Re: proper realloc(p,0) behavior?

[Jim Meyering]

Paul Eggert wrote:
> On 03/24/2011 10:17 AM, Eric Blake wrote:
>> How should gnulib react?  And are there any existing GNU programs that
>> would break if C99 realloc semantics were enforced?
>
> My kneejerk reaction is that
> programs should be written so as to be portable to
> both C89 (which GNU implements) and C99 realloc.
> Programs that rely on GNU behavior but get C99 behavior
> will have memory leaks, but that's better than crashing.
> GNU programs should not be written to rely on C99 behavior,
> because they may crash.
>
> I don't offhand know of any existing GNU code that expects C99
> behavior and would crash with GNU behavior.
>
> The issue is well worth documenting in the gnulib manual.
>
> It should be fairly easy to write code that is portable
> to both C89 and C99 and does not leak, by using free (p) rather
> than realloc (p, 0).  For example, we could apply the following
> patch to xmalloc.c:
>
> --- a/lib/xmalloc.c
> +++ b/lib/xmalloc.c
> @@ -52,10 +52,18 @@ xmalloc (size_t n)
>  void *
>  xrealloc (void *p, size_t n)
>  {
> -  p = realloc (p, n);
> -  if (!p && n != 0)
> -    xalloc_die ();
> -  return p;
> +  if (n)
> +    {
> +      p = realloc (p, n);
> +      if (!p)
> +        xalloc_die ();
> +      return p;
> +    }
> +  else
> +    {
> +      free (p);
> +      return NULL;
> +    }
>  }
>
>  /* If P is null, allocate a block of at least *PN bytes; otherwise,

Good idea.
Maybe write it like this instead?

  if (n == 0)
    {
      free (p);
      return NULL;
    }

  p = realloc (p, n);
  if (!p)
    xalloc_die ();
  return p;

-------------
or like this?
-------------

  if (n == 0)
    {
      free (p);
      p = NULL
    }
  else
    {
      p = realloc (p, n);
      if (!p)
        xalloc_die ();
    }
  return p;


FYI, I searched for examples with a literal 0, using this:

  http://codesearch.google.com/codesearch?sa=N&q=realloc\+*\%28[^,],\+*0\%29%3B

This is the only significant hit:

    daikon/kvasir/valgrind/massif/tests/realloc.c - 5 identical
    16:   x = realloc(x, 0);            // equivalent to free(x), and ends up
    17:                                 // calling Valgrind's (and Massif's) 
free
    pag.csail.mit.edu/daikon/download/daikon.tar.gz - GPL - C

There are plenty of others, but they're in configure tests.