Date: Mon, 22 Nov 2004 12:39:54 +0100 (MET)
From: Jan Engelhardt <jengelh@linux01.gwdg.de>
To: Jakub Jelinek <jakub@redhat.com>
cc: linux-kernel@vger.kernel.org
Subject: Re: var args in kernel?
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>> 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

Yeah I originally had that in mind but did not write it out ;)
What would the struct look like? Because if the following was true, then you
could also use a pointer:

int my_printf(const char *fmt, struct { some ints, some char's, whatever } arg)
{
    struct giveItAName *ap = &arg;
    callanotherfunc(fmt, ap);
    callanotherfunc(fmt, ap);
}

>In the latter case, you obviously can't do va_list dest = src and

Mmh, I could, because GCC is wise enough to copy whole structs. This is BTW
done a LOT in the kernel sources. Just compile it with -Waggreagte-return and
ye'll get a whole whopping lot of warnings from GCC. For
linux-2.[46].\d+.tar.bz2.

>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

Who said the struct is changed? The compiler could use some magic to copy
above-metioned "arg" to a temporary which is only valid during my_printf() and
callees.
Who says the struct has to be changed? And even if it was about to, you could
copy it.

>(as it depends on where exactly the called function stopped with va_arg).

Don't get me wrong, I'm just asking... in a pessimistic fashion.


Jan Engelhardt
-- 
Gesellschaft für Wissenschaftliche Datenverarbeitung
Am Fassberg, 37077 Göttingen, www.gwdg.de
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