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

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

Maciej Sobczak <see.my.homepage@gmail.com> writes:

> And how the implementation can guarantee it without mandating the
> representation on the C compiler?

The idea is that the Ada compiler writer chooses a particular
implementation of C, and supports interfacing to that.
No need to "mandate" -- just write the Ada compiler so it
mimics what that particular C compiler does.  And then the
Ada compiler writer writes documentation "this Ada implementation
supports interface to the Mumble C compiler, version 1.2.3".
The Ada implementation could support intefacing with multiple C
implementations, but if the Ada implementation claims to support
interfacing to C, it has to support at least one.

> Consider a C compiler that has a switch that selects the
> representation for fundamental types. It is not uncommon.
> I can have *the same* program compiled twice by *the same* compiler
> and the two versions will differ in representation of their
> fundamental types.

That's conceptually two (or more) C implementations.  The Ada compiler
writer would document the switches that must be used on the C side.
Or maybe there would be similar switches on the Ada side,
and the documentation would require them to match.

> How Ada implementation can guarantee anything in this area?

An Ada implementation guarantees that Interfaces.C.C_Float matches the
representation of float chosen by a particular C implementation, such as
version 1.2.3 of the Mumble C compiler.  Not so hard.  And quite
useful.

- Bob