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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 10:53:32 +0300
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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.

-- 
Mikko