Re: Keyword "null" Question

[Mark Johnson <mark_h_johnson@raytheon.com>]

Michael Bustillo wrote:
> 
> While reviewing the Ada LRM, it makes note that the Ada keyword "null"
> does nothing except pass to the next statement.  This seems intuitive
> enough, however...
> 
> I'm currently attempting to reverse engineer some code so that it can be
> converted to another language, and I've run across the following:
> 
>     if SOME_EVENT_TOOK_PLACE then
>         null;
>     end if;
> 
> This statement is simply standing alone by itself, not part of any
> "case-like" structure or anything sneaky.  I guess my question is when
> it says "...it does nothing..." does it actually mean that it is putting
> a NO OP there?  Because then this would actually be doing something,
> most likely a timing issue, and that's what I need to know.

Hmm. Actually in 5.1(13) I read...
  The execution of a null_statement has no effect.
so the NO OP is not required (and I could argue should not even appear).
Without knowing what "SOME_EVENT_TOOK_PLACE" does, it appears to me that
this code is calling the function and discarding the return value (to
avoid the generation of a local variable).

If it is a timing issue - it appears to be a quite subtle one (and
fraught with error in the case of translation). However, now that I
think about it some more, the code may have been present to provide a
"hook" for a breakpoint. You could break on the null statement to then
walk through the condition you are trying to debug.
  --Mark