On Fri, 2 Sep 2011, Robert Dewar wrote: > On 9/2/2011 8:58 AM, Arnaud Charlet wrote: > > > > In Ada, it's quite natural to end up with a dynamically sized object of > > > > size 0. For instance, if you declare an array with a dynamic bound: > > > > > > > > Table : Unit_Table (1 .. Last_Unit); > > > > > > > > and Last_Unit happens to be 0 at run-time > > > > > > But are we expected to read/store from the storage? > > > > No, that shouldn't happen, although you can e.g. reference Table'Address > > and expect it to be non null. > > Actually I am not sure of this, I discussed this with Bob, Address > is defined as the pointing to the first storage unit allocated for > an object. Not clear what this means when the object has no storage > units. This is a gap in the RM. Bob's view is that it must return > some random valid address (what exactly *is* a valid address?) > > > > > I'd have > > > expected that alloca (0) returning NULL shouldn't break > > > anything at runtime ... > > > > Not sure exactly what failed here, probably something relatively subtle > > (perhaps related to passing this variable or a "slice" of this variable > > to another procedure). > > But that wouldn't cause a dereference, however, it might cause an > explicit test that the argument was not null, and perhaps that's > what is causing the trouble. > > For example, if you have something like > > type S is aliased array (1 .. N); > type P is access all S; > B : S; > > procedure Q is (A : not null Astring) is > begin > null; > end; > > Q (B'Access); > > Then there will be an explicit check that B is not null
The bootstrap failure showed NULL pointer dereferences (which probably easily points to the affected part of the RTS). Richard.
