Date: Tue, 24 Aug 2004 10:29:19 -0700 From: Sean McNeil <sean@mcneil.com> To: Peter Wemm <peter@wemm.org> Cc: freebsd-amd64@freebsd.org Subject: Re: va_list structure passing as argument Message-ID: <1093368559.9914.15.camel@server.mcneil.com> In-Reply-To: <200408240220.45554.peter@wemm.org> References: <1093328434.6603.21.camel@server.mcneil.com> <200408240220.45554.peter@wemm.org>
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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.
Thanks,
Sean
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