Fortran90: present() function doesn't work in subroutines

[isotoper]

I am using Gfortran 4.4.3 on Ubuntu 10.04 operating system.
Can someone explain why the following code works correctly (a):-

program testpresent
real :: a,b,c
call test_present(a,b,c)
contains
subroutine test_present(x1,x2,x3,x4,x5,x6)
real, intent(inout), optional :: x1,x2,x3,x4,x5,x6
write(6,*) present(x1),present(x2),present(x3),present(x4),present(x5),present(x6)
return
end subroutine test_present
end program

works correctly (T T T F F F) while (b):-

program testpresent
real :: a,b,c
call test_present(a,b,c)
end program

subroutine test_present(x1,x2,x3,x4,x5,x6)
real, intent(inout), optional :: x1,x2,x3,x4,x5,x6
write(6,*) present(x1),present(x2),present(x3),present(x4),present(x5),present(x6)
end subroutine test_present

does not. There is nothing in the documentation that says (b) is wrong, and it compiles without error messages.

 

[Future Bruno]

The reason why the code in (b) does not work correctly is because the subroutine containing the call to the present function is not included in the program block. In Fortran, any subroutines containing calls to functions must be declared within the program block, otherwise the program will not be able to access the subroutine and thus will not be able to execute the desired code.