reserved fields; was Re: Interfacing with C ; an ununsed fields dilemma

[tmoran@acm.org]

> A lot of C structure are defined with so called unused field
> reserved for future use (oftenly never used in any future). This
> fields are generally to be initialized to zero, what is required, to
> unsure it will not be errorneously interpreted ...

  So such a field is not unused - it's currently a version field whose
value is zero, but in the future it may take non-zero values with
meanings other than version number, so "Version_Number" would not
be an appropriate name. "Reserved", or possibly "TBD", would be
appropriate names.

   type C_Rec is record
      I1       : Integer  := 0;
      Reserved : Integer range 0 .. 0 := 0;
      F1       : Float    := 0.0;
   end record;
   for C_Rec use record
      I1       at 0 range  0 .. 31;
      Reserved at 4 range 32 .. 63;
      F1       at 8 range 64 .. 95;
   end record;