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From: olcott <polcott333@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: Can someone please verify the execution trace of this?
Date: Sat, 18 May 2024 23:58:10 -0500
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On 5/18/2024 11:25 PM, Rosario19 wrote:
> On Sat, 18 May 2024 16:40:45 -0500, olcott  wrote:
> 
>> People are saying that they have no idea what this code does
>> because they do not believe it conforms to c11 or c17.
>>
>> typedef int (*ptr)();  // ptr is pointer to int function
>> 00 int H(ptr x, ptr y);
>> 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 }
> 
> here you call H(D,D) D is just a pointer, but what is H? if it is a
> ponter for me the program is not definited and has to stop here
> im not one expert of C
> 

I explain that below in the part that you skipped.

Every element of an infinite set of H/D pairs matching a template
where H correctly simulates 1 to ∞ steps of D thus including
0 to ∞ recursive simulations of H simulating itself simulating D.

My claim is that D correctly simulated by H cannot possibly
reach its own line 06 and halt.

>> In the above case a simulator is an x86 emulator that correctly emulates
>> at least one of the x86 instructions of D in the order specified by the
>> x86 instructions of D.
>>
>> This may include correctly emulating the x86 instructions of H in the
>> order specified by the x86 instructions of H thus calling H(D,D) in
>> recursive simulation.
>>
>> *Execution Trace*
>> Line 11: main() invokes H(D,D);
>>
>> *keeps repeating* (unless aborted)
>> Line 01:
>> Line 02:
>> Line 03: simulated D(D) invokes simulated H(D,D) that simulates D(D)
>>
>> *Simulation invariant*
>> D correctly simulated by H cannot possibly reach past its own line 03.
>>
>> The key thing to note is that no D correctly simulated by any H of every
>> H/D pair specified by the above template ever reaches its own line 06
>> and halts.
> 

-- 
Copyright 2024 Olcott "Talent hits a target no one else can hit; Genius
hits a target no one else can see." Arthur Schopenhauer