[vox-tech] C question: global vs local const

[Micah Cowan]

On Fri, Jan 18, 2002 at 01:42:11AM -0800, Peter Jay Salzman wrote:
> begin Mark K. Kim <markslist@cbreak.org> 
> > On Thu, 17 Jan 2002, Peter Jay Salzman wrote:
> > 
> > > begin Mark K. Kim <markslist@cbreak.org>
> > > > You're initializing K with a variable.  Because globals are calculated at
> > > > compile time,
> > 
> > Since Jeff will sooner or later jump in to correct me, I'll correct myself
> > before that happens:
> > 
> > I didn't meant to say that globals are calculated at compile time -- they
> > can certainly be modified during runtime.  What I meant was that initial
> > values of globals have to be calculated at compiletime -- due to the way
> > they are stored in memory.
>  
> does this apply to static variables too?  i have a situation where a
> function is called many times over:
> 
> void function( ..., long double dr)
> {
> 	long double variable = expensive_calculation * dr;
> 	...
> }

It does apply to static variables.

> i'd like to declare variable as static, since both expensive_calculation
> and dr remain constant through the entire program.
>
> i can't declare dr
> as being global because it depends on other parameters that need to be
> calculated at run time (but otherwise don't change).

What I would do is perform a test at the beginning of function as to
whether variable should be assigned an initial value or not.

  void function ( ..., long double dr)
  {
    static bool inited = false;  /* Need <stdbool.h> for this - it's
                                    C99-specific, so don't #include it
                                    if you intend for it to be 
                                    portable to C90: instead, define
                                    bool and false yourself. */
    static long double variable;

    if (!inited)
    {
      variable = expensive_calculation * dr;
    }

    ...
  }

Micah