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From: Richard Heathfield <rjh@cpax.org.uk>
Newsgroups: comp.theory
Subject: Re: Every sufficiently competent C programmer knows
Date: Tue, 11 Mar 2025 14:13:34 +0000
Organization: Fix this later
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On 11/03/2025 14:02, Mikko wrote:
> On 2025-03-11 13:26:55 +0000, olcott said:
> 
>> On 3/11/2025 5:01 AM, Richard Heathfield wrote:
>>> On 11/03/2025 08:55, Fred. Zwarts wrote:
>>>> Op 11.mrt.2025 om 00:41 schreef olcott:
>>>>> typedef void (*ptr)();
>>>>> int HHH(ptr P);
>>>>>
>>>>> void Infinite_Loop()
>>>>> {
>>>>>    HERE: goto HERE;
>>>>>    return;
>>>>> }
>>>>>
>>>>> void Infinite_Recursion()
>>>>> {
>>>>>    Infinite_Recursion();
>>>>>    return;
>>>>> }
>>>>>
>>>>> void DDD()
>>>>> {
>>>>>    HHH(DDD);
>>>>>    return;
>>>>> }
>>>>>
>>>>> int DD()
>>>>> {
>>>>>    int Halt_Status = HHH(DD);
>>>>>    if (Halt_Status)
>>>>>      HERE: goto HERE;
>>>>>    return Halt_Status;
>>>>> }
>>>>>
>>>>> That when HHH correctly emulates N steps of the
>>>>> above functions that none of these functions can
>>>>> possibly reach their own "return" instruction
>>>>> and terminate normally.
>>>>>>
>>>>>
>>>>> Since HHH does see that same pattern that competent
>>>>> C programmers see it correctly aborts its emulation
>>>>> and rejects these inputs as non terminating.
>>>>
>>>> All competent C programmers see that HHH correctly reports 
>>>> that it cannot possibly reach the 'return' instruction.
>>>
>>> First, my credentials. I've been programming in C for over 35 
>>> years; I'm told that my book on C has been used on two 
>>> undergraduate Comp Sci courses (one in the States and one in 
>>> the UK); and I have my Knuth cheque. I don't claim to be any 
>>> kind of programming guru, but I hope I do not overstate the 
>>> case when I suggest that I can be regarded as competent not 
>>> just as a programmer but specifically in the C language.
>>>
>>> And yet I can't even /see/ HHH, let alone judge what it does 
>>> or does not do correctly. All I see is a call to it.
>>
>> It is stipulated that HHH correctly emulates N
>> steps of the x86 machine code of its input functions.
>> This may or may not include HHH emulating itself
>> emulating an input.
> 
> The stipulation does not include the value of N (although a 
> reasonable
> interpretation is that it is finite). Nor does the stipulation 
> specify
> what HHH does after the emulation.

Nor does the stipulation hold any water.

-- 
Richard Heathfield
Email: rjh at cpax dot org dot uk
"Usenet is a strange place" - dmr 29 July 1999
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