Re: Every D(D) simulated by H presents non-halting behavior to H ###

On 5/18/24 12:32 PM, James Kuyper wrote:

On 5/18/24 11:44, Richard Damon wrote:

On 5/18/24 11:34 AM, James Kuyper wrote:

On 5/18/24 09:02, Mikko wrote:

On 2024-05-17 17:14:01 +0000, olcott said:...>>>> Fully operational

software that runs under Widows and Linux

proves that the above is true EMPIRICALLY.

 You cannot prove that code is strictly conforming empirically. It might
simply happen to work on every system you've tried it on. Have you ever
tested it on a system where code and data pointers are different sizes?
 

No, it does not. As the program is not strictly conforming
and uses a non-standard extension some implementation may
execute it differently or refuse to execute.

>
Which non-standard extension does it use?

>
I think the issue is the casting of a pointer to function to a pointer
to object, which is one of the grey areas in the standard. (which occurs
in code not shown)
>
It is not specified that such a cast is allowed, but it also isn't
specifically disallowed, it is just omitted as a case in the listing of
te possibilities for casting.

 Such code has undefined behavior "by the omission of any explicit
definition of behavior." (4p2).
Strictly conforming programs cannot have undefined behavior (4p5).

Undefined by the C Standard, but likely defined by the implementation.
Most programs don't need to be "Strictly Conforming" and most actual practical programs are not, because they use some aspect of implementation defined behavior.