Re: var args in kernel?

[Bill Davidsen]

Jakub Jelinek wrote:
> On Mon, Nov 22, 2004 at 12:03:56PM +0100, Jan Engelhardt wrote:
>
> > > > What you can't do is e.g.
> > > >  va_list ap;
> > > >  va_start (ap, x);
> > > >  bar (x, ap);
> > > >  bar (x, ap);
> > > >  va_end (ap);
> >
> > In theory, you can't. But the way how GCC (and probably other compilers)
> > implement it, you can. Because "ap" is just a pointer (which fits into a
> > register, if I may add). As such, you can copy it, pass it multiple times, use
> > it multiple times, and whatever you like.
>
>
> That's exactly the wrong assumption.
> On some Linux architectures you can, on others you can't.
> Architectures where va_list is a char or void pointer include e.g.:
> i386, sparc*, ppc64, ia64
> Architectures where va_list is something different, usually struct { ... } va_list[1];
> or something similar:
> x86_64, ppc32, alpha, s390, s390x
>
> In the latter case, you obviously can't do va_list dest = src and
> if you do bar (x, ap); the content of the struct pointed by ap is changed
> after the call, therefore you can't use it for other routines
> (as it depends on where exactly the called function stopped with va_arg).

Why can't you do dest=src? Assignment of struct to struct has been a 
part of C since earliest times. I used it in ~1990 in code which ran on 
Z80, Multics, M68k, VAX and Cray2, and it worked without any ifdefs (for 
that, there were "just a few" for other issues like 8 vs. 9 bit char, etc).

--
   -bill davidsen (davidsen@xxxxxxx)
"The secret to procrastination is to put things off until the
 last possible moment - but no longer"  -me

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