Subj : Re: returning arguments
To   : comp.programming.threads
From : Sergey P. Derevyago
Date : Fri Jul 01 2005 05:38 pm

David Butenhof wrote:
> > The effect of using an invalid pointer value ... is undefined (On some
> > implementations, it causes a system-generated runtime fault).
> 
> I usually take care to point out that the semantics of casting between
> integer and pointer types itself is not completely defined by the
> standard. The clause you point out is also valid, but generally of less
> concern because it applies only if you dereference the "not really a
> pointer" value. 
>
	Not correct, sorry. The standard clearly stands: "effect of _using_ an
invalid pointer value" and copying *is* using.
	That is even:

 void f(void*);

 void g(int i)
 {
  f((void*)i);  // undefined behavior!
 }

is undefined behavior. It's allowed to assign _to_ pointers containing invalid
pointer values, though:

 void h()
 {
  int* pi=new int(1);
  delete pi;      // now pi contains invalid pointer value

  int* pi2=pi;    // undefined behavior!
  pi=new int(2);  // ok, now pi is valid again
 }

> HOWEVER, this stuff is safe on any mainstream C/UNIX class of system
> because it's always been presumed to be safe by a vast body of extant
> code -- breaking that would be severely limiting no matter how legal it
> is. 
>
	Agreed.
--
         With all respect, Sergey.               http://ders.angen.net/
         mailto : ders at skeptik.net

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