Re: Undefined behavior or not?

[Martin Sebor <msebor at gmail dot com>]

On 03/10/2017 09:12 AM, Segher Boessenkool wrote:
> On Fri, Mar 10, 2017 at 11:57:28PM +0800, Liu Hao wrote:
> > > So does this mean, that the above C code uses undefined behavior?
> > Yes.
> >
> > 6.2.4 Storage durations of objects
> > 1 ... Allocated storage is described in 7.22.3.
> > 2 ... The value of a pointer becomes indeterminate when the object it
> > points to (or just past) reaches the end of its lifetime.
> >
> > 7.22.3 Memory management functions
> > 1 ... The lifetime of an allocated object extends from the allocation
> > until the deallocation. ...
>
> But a null pointer does not point to an (allocated) object.  So the
> code has undefined behaviour only if ever the pointer is not a null
> pointer?

That's right.  Same as in:

    char *p = 0;

    if (rand () & 1) {
        char c;
        p = &c;
    }

    if (p)
      puts ("p i non-null");
    else
      puts (p is null");

Martin