Wednesday, November 17, 2021

Digest for comp.lang.c++@googlegroups.com - 8 updates in 1 topic

olcott <NoOne@NoWhere.com>: Nov 16 09:33PM -0600

#include <stdint.h>
typedef void (*ptr)();
 
int H(ptr x, ptr y)
{
x(y); // direct execution of P(P)
return 1;
}
 
int P(ptr x)
{
H(x, x);
return 1;
}
 
int main(void)
{
H(P, P);
}
 
For every H that simulates or executes its input and aborts or does not
abort its input P never reaches its last instruction.
 
 
--
Copyright 2021 Pete Olcott
 
Talent hits a target no one else can hit;
Genius hits a target no one else can see.
Arthur Schopenhauer
wij <wyniijj@gmail.com>: Nov 16 08:10PM -0800

On Wednesday, 17 November 2021 at 11:33:49 UTC+8, olcott wrote:
 
> Talent hits a target no one else can hit;
> Genius hits a target no one else can see.
> Arthur Schopenhauer
 
I would guess yes, it is specified: BAD, illegal program
 
[]$ g++ t.cpp
t.cpp: In function 'int H(ptr, ptr)':
t.cpp:6:2: error: too many arguments to function
6 | x(y); // direct execution of P(P)
| ~^~~
t.cpp: In function 'int main()':
t.cpp:18:3: error: invalid conversion from 'int (*)(ptr)' {aka 'int (*)(void (*)())'} to 'ptr' {aka 'void (*)()'} [-fpermissive]
18 | H(P, P);
| ^
| |
| int (*)(ptr) {aka int (*)(void (*)())}
t.cpp:4:11: note: initializing argument 1 of 'int H(ptr, ptr)'
4 | int H(ptr x, ptr y)
| ~~~~^
t.cpp:18:6: error: invalid conversion from 'int (*)(ptr)' {aka 'int (*)(void (*)())'} to 'ptr' {aka 'void (*)()'} [-fpermissive]
18 | H(P, P);
| ^
| |
| int (*)(ptr) {aka int (*)(void (*)())}
t.cpp:4:18: note: initializing argument 2 of 'int H(ptr, ptr)'
4 | int H(ptr x, ptr y)
| ~~~~^
olcott <NoOne@NoWhere.com>: Nov 16 10:46PM -0600

On 11/16/2021 10:10 PM, wij wrote:
> On Wednesday, 17 November 2021 at 11:33:49 UTC+8, olcott wrote:
 
#include <stdint.h>
#include <stdio.h>
typedef int (*ptr)();
 
int H(ptr x, ptr y)
{
x(y); // direct execution of P(P)
return 1;
}
 
int P(ptr x)
{
H(x, x);
return 1;
}
 
int main(void)
{
H(P, P);
}
 
For every H that simulates or executes its input and aborts or does not
abort its input P never reaches its last instruction.
 
> I would guess yes, it is specified: BAD, illegal program
 
It must be compiled as C and the typedef was incorrect.
I can't find a way to specify the "C" calling convention for g++
 
 
--
Copyright 2021 Pete Olcott
 
Talent hits a target no one else can hit;
Genius hits a target no one else can see.
Arthur Schopenhauer
"Chris M. Thomasson" <chris.m.thomasson.1@gmail.com>: Nov 16 09:12PM -0800

On 11/16/2021 8:46 PM, olcott wrote:
> {
>   H(P, P);
> }
 
 
Stack overflow...
Richard Damon <Richard@Damon-Family.org>: Nov 17 06:53AM -0500

On 11/16/21 11:46 PM, olcott wrote:
 
>> I would guess yes, it is specified: BAD, illegal program
 
> It must be compiled as C and the typedef was incorrect.
> I can't find a way to specify the "C" calling convention for g++
 
Shows you don't understand C++.
 
Making it 'C' calling convention doesn't change the syntax for the call.
 
In C++, an empty parameter list specifies a void parameter list, while
in C it specifies tha mostly obsolete concept that this is a
non-prototype declaration that doesn't specify the parameters.
 
Ultimately the problem is that this runs into the issue that it is
impossible to express in C a function that takes or returns a pointer to
the same type as the function itself is, as that requires self
references in the type definition.
olcott <NoOne@NoWhere.com>: Nov 17 08:33AM -0600

On 11/17/2021 2:28 AM, Mark Bluemel wrote:
>>> }
>> Stack overflow...
 
> Surely a compiler can optimise this to remove the recursion, and for that matter, remove any functionality at all, as the code has no observable behaviour, as far as I can see (I'd be happy to be corrected on this!).
 
This is the question:
For every H that simulates or executes its input and aborts or does not
abort its input does any P ever reach its last instruction?
 
 
 
--
Copyright 2021 Pete Olcott
 
Talent hits a target no one else can hit;
Genius hits a target no one else can see.
Arthur Schopenhauer
olcott <NoOne@NoWhere.com>: Nov 17 08:49AM -0600

On 11/17/2021 5:53 AM, Richard Damon wrote:
> impossible to express in C a function that takes or returns a pointer to
> the same type as the function itself is, as that requires self
> references in the type definition.
 
This code does compile and run correctly.
The only difference from Ben's code is that
void (*ptr)(); was changed to int (*ptr)();
 
In comp.lang.c On 11/11/2021 2:31 PM, Ben Bacarisse:
transformed my syntax into his syntax
 
 
 
 
--
Copyright 2021 Pete Olcott
 
Talent hits a target no one else can hit;
Genius hits a target no one else can see.
Arthur Schopenhauer
Anurag Ranjan <anuragranjan630@gmail.com>: Nov 17 10:00PM

On 17/11/2021 04:46, Wolcott wrote:
> On 11/16/2021 10:10 PM, wij wrote:
>> On Wednesday, 17 November 2021 at 11:33:49 UTC+8, olcott wrote:
 
#include <stdio.h>
 
void count(int n)
{
static int d = 1;
printf("n = %d\n", n);
printf("d = %d\n", d);
 
d++;
 
if (n > 1)
{
count(n - 1);
}
printf("d = %d\n", d);
}
int main()
{
count(5);
return 0;
}
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