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From: Mikko <mikko.levanto@iki.fi>
Newsgroups: comp.theory
Subject: Re: Every D(D) simulated by H presents non-halting behavior to H ###
Date: Sun, 19 May 2024 16:48:08 +0300
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On 2024-05-19 12:34:08 +0000, olcott said:

> On 5/19/2024 2:53 AM, Mikko wrote:
>> On 2024-05-18 15:34:36 +0000, James Kuyper said:
>> 
>>> On 5/18/24 09:02, Mikko wrote:
>>>> On 2024-05-17 17:14:01 +0000, olcott said:
>>> 
>>> I recommend ignoring olcott - nothing good ever comes from paying
>>> attention to him.
>>> 
>>>>> On 5/17/2024 5:53 AM, Mikko wrote:
>>>>>> On 2024-05-16 14:50:19 +0000, olcott said:
>>>>>> 
>>>>>>> On 5/16/2024 5:48 AM, Mikko wrote:
>>>>>>>> On 2024-05-15 15:24:57 +0000, olcott said:
>>> ...
>>>>>>>>> typedef int (*ptr)();  // ptr is pointer to int function
>>>>>>>>> 00 int H(ptr x, ptr x);
>>>>>>>>> 01 int D(ptr x)
>>>>>>>>> 02 {
>>>>>>>>> 03   int Halt_Status = H(x, x);
>>>>>>>>> 04   if (Halt_Status)
>>>>>>>>> 05     HERE: goto HERE;
>>>>>>>>> 06   return Halt_Status;
>>>>>>>>> 07 }
>>>>>>>>> 08
>>>>>>>>> 09 int main()
>>>>>>>>> 10 {
>>>>>>>>> 11   H(D,D);
>>>>>>>>> 12   return 0;
>>>>>>>>> 13 }
>>>>>>>> 
>>>>>>>> Can you find any compiler that is liberal enough to accept that?
>>>>>>>> 
>>>>>>> 
>>>>>>> It has been fully operational code under Windows and
>>>>>>> Linux for two years.
>>>>>> 
>>>>>> If your compiler does not reject that program it is not a conforming
>>>>>> C compiler. The semantics according to C standard is that a diagnostic
>>>>>> message must be given. The standard does not specify what happens if
>>>>>> you execute that program anyway.
>>>>>> 
>>>>> 
>>>>> It is not nit picky syntax that is the issue here.
>>>>> The SEMANTICS OF THE C PROGRAMMING LANGUAGE SPECIFIES
>>>>> 
>>>>> No D simulated correctly by any H of every H/D pair specified
>>>>> by the above template ever reaches its own line 06 and halts.
>>>> 
>>>> The standard allows that an program is executed but does not
>>>> specify what happens when an invalid program is executed.
>>> 
>>> You've cross-posted this to comp.lang.c after a long-running discussion
>>> solely on comp.theory. Presumably you're doing that because you want
>>> some discussion about what the standard says about this code. For the
>>> sake of those of us who have not been following that discussion on
>>> comp.theory, could you please identify what it is that you think renders
>>> this code invalid? Offhand, I don't see anything wrong with it, but I'm
>>> far more reliable when I say "I see an error" than when I say "I don't
>>> see an error".
>>> 
>>> 
>>>>> Fully operational software that runs under Widows and Linux
>>>>> proves that the above is true EMPIRICALLY.
>>>> 
>>>> No, it does not. As the program is not strictly comforming
>>>> and uses a non-standard extension some implementation may
>>>> execute it differently or refuse to execute.
>>> 
>>> Which non-standard extension does it use?
>> 
>> The main question is whether both arguments of H on the line 00 can have
>> the same name.
> 
> That was a typo that I did not believe when told because so may people
> continue to lie about the behavior of D correctly simulated by H.

How does the D that is correctly simulated by H different from any
D that is incorrectly simulated by H nor not simulated by H?

-- 
Mikko