Re: [Qemu-devel] [PATCH] loader: don't call realloc(O) when no symbols are present

[malc <av1474@comtv.ru>]

On Thu, 21 Jan 2010, Jamie Lokier wrote:

> Markus Armbruster wrote:
> > malc <av1474@comtv.ru> writes:
> > 
> > > On Tue, 29 Dec 2009, Jamie Lokier wrote:
> > >
> > >> malc wrote:
> > >> > On Mon, 28 Dec 2009, Jamie Lokier wrote:
> > >> > 
> > >> > > Aurelien Jarno wrote:
> > >> > > > This fixes the loading of a stripped kernel with zero malloc disabled.
> > >> > > 
> > >> > > *Raises an eyebrow*
> > >> > > 
> > >> > > Even though there's different perspectives over whether qemu_malloc(0)
> > >> > > should be allowed, inherited from ambiguity over malloc(0),
> > >> > > realloc(p,0) has always had a standard, well-defined meaning.
> > >> > 
> > >> > No.
> > >> > http://groups.google.com/group/comp.std.c/browse_thread/thread/4e9af8847613d71f/6f75ad22e0768a0b?q=realloc++group:comp.std.c#6f75ad22e0768a0b
> > >> 
> > >> Wow, thanks for that.  It's a real surprise.  Looks like C99's own
> > >> rationale is not consistent with itself on the subject, and differs
> > >> from C90 where the "standard, well-defined meaning" I referred to was
> > >> defined.
> > >
> > > Yep.
> > 
> > No, this is a misinterpretation of the C99 standard, made possible by
> > its poor wording.  The C99 Rationale is perfectly clear, though:
> > 
> >     7.20.3.4 The realloc function
> > 
> >     A null first argument is permissible.  If the first argument is not
> >     null, and the second argument is 0, then the call frees the memory
> >     pointed to by the first argument, and a null argument may be
> >     returned; [...]
> 
> The rationale above does not match C89 behaviour.  It says the call
> frees the memory, but it does not forbid the call from then proceeding
> to do the same as malloc(0) and return a non-NULL pointer.  It's quite
> explicit: a null argument *may* be returned.  Which means the
> rationale does not require realloc(p,0) to do the same as C89, which
> always frees the memory and doesn't allocate anything.
> 
> > This is hardly surprising, because anything else would break working C89
> > programs, and that would squarely contradict the standard's mission,
> 
> Understood.  But it doesn't really matter what's intended or what's
> misinterpreted.  If there are any significant implementations out
> there based on the "misinterpretation", or even based on the
> rationale, that's enough of a reason to not depend on realloc(p,0).
> 

My sentiment exactly.

An example:

Dinkum Unabridged Library was certified by Perennial 
(http://peren.com/pages/aboutus_set.htm) to conform
to ISO/IEC 9899:1999.

Documentation for realloc:
http://www.dinkumware.com/manuals/?manual=compleat&Search=realloc&page=stdlib.html#realloc

Hallvard B Furuseth analysis fully applies i believe...

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