XML CO538 Anonymous Questions and Answers Keyword Index

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Keyword reference for global-variables

Question 35 (2003):

I have:

    VAL INT state IS 0:

I'm hoping this is like an external field in Java, but I can't work out how to alter the value of state. ``state := 1'' and many other ways I can't get to work. How can I alter the value, or is there another way or storing a value outside of a proc.

Answer 35:

In occam, the only way you can make a variable global to a PROC is to have it in-scope. The `VAL' qualifier in occam effectively means constant, so your `state' isn't actually a variable -- it's a constant equal to zero. Removing the `VAL' won't work either, since occam does not permit variables to be declared at the outermost level.

You can do this sort of thing, but it probably won't work in the way you expect it to.. e.g.:

    PROC thing (...)
      INT state:

      PROC fiddle.state (VAL INT new.state)
        state := new.state
      :

      ...  body of `thing'
    :

In this code, the PROC `fiddle.state' has access to the `state' variable. However, the compiler's alias and parallel usage checking might prevent you from using `state' in the `body of `thing'' -- depending on usage of `fiddle.state'. In short, this is generally not a good way forward.

If you need a state variable for a PROC, just declare it inside the PROC and initialise suitably, e.g.:

    PROC thing (...)
      INT state:
      SEQ
        state := 0         -- default state
        ...  body of `thing'
    :

Only useful if the `body of `thing'' is long-lived, however.

Keywords: global-variables

Question 11 (2003):

I was wondering if it is possible to create a global boolean value that can be changed. I know that you can write:

    VAL BOOL name IS TRUE:

and this can be used by any method, but you can't change this. So is there another version of writing this which will allow me to change it?

Answer 11:

No; the compiler will not allow you to declare variables at the outermost level.

If you REALLY want to do this, you could re-structure your program so that everything is inside the main PROC; for example:

    PROC main (CHAN OF BYTE in, out, error)

      BOOL b:               -- local to main, but
                            -- global to set & clear

      PROC clear ()         -- nested PROCs
        b := FALSE
      :

      PROC set ()
        b := TRUE
      :

      SEQ                   -- body of main
        clear ()
        set ()
    :

But this sort of global variable is a very Bad Thing!

Any attempt to use such a variable in parallel (that is not CREW) will result in the usual compiler errors.

Keywords: global-variables