety": you can also use the "value" attribute in the declaration of a dummy argument to essentially make a local copy of the argument and guarantee that the actual argument won't be altered.

Edit: As kemiisto pointed out, "contains" also makes the interface known. I don't like "contains" because the variable scoping ... all variables of the parent program are visible. Try this test code out:

PROGRAM contains_tst

  INTEGER :: i, m

  i = 21
  m = 22
  CALL test(m)

  CONTAINS

    SUBROUTINE test(j)
      INTEGER, INTENT(IN) :: j
      write (*, *) i, j
    END SUBROUTINE test

END PROGRAM contains_tst