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

[Mike Frysinger <vapier-aBrp7R+bbdUdnm+yROfE0A@public.gmane.org>]

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On Friday 22 February 2008, Michael Kerrisk wrote:
> On Thu, Feb 21, 2008 at 7:59 AM, Mike Frysinger <vapier-aBrp7R+bbdUdnm+yROfE0A@public.gmane.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.

a quick read and it leaves me wondering "what if i call realloc(NULL, 0) ?".  
if i re-read it a few times, i can work out that the documentation, by not 
saying anything about size in the first statement, implies that malloc(size) 
is for all values of size.  but perhaps that should be made explicit ?

... If ptr is NULL, then the call is equivalent to malloc(size) for all values 
of size; ...

otherwise, it looks good to me, thanks
-mike

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