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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: Fri, 17 May 2024 13:53:57 +0300
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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:
>> 
>>> On 5/15/2024 3:18 AM, Mikko wrote:
>>>> On 2024-05-14 14:35:42 +0000, olcott said:
>>>> 
>>>>> On 5/14/2024 4:52 AM, Mikko wrote:
>>>>>> On 2024-05-13 14:43:09 +0000, olcott said:
>>>>>> 
>>>>>>> On 5/13/2024 4:14 AM, Mikko wrote:
>>>>>>>> On 2024-05-13 04:54:30 +0000, immibis said:
>>>>>>>> 
>>>>>>>>> On 10/05/24 19:55, Alan Mackenzie wrote:
>>>>>>>>>> [ Followup-To: set ]
>>>>>>>>>> 
>>>>>>>>>> In comp.theory olcott <polcott333@gmail.com> wrote:
>>>>>>>>>> 
>>>>>>>>>> [ .... ]
>>>>>>>>>> 
>>>>>>>>>> I've tried out your much spammed code on GCC (see below).  It is clear
>>>>>>>>>> you have never built or run this code, which ironically can't reach Line
>>>>>>>>>> 06.  It can't even reach line 00.
>>>>>>>>>> 
>>>>>>>>>>> Richard tried to get away with D never simulated by H as an example
>>>>>>>>>>> of D simulated by H:
>>>>>>>>>> 
>>>>>>>>>>> Message-ID: <v0ummt$2qov3$2@i2pn2.org>
>>>>>>>>>>> On 5/1/2024 7:28 PM, Richard Damon wrote:
>>>>>>>>>> 
>>>>>>>>>>> *That people say they know I am wrong yet will not show the detailed*
>>>>>>>>>>> *steps of how I am wrong indicates that they are probably liars*
>>>>>>>>>> 
>>>>>>>>>> You have said, or at least implied that your code fragment is runnable.
>>>>>>>>>> I think you are the liar, here.
>>>>>>>>>> 
>>>>>>>>>>> 00 int H(ptr x, ptr x)  // ptr is pointer to int function
>>>>>>>>>>> 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 }
>>>>>>>>>> 
>>>>>>>>>> 
>>>>>>>>>> .  These are the diagnostics generated by GCC:
>>>>>>>>>> 
>>>>>>>>> And the halting problem is about Turing machines, anyway.
>>>>>>>> 
>>>>>>>> Not necessarily. The same question can be asked about different machines.
>>>>>>>> The restriction to Turing machines is just one way to ensure that the
>>>>>>>> problem is well defined.
>>>>>>>> 
>>>>>>> 
>>>>>>> Well put. All computable functions even if written in C.
>>>>>> 
>>>>>> You still must ensure that the problem is well defined. With C there is
>>>>>> the problem that conforming C programs often fail to be strictly conforming.
>>>>>> If a program is not strictly conforming it may be interpreted as halting
>>>>>> by some implementation and non-halting or incorrect by another.
>>>>>> 
>>>>> 
>>>>> 00 int H(ptr x, ptr x)  // ptr is pointer to int function
>>>>> 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 }
>>>>> 
>>>>> One can correctly determine that every D simulated by H
>>>>> never reaches past its own line 03.
>>>> 
>>>> That is not strictly conforming and hardly conforming at all.
>>>> 
>>> 
>>> What trivial syntactic error that has no effect
>>> what-so-ever on the semantics do you believe remains?
>> 
>> A trivial sysntactic error means that the program is not strictly
>> conforming. It may be conforming if some liberal compiler accepts
>> it.
>> 
>>> A missing semi-colon?
>> 
>> Normal semantics is that the program is not executed.
>> 
>>> 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.

-- 
Mikko