From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43)
	id 1NGyRY-00062A-Do
	for qemu-devel@nongnu.org; Sat, 05 Dec 2009 12:29:16 -0500
Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43)
	id 1NGyRT-0005yn-Iv
	for qemu-devel@nongnu.org; Sat, 05 Dec 2009 12:29:15 -0500
Received: from [199.232.76.173] (port=34009 helo=monty-python.gnu.org)
	by lists.gnu.org with esmtp (Exim 4.43) id 1NGyRT-0005yk-D0
	for qemu-devel@nongnu.org; Sat, 05 Dec 2009 12:29:11 -0500
Received: from mail-gx0-f223.google.com ([209.85.217.223]:34501)
	by monty-python.gnu.org with esmtp (Exim 4.60)
	(envelope-from <blauwirbel@gmail.com>) id 1NGyRT-0008Pq-8d
	for qemu-devel@nongnu.org; Sat, 05 Dec 2009 12:29:11 -0500
Received: by gxk23 with SMTP id 23so704881gxk.2
	for <qemu-devel@nongnu.org>; Sat, 05 Dec 2009 09:29:10 -0800 (PST)
MIME-Version: 1.0
In-Reply-To: <4B1A9359.8080305@redhat.com>
References: <m3skbwxgkp.fsf@crossbow.pond.sub.org>
	<4B193DA5.6040507@codemonkey.ws> <4B1A9359.8080305@redhat.com>
From: Blue Swirl <blauwirbel@gmail.com>
Date: Sat, 5 Dec 2009 17:28:50 +0000
Message-ID: <f43fc5580912050928r47dbf4f4l721047ca9f6654fd@mail.gmail.com>
Subject: Re: [Qemu-devel] [PATCH] Permit zero-sized qemu_malloc() & friends
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable
List-Id: qemu-devel.nongnu.org
List-Unsubscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.gnu.org/pipermail/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Avi Kivity <avi@redhat.com>
Cc: qemu-devel@nongnu.org, Paul Brook <paul@codesourcery.com>, Markus Armbruster <armbru@redhat.com>

On Sat, Dec 5, 2009 at 5:07 PM, Avi Kivity <avi@redhat.com> wrote:
> On 12/04/2009 06:49 PM, Anthony Liguori wrote:
>>
>> I still believe that it is poor practice to pass size=3D=3D0 to *malloc(=
). =C2=A0I
>> think actively discouraging this in qemu is a good thing because it's a
>> broken idiom.
>
> Why? =C2=A0Unless we have a separate array allocator (like C++'s new and =
new[]),
> we need to support zero-element arrays without pushing the burden to call=
ers
> (in the same way that for () supports zero iteration loops without a
> separate if ()).

Running a loop zero or nonzero number of times always has a very clear
and precise meaning. A pointer returned from allocating zero or
nonzero number of items may be completely unusable or usable,
respectively.

I think Laurent's proposal would work. We even could go so far as
rename the current function as qemu_malloc_possibly_broken (and adjust
callers mechanically) and introduce two new versions, which handle the
zero case in clearly advertised ways. Patches would fix the callers to
use the correct one.