Re: Possible compiler bug with this simple program

["Randy Brukardt" <randy@rrsoftware.com>]

"Niklas Holsti" <niklas.holsti@tidorum.invalid> wrote in message 
news:48b65b3b$0$25384$4f793bc4@news.tdc.fi...
...
> In fact, when an Ada subprogram has an unconstrained array parameter with 
> Convention C, it seems to me that the subprogram's body cannot make any 
> use of individual elements of the array, because it doesn't know the index 
> range, so the compiler should reject any indexing of such an array 
> parameter, as well as any attempt to pass it on as a Convention Ada 
> parameter.

This is the subject of AI05-0002-1. (It was carried over from the Ada 95.) 
[Now, I have to go look this one up because I don't remember anything about 
what we decided...] Ah, yes:

"We do not require support for C convention interfacing pragmas for 
unconstrained
array objects, unconstrained array function results, and most unconstrained
array parameters."

In particular, "An implementation need not support ... an Export or 
Convention pragma applied to a subprogram which has a parameter of an 
unconstrained array subtype;". The wording goes on to include unconstrained 
array objects and function results as well.

Note that an implementation *can* support this if it wants; some 
implementations do implement this with various meanings (Tucker reported 
that their compiler gives the array maximum bounds) and it was thought to be 
bad to break user programs that depend on such behaviors. But if it does 
support it, it ought to do something sensible (raising random exceptions 
doesn't count). (Also note that it is required to support pragma Import in 
this case, as C doesn't care about the bounds and they can just be dropped.)

> Conclusion: Your program tries to do something that cannot possibly work, 
> but the compiler should have told you so.

Well, not necessarily (see Tucker's implementation, for instance). But 
either it should do something defined *or* reject it at compile-time. 
(Janus/Ada would have rejected the Convention pragma.) In any case, it is 
not required to support this in any useful way, and, as it is not portable, 
it should be avoided.

                                       Randy.