Re: [PATCH] Make qemu_alloc()/qemu_realloc() return NULL for size==0 (was Re: [Qemu-devel] [PATCH] fix qemu_malloc() error check for size==0)

[malc]

On Tue, 19 May 2009, Eduardo Habkost wrote:

> On Tue, May 19, 2009 at 06:55:11PM +0400, malc wrote:
> <snip>
> > > > 
> > > > That's the problem standard C does _not_ define the behaviour, and 
> > > > leaves
> > > > that to implementation.
> > > 
> > > The only thing it doesn't define is either the returned pointer is NULL
> > > or not, and that doesn't make malloc(0) automatically unportable,
> > > because all the rest is perfectly defined:
> > > 
> > > 1) You can't dereference the pointer (just like you can't
> > >    dereference p[n] on a malloc(n) block)
> > > 2) You should pass the returned pointer to free() later
> > > 
> > 
> > Alas your list is not exhaustive:
> > 
> >   3) Test the returned value against NULL
> > 
> > [Which is precisely what the qcow2 code did]
> > 
> [...]
> > > 
> > > I agree that expecting the Linux behaviour (non-NULL) is a bug. My point
> > > is that there is no reason to consider malloc(0) a bug.
> > 
> > There is, due to the possibility of performing a 3) and a hard time 
> > catching that (unless someone solves halting problem or subset applicable
> > to QEMU thereof)
> 
> This is probably the only of your points which I agree with. What about
> the following, then?
> 
> That would catch the cases you are worried about, but won't break
> existing cases where malloc(0) is used correctly, and we won't be
> creating a new malloc/free API that is incompabible from every other
> malloc/free API out there.

Thanks for an attempt, but i don't like it either, since it sortof
breaks the (unspoken?) qemu_malloc/realloc contract that those will
never return NULL. I've commited the thing i had in mind.

[..snip..]

P.S. It probably says something about my experience in the field, but the
     only time i've encountered real-life usage of malloc(0) is in
     that qcow2 case, the code was produced by a programmer who among
     (numerous) other things wrote full C compiler and won IOCCC
     twice, yet even his code failed to work.

-- 
mailto:address@hidden