Messages in this thread | | | Date | Sun, 22 Dec 2013 17:03:21 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH] pid: change task_struct::pid to read-only |
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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.
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