On 12/20, Peter Zijlstra wrote:
>
> On Fri, Dec 20, 2013 at 08:01:57PM +0100, Oleg Nesterov wrote:
> > The only problem is that
> >
> > #define ASSIGN_CONST(l, r) (*(typeof(r) *)&(l) = (r))
> >
> > obviously can't work in this case ;) We need something more clever.
>
> Hmm indeed, C++ has both the const_cast<>() thingy and the template
> system is powerful enough to actually implement const_cast<>() inside
> the language.
>
> But I cannot find anything useful for C. Your attempt to use the rvalue
> type to hopefully obtain a const-less lvalue type is clever, but does
> indeed fail where the rvalue is const too.
Yes.
We can probably do
#define ASSIGN_CONST(l, r) ({ \
typeof (l) const r__ = (r); \
memcpy((void *)&(l), &(r__), sizeof(l)); \
(l); \
})
gcc can actually avoid a temporary if you do
ASSIGN_CONST(tsk->pid, leader->pid);
but this doesn't look very nice and assumes __builtin_memcpy().
So, how about
#define CONST_CAST(type, lval) \
(*({ \
(void)((type *)0 == &(lval)); \
(type *)&(lval); \
})) \
?
de_thread/copy_process can do
CONST_CAST(pid_t, tsk->pid) = leader->pid;
and if "type" is wrong we have a warning.
Oleg.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
