Re: Should representation clauses be complete for each bit?

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Georg Bauhaus <rm.dash-bauhaus@futureapps.de> writes:

> More specifically, if the original example of "not all bits used and some
> gaps between them" is unusual, is it so
>
> - when mapping to a set of hardware pins, say?

Whenever I see documentation for such things, they have rectangular pictures
showing what every bit means.  They don't have implicit gaps.  If there
are any gaps, they are shown explicitly, as "must be zero" or "reserved
for future hardware extensions" or whatever.

My claim is that the Ada code interfacing to such a thing should be
similarly explicit about the gaps.

Otherwise, you end up with nasty bugs: one compiler just happens to
set the gaps to zero, and you write to a file, and binary comparisons
of those files work.  Then you port to a different compiler, and it
puts arbitrary junk in the gaps.

> - or when "interfacing" to C structs?
>
> For C structs, declaration order seems very convenient, as C keeps
> declaration order.  But C may well introduce gaps between structure
> components whenever its alignment rules requires.  Then I will want
> gaps in Ada, too.  Is a convention pragma sufficient, then?

Right, pragma Convention(C) will properly interface to a C struct,
so long as the Ada record matches the C struct component-for-component,
and you're using a C compiler supported (for interfacing) by the Ada
compiler -- it's the Ada compiler's job to make the layout match
what that particular C compiler does.  And Convention(C) will prevent
reordering of components.

- Bob