Re: xfree() -- set ptr to nil (fwd)

["John R. Moore" <jmoore@cygnus.com>]

Yes, I've usually seen this as a macro

#define XFREE(ptr) do \
{ \
 if (ptr) \
  { \
    free (ptr); \
    ptr = NULL; \
  } \
} while (0)

Ok, then, do we want to replace xfree() with something like XFREE() ?

John   (Can I hide my first post? :-)  )


On Mon, 12 Feb 2001, Kevin Buettner wrote:

> On Feb 12,  3:07pm, John R. Moore wrote:
>
> > Whilst fixing xfree() callsI noticed that xfree() itself has a peculiarity
> > that needs attention:
> >
> > The call goes like this:
> >
> > if (ptr != NULL)
> >   free(ptr);
> >
> > Nice, but why not the following:
> >
> > if (ptr)
> >   {
> >      free (ptr);
> >      prt = NULL);
> >   }
> >
> > The latter catches any re-calls to xfree(), unless the compiler sets the
> > ptr to nil for one (gcc doesn't appear to). Anyhow, it's a good practice
> > to do this anyhow.
> >
> > Any opinions?  The only reason I can think not to is to insure that gdb
> > core dumps on succesive xfree() calls to the same pointer (and hence
> > insure efficient code, but in that case, why bother with xfree() in the
> > first place.
>
> Let me see if I understand you correctly.  You'd like to replace
>
>     void
>     xfree (void *ptr)
>     {
>       if (ptr != NULL)
> 	free (ptr);
>     }
>
> with
>
>     void
>     xfree (void *ptr)
>     {
>       if (ptr)
> 	{
> 	  free (ptr);
> 	  ptr = NULL;
> 	}
>     }
>
> right?
>
> If so, how will this work?  ``ptr'' is a local variable and will not
> be modified outside the scope of xfree().
>
> What you have in mind could be done with a macro and I have seen
> this done in other programs.  (But rather than insuring that gdb
> core dumps on successive xfree() calls, it instead causes gdb to
> core dump when attempting to use an already freed-and-nulled pointer.)
>
> Kevin
>