On Tuesday 2010-06-01 08:49, Ian Lance Taylor wrote:
>
>struct list_head {
> struct list_head *next, *prev;
>};
>
>int main(void)
>{
> struct list_head clh = {&clh, &clh};
> struct item *pos;
>
> list_for_each_entry(pos, &clh, list)
>
>This is going to expand into
>
> for ((pos) = list_entry((&clh)->next, typeof(*(pos)), member);
>==>
> for ((pos) = containerof(((&clh)->next), typeof(*(pos)), member)
>==>
> for ((pos) = (typeof(*(pos)) *)((char *)((&clh)->next) - offsetof(...)
>
>At this point you are starting with the field clh.next, which has type
>"struct list_head", you are casting to to "char *", and you are
>accessing it as type "struct item *". That is an aliasing violation.
I could make the containerof macro into a separate, extern, function,
so that the optimizer does not cut through the aliasing violation
that was intended to be none.
extern void *sub(void *x, ptrdiff_t d) { return (char *)x - d; }
#define containerof(var, type, member) \
sub(var, offsetof(type, member))
for (pos = containerof(clh.next, typeof(*pos), member)); ...
Is there a way I could still make this work, without either
making clh a full struct item, or the extern function?
Jan
