Re: Every sufficiently competent C programmer knows --- original post appended at end

On 3/16/2025 2:26 PM, Keith Thompson wrote:

Richard Heathfield <rjh@cpax.org.uk> writes:

On 16/03/2025 10:31, Mikko wrote:

False. You didn't say that HHH is a C function. In particular, the
code shown above does not say so.

>
It scarcely qualifies as C.
For example, it begins by a goto this code:
>
__asm__("lea eax, DATA1");
   __asm__("mov Aborted, eax"); __asm__("lea eax, DATA2");
   __asm__("mov execution_trace, eax");
   __asm__("mov eax, END_OF_CODE"); __asm__("mov End_Of_Code, eax");
>
which any C compiler is free to reject.
C99 introduced the asm keyword, but that's spelled asm, not __asm__,
and of course it's not a magic wand, so it can't make an inherently
unportable program work on every platform supported by C compilers.
C/370, for example, would have a fit.

 
No, C99 didn't introduce the asm keyword.  Both C90 and C99 (and all
later editions) document the "asm" keyword as a common extension, but
it's not in the list of keywords.  K&R1 (1978) mentions that some
implementations reserve "fortran" and "asm".  A conforming C compiler
must accept "asm" as an ordinary identifier.
I don't think Olcott intends HHH to be fully portable C (assuming he
knows what that means).  In any case, his claims about "Every
sufficiently competent C programmer" are ludicrous.
 

*Every sufficiently competent C programmer knows*
 
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;
}
 
When HHH correctly emulates N steps of the above functions none of them
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.