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From: Mikko <mikko.levanto@iki.fi>
Newsgroups: comp.theory
Subject: Re: Every sufficiently competent C programmer knows
Date: Tue, 11 Mar 2025 16:02:49 +0200
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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.

-- 
Mikko