Re: Ada array vs C pointer (call by reference)

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

Adam Beneschan <adam@irvine.com> writes:

> On Jun 28, 10:52 am, Robert A Duff <bobd...@shell01.TheWorld.com>
> wrote:
>> Adam Beneschan <a...@irvine.com> writes:
>> > On Jun 27, 3:14 pm, Keith Thompson <ks...@mib.org> wrote:
>> >> It doesn't *mandate* the representation used by the C compiler; the
>> >> author of the Interfaces.C implementation has to *know* the
>> >> representation used by the C compiler.
>>
>> > Which is, of course, impossible without a crystal ball.
>>
>> No need for crystal balls.  Just read the documentation of the C
>> compiler.
>
> What do you mean by "the" C compiler?

The one the compiler writer chooses to support, for interfacing.

Or more than one, if the compiler writer so chooses.

>...My impression was that more
> than one C compiler exists in the world.  And even for particular
> target architectures, more than one C compiler exists for many of
> these.  How is the Ada compiler supposed to know which one you're
> using, without a crystal ball?

The supported C compiler(s) should be documented.  If you use some other
C compiler, you're not playing by the rules of the game.

It's the same with interfacing to hardware -- if I give you an Ada
compiler that generates code for an x86, and you try to run programs
on a SPARC, it won't work.  That should not be a surprise!  ;-)

>...Some people seem to think the Ada compiler will
> know how the C compiler works, ...

Yes, of course it will.  It won't know how ALL C compilers in the world
work (of course), but it will know about the one (or ones) that are
supported.

>... and some seem to go far enough to say
> that the Ada compiler should be able to *guarantee* that types in
> Interfaces.C will have the same representation, and that the RM
> requires this.

Yes, the RM requires this.  But you have to obey the Ada compiler's
documentation.  If it says "compile the C part of your program with gcc
version xxx using so-and-so switches", and use some other C compiler, or
some other switches, it might not work.

> I also don't think there's any requirement for Ada to dig around
> external files to figure out what the representations are.  I checked
> a C book we had lying around, and while it gave no specific definition
> for the representations of "int", "float", etc., it did say that the
> boundaries of those types are available in <limits.h>.  This is (or
> was) apparently part of the ANSI standard.  Does this mean that an Ada
> compiler has to read <limits.h> [assuming it knows what the default
> #include directory is] to find information about the types used by the
> C compiler?  I really, really do not think there is any such RM
> requirement.

The compiler writer can read limits.h, just as well as any other
documentation.

>> This whole argument started because somebody wanted to use Float instead
>> of C_Float.  But there is nothing in the Ada RM saying that Float should
>> correspond to anything in particular.  There IS something in the RM
>> saying that C_Float corresponds to float as implemented by the C
>> compiler(s) that the Ada compiler claims to support interfacing to.
>> And the Ada compiler documentation will tell you which C compiler(s)
>> are supported.
>
> Yeah, I think those last two sentences are the key here.

OK.  So what is the confusion?

- Bob