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NNTP-Posting-Date: Mon, 19 Sep 2011 18:17:44 -0500
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Date: Mon, 19 Sep 2011 19:17:42 -0400
From: pete <pfiland@mindspring.com>
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Subject: Re: Testing if a pointer is valid
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.c:10543

Joe Wright wrote:
> 
> On 9/18/2011 20:32, Keith Thompson wrote:
> > cri@tiac.net (Richard Harter) writes:
> > [...]
> >> Any language in which it is possible
> >> for a pointer to be invalid but
> >> which provides no way to test for validity is fundamentally flawed.
> >
> > What existing language has pointers
> > and is not "fundamentally flawed"?
> >
> Indeed. Testing a pointer by simple inspection
> can tell you whether it is null or not.
> If it is null it is not valid.
> If not you simply can't tell.

"null" isn't the last word as to whether or not a pointer is valid.
"validity" is context dependent.

The most invalid pointer value, is an indeterminate pointer value.
You can't do anything with an indeterminate pointer value.

Next on the list, is a null pointer value.
There's nothing wrong with
    int *a = 0;
    int *b = a;
    if (b == a) free(a);

Next on the list, is the one past pointer.
You can do pointer arithmetic with the one past pointer.
    int a;
    int *one_past = &a + 1;
    if (one_past - 1 == &a) one_past[-1] = 5;

And there are also operators, such as greater than (>)
which have their own specifications 
as to what's valid and what isn't.
Given:
    int a;
    int b;
    int *one_past = &a + 1;

This is valid:
    if (one_past > &a) ;

This isn't valid:
    if (&b > &a) ;

-- 
pete