NetNews Usenet Archive 1992 #16/NN_1992_16.iso/spool/comp/lang/c/11746

Internet Message Format | 1992-07-30 | 1.6 KB

Path: sparky!uunet!mcsun!sun4nl!and!jos
From: [e-mail] (Jos Horsmeier)
Newsgroups: comp.lang.c
Subject: Re: Pointers to freed memory
Message-ID: <[e-mail]>
Date: 30 Jul 92 08:20:58 GMT
References: <[e-mail]>
Organization: AND Software BV Rotterdam
Lines: 33

In article <[e-mail]> [e-mail] (M. J. Saltzman) writes:
|I know that a pointer to freed memory should never be dereferenced,
|but is it standard-conforming to assign or do arithmetic with such
|pointers?
|
|The application is a routine to free the elements of a circular list.
|Since I have a count of the elements, I can simply run a pair of pointers
|around the list, like so:
|
|    for ( p = head, i = 0;  i < n;  i++ ) {
|        q = p;
|        p = p->next;
|        free(q);
|    }
|
|The question comes up because on the last iteration, p->next points
|to the element freed in the first iteration.  Do I need to break the
|circle before I start the loop? Or am I just being paranoid after the
|discussion of pointers to invalid array elements?

Erm, yes, I hate to say it, but you're getting paranoid ;-)

As long as you do not dereference such a pointer (as you indicated
yourself,) everything is fine.  If pointer q contained a valid pointer
value _before_ the call to free(), it'll contain a valid pointer value
_after_ the call. In the last iteration, the value p->next points to
a location in memory where the first element of the list _would have
been_ if it weren't freed in the first iteration. And, again, if you
don't dereference that pointer value, everything is safe and sound.

kind regards,

Jos aka [e-mail]