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Date: Thu, 20 May 2004 23:36:39 -0700
From: Andrew Morton <akpm@osdl.org>
To: Ulrich Drepper <drepper@redhat.com>
Cc: jakub@redhat.com, linux-kernel@vger.kernel.org, mingo@redhat.com
Subject: Re: [PATCH] Add FUTEX_CMP_REQUEUE futex op
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Ulrich Drepper <drepper@redhat.com> wrote:
>
> Andrew Morton wrote:
> 
> > Is it safe to go adding a new argument to an existing syscall in this manner?
> 
> Yes.  This is a multiplexed syscall and the opcode decides which syscall
> parameter is used.
> 

Of course.

> > It'll work OK on x86 because of the stack layout but is the same true of
> > all other supported architectures?
> 
> We add parameters at the end.  This does not influence how previous
> values are passed.  And especially for syscalls it makes no difference.
> 

what we're effectively doing is:

void foo(int a, int b, int c)
{
}

and from another compilation unit:

	foo(a, b);

and we're expecting the a's and b's to line up across all architectures and
compiler options.  I thought that on some architectures that only works out
if the function has a vararg declaration.

Does it do the right thing on stack-grows-up machines?

If the compiler passes the first few args via registers and the rest on the
stack, are we sure that it won't at some level of complexity decide to pass
_all_ the args on the stack?  It's free to do so, I think.

I have a vague memory of getting bitten by this trick once...