Protected global variables in Fortran

Question

I wonder if there is a way of having a global variable in Fortran, which can be stated as some kind of 'protected'. I am thinking of a module A that contains a list of variables. Every other module or subroutine that uses A can use it's variables. If you know what the value of the variable is, you could use parameter to achieve that it can't be overwritten. But what if you have to run code first to determine the variables value? You could not state it as parameter since you need to change it. Is there a way to do something similar but at a specific point at runtime?

Answer 1

You could use the PROTECTEDattribute in a module. It has been introduced with the Fortran 2003 standard. The procedures in the module can change PROTECTED objects, but not procedures in modules or programes that USE your module.

Example:

module m_test
    integer, protected :: a
    contains
        subroutine init(val)
            integer val            
            a = val
        end subroutine
end module m_test

program test
    use m_test

    call init(5)
    print *, a
    ! if you uncomment these lines, the compiler should flag an error
    !a = 10
    !print *, a
    call init(10)
    print *, a
end program