va_list structure passing as argument

[Peter Wemm]

On Tuesday 24 August 2004 10:29 am, Sean McNeil wrote:
> On Tue, 2004-08-24 at 02:20, Peter Wemm wrote:
> > On Monday 23 August 2004 11:20 pm, Sean McNeil wrote:
> > > I'm looking at a problem I have on the amd64 with bsdtar.
> > > Essentially, you get a core dump if you try to run the following:
> > >
> > > tar zxvvf nonexistent.tar.gz
> > >
> > > I've tracked it down to an issue where the ap is getting changed
> > > as a side-effect of calling __vfprintf.  It looks like this is
> > > happening because the va_list structure is being passed by
> > > reference.  The va_list structure on amd64 is 24 bytes.  I'm
> > > guessing that it is 16 bytes or less for i386.  It has been a
> > > while since I've looked at the macro that determines when a
> > > structure is passed by reference or value.  Does anyone know what
> > > that is?  I'm guessing that 24 passes that cutoff but 16 does not
> > > and that is why I see this bug on amd64 and not i386.
> >
> > Yes, its an external value.  Consider it a pointer.  It is the same
> > on both ppc and amd64.
> >
> > The problem is that vfprintf  "consumes" the values and advances
> > the counters in the structure.  (The argument passing ABI is very
> > complex)
> >
> > What you need to do is this:
> > myfunc(va_list ap)
> > {
> > 	va_list apcopy;
> >
> > 	va_copy(apcopy, ap);
> > 	vprintf(stuff, ap1);
> > 	va_copy(apcopy, ap);
> > 	do_stuff_with(ap1);
> > }
> > etc.  Using va_copy is "correct" for all our platforms, but
> > neglecting to use it is only fatal for amd64 and ppc.
> >
> > Does that make sense?
>
> Makes sense and is what I tried originally.  I figured this wasn't
> correct, however, as it means that a side-effect of the call is
> allowable.  What you are indicating is that it is allowable and
> expected.  The problem is, the side-effect will only happen on ppc
> and amd64 since i386 will pass the va_list by value.

Just because you can get away with it on i386, that doesn't mean it is 
allowed.  With C99, you're supposed to use va_copy, even if you can get 
away with not using it on some platforms.
-- 
Peter Wemm - peter at wemm.org; peter at FreeBSD.org; peter at yahoo-inc.com
"All of this is for nothing if we don't go to the stars" - JMS/B5