Re: Error in man page: realloc(ptr, 0) is not equivalent to free(ptr)

["Michael Kerrisk" <mtk.manpages-gM/Ye1E23mwN+BqQ9rBEUg@public.gmane.org>]

Chris,

Ping!  Did you have anything further to add?

Cheers,

Michael

On Tue, Feb 26, 2008 at 4:12 PM, Michael Kerrisk
<mtk.manpages@googlemail.com> wrote:
>
> Chris ク Heath wrote:
> > On Fri, 2008-02-22 at 11:15 +0100, Michael Kerrisk wrote:
> >> On Thu, Feb 21, 2008 at 7:59 AM, Mike Frysinger <vapier@gentoo.org> wrote:
> >>> On Thursday 21 February 2008, Lasse Kärkkäinen wrote:
> >>>  > The man page says that realloc(ptr, 0) is equivalent to free, even
> >>>  > though it isn't. The text on the man page says
> >>>  >
> >>>  > ---
> >>>  > realloc()  changes the size of the memory block pointed to by ptr to
> >>>  > size bytes.  The contents will be unchanged to the minimum of the old
> >>>  > and new sizes;  newly  allocated memory  will  be  uninitialized.   If
> >>>  > ptr  is  NULL,  the call is equivalent to malloc(size); if size is equal
> >>>  > to zero, the call is equivalent to free(ptr).  Unless ptr is NULL, it
> >>>  > must have been returned by an earlier call to malloc(), calloc() or
> >>>  > realloc().  If the area pointed to was moved, a free(ptr) is done.
> >>>  > [...]
> >>>  > realloc()  returns a pointer to the newly allocated memory, which is
> >>>  > suitably aligned for any kind of variable and may be different from ptr,
> >>>  > or NULL if the request fails.  If  size  was equal to 0, either NULL or
> >>>  > a pointer suitable to be passed to free() is returned.  If realloc()
> >>>  > fails the original block is left untouched; it is not freed or moved.
> >>>
> >>>  i would just word it to say that when realloc() is given a size of 0, it is
> >>>  implementation defined as to the behavior, but it tends to match the behavior
> >>>  of malloc(0) (which too is implementation defined).  POSIX and C99 allow both
> >>>  cases to return either a NULL pointer or a "unique" pointer.  glibc will
> >>>  return a unique pointer (which cannot actually be used to store anything),
> >>>  but uClibc may return NULL.
> >>>  -mike
> >> Lasse, thanks for raising this; Mike, thanks for your input.
> >>
> >> For man-pages-2.79, I propose to amend the description of realloc() to be:
> >>
> >>        realloc() changes the size of the memory block pointed to
> >>        by  ptr to size bytes.  The contents will be unchanged to
> >>        the minimum of the old and  new  sizes;  newly  allocated
> >>        memory  will  be uninitialized.  If ptr is NULL, then the
> >>        call is equivalent to malloc(size); if size is  equal  to
> >>        zero, and ptr is not NULL, then the call is equivalent to
> >>        free(ptr).   Unless  ptr  is  NULL,  it  must  have  been
> >>        returned  by  an  earlier  call  to malloc(), calloc() or
> >>        realloc().  If the area pointed to was moved, a free(ptr)
> >>        is done.
> >
> > Hmmm.  The phrase
> >
> >                                            if size is  equal  to
> >        zero, and ptr is not NULL, then the call is equivalent to
> >        free(ptr).
> >
> > seems to contradict the following sentence, found under RETURN VALUES:
> >
> >                               If size was equal to 0, either NULL
> >         or a pointer suitable to be passed to free() is returned.
> >
> > If realloc(ptr, 0) can return a non-NULL pointer, then it isn't
> > equivalent to free(ptr).
> >
> > So which one is correct?  My tests with glibc 2.6 indicate that
> > realloc(ptr, 0) always returns NULL, so it IS equivalent to free(ptr).
> > However, I don't know if that is guaranteed to always be the case.
>
> Chris,
>
> The text under RETURN VALUE is quite close to the POSIX.1 spec for
> realloc(), which says this:
>
>       If size is 0, either a null pointer or a
>       unique pointer that can be successfully passed to  free()
>       shall be returned.
>
> After Mike's last suggestion, the body of the Linux man page says this:
>
>       If ptr is NULL, then the
>       call is equivalent to malloc(size),  for  all  values  of
>       size; if size is equal to zero, and ptr is not NULL, then
>       the call is equivalent to free(ptr).
>
> So:
>
> a) If ptr is NULL, and size is zero, then realloc() is equivalent to
> malloc(0), which does the following (as already documented in the Linux
> malloc.3 man page):
>
>       If size is 0, then malloc() returns either NULL, or a unique
>       pointer value that can later be successfully passed to free().
>
> That seems entirely consistent with the text under RETURN VALUE for the
> Linux malloc.3 man page.
>
> b) If ptr is not NULL, and size is zero, then the standard isn't (AFAICS)
> explicit about whether "a null pointer or a unique pointer that can be
> successfully passed to free()" shall be returned, but the former certainly
> seems more reasonable, and is what you observed for glibc.
>
>
> Cheers,
>
> Michael
>
> --
> Michael Kerrisk
> Maintainer of the Linux man-pages project
> http://www.kernel.org/doc/man-pages/
> Want to report a man-pages bug?  Look here:
> http://www.kernel.org/doc/man-pages/reporting_bugs.html
>
>
>



-- 
Michael Kerrisk
Maintainer of the Linux man-pages project
http://www.kernel.org/doc/man-pages/
Want to report a man-pages bug?  Look here:
http://www.kernel.org/doc/man-pages/reporting_bugs.html