ipc.h struct layout fix

[Mike Frysinger]

On Friday, July 08, 2011 10:05:35 Peter Mazinger wrote:
> > On 07/07/11 20:41, Peter Mazinger wrote:
> > >> This fixes a problem identified by Mark Salter on C6X big-endian
> > >> systems. We should not use "unsigned short int mode"; the proper
> > >> type is in theory mode_t, but here we need to use kernel_mode_t
> > >> since this structure is used for communication with the kernel, and
> > >> some targets have a 16-bit kernel_mode_t and 32-bit mode_t.
> > > 
> > > haven't looked closely, but if this header gets installed on target,
> > > [__]kernel_mode_t is not/should not be defined. If the structure is
> > > not meant to be used on target, than it should be guarded (like
> > > _LIBC), else we should use mode_t here and handle accordingly in
> > > code.
> > 
> > Accordingly meaning...? Should we translate kernel structures to user
> > structures here? That would get really ugly.
> 
> It is, we do that in some places already like xstatconv.c, where we provide
> a common stat structure. The sigaction implementation had also similar
> problems, at least 3 types of arch-specific structures are present, the
> members are not even the same (or in same order), vda intended to make
> changes, but sincerely I did not follow up how far the changes went.
> 
> > Can't we ensure that the necessary type is defined somewhere?
> 
> My understanding for a libc is that we try to expose to userspace a common
> interface, hiding specialities of the kernel. Let's see what others
> think...

we resort to adding this layer of indirection between the kernel and C library 
users only when we dont have a choice.  shuffling bytes around from one struct 
layout to the other is wasteful overhead ... i wouldnt take xstatconv as a 
golden standard to be replicated.

also, Bernd is modifying a struct that has already been shipped to C library 
users.  we can change very few things now without breaking the ABI.
-mike
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