Re: value of dangling pointer

[Igor Khavkine]

On Wed, Aug 01, 2001 at 01:47:27AM +0300, Kalle Olavi Niemitalo wrote:
> hostmux/mux.c (free_name) does this:
> 
>   free ((char *)nm->name);
>   if (nm->name != nm->canon)
>     free ((char *)nm->canon);
> 
> However, according to the C99 N869 draft, "The value of a pointer
> that refers to freed space is indeterminate" (7.20.3 [#1]), which
> causes undefined behavior if used (3.18 [#1]).  IIRC, the final
> standard has a similar rule.
> 
> Does some undocumented GCC extension make such use valid, or
> should the two statements be swapped?

You have to remember that the function free() cannot modify the value
of its argument (nm->name) since it's only passed by value. So before
or after nm->name has the same value, and so can be compared to
nm->canon since neither of them has been modified.

Igor