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From: Mikko <mikko.levanto@iki.fi>
Newsgroups: comp.theory
Subject: Re: Every sufficiently competent C programmer knows
Date: Wed, 12 Mar 2025 11:32:08 +0200
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On 2025-03-11 19:16:35 +0000, olcott said:

> On 3/11/2025 9:02 AM, 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.
>> 
> 
> It need not those are totally
> irrelevant to the primary point:
> 
> When HHH correctly emulates N steps of the
> above functions none of these functions can
> possibly reach their own "return" instruction
> and terminate normally.

That's almost self-evident. Unless some special fieatures of C are used,
and none of these are used above, no other function can do anything when
one functin does something.

> For HHH(DDD) and HHH(DD) it is stipulated that
> HHH does correctly emulate itself emulating
> these inputs.

But not that the emulation is complete, the emulation is discontinued
when the total step count is N.

-- 
Mikko