Re: auto const ints and pointer issue

[Andrew Haley <aph at redhat dot com>]

Karen Shaeffer wrote:
> On Tue, Jun 17, 2008 at 11:01:31AM -0700, Ian Lance Taylor wrote:
>>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>> ~~~~~~~~~~~~~~~~ output ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>> $ const_ints 
>>> const int ic = 0   *cip = 5   *ip = 5
>>> &ic = 0xbfbd72a0    cip = 0xbfbd72a0    ip = 0xbfbd72a0
>>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>
>>> The global variable code would segfault, as I expected. But the
>>> auto variable code gives this illogical result. I would appreciate
>>> comments. I am not on this list, so please ensure I am cc'd with
>>> any responses. I'll be happy to file a bug, if it is a bug.
>> Modifying a variable which is declared const is undefined behaviour.
>> You can not predict what happens.

> I can live with that. My problem was that the addresses cannot
> be correct. In my opinion, the undefined behavior should be
> limited to the value in the address or in some form of error.

Your opinion about undefined behaviour is not shared by the C
Standard committee: undefined code may do anything.  As the
saying goes, "Demons might fly our of your nose."

> But to let the buggy code execute with addresses that are not
> accurate is a liberty I would hope could have been avoided. It
> just looks bad. I do realize, no one should have a gripe, because
> the code is buggy to begin with. But addresses should always be
> reported accurately IMHO. Of course, I obviously know nothing
> about compilers. (smiles ;)

For what it's worth, this is a common misunderstanding.  A proper
understanding of the true meaning of "undefined behaviour" comes
later.

Andrew.