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From: Mikko <mikko.levanto@iki.fi>
Newsgroups: comp.theory
Subject: Re: Who here understands that the last paragraph is Necessarily true?
Date: Sun, 14 Jul 2024 11:09:35 +0300
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On 2024-07-13 20:15:56 +0000, olcott said:

> typedef void (*ptr)();
> int HHH(ptr P);
> 
> void Infinite_Loop()
> {
>    HERE: goto HERE;
> }
> 
> void Infinite_Recursion()
> {
>    Infinite_Recursion();
> }
> 
> void DDD()
> {
>    HHH(DDD);
> }
> 
> int main()
> {
>    HHH(Infinite_Loop);
>    HHH(Infinite_Recursion);
>    HHH(DDD);
> }
> 
> Any input that must be aborted to prevent the non
> termination of HHH necessarily specifies non-halting
> behavior or it would never need to be aborted.

Everyone understands that DDD specifies a halting behaviour if HHH(DDD) does,
as can be verified by running

#include <stdio.h>
int main(void) {
  printf("calling HHH(DDD)\n");
  HHH(DDD);
  printf("HHH(DDD) halted\n");
  printf("calling DDD()\n");
  DDD();
  printf("DDD halted\n");
  return 0;
}

-- 
Mikko