On Mon, 28 Feb 2022 16:41:04 -0800 Linus Torvalds wrote:
So yes, initially my idea had been to just move the iterator entirely inside the macro. But specifying the type got so ugly that I think that
typeof (pos) postrick inside the macro really ends up giving us the best of all worlds:
(a) let's us keep the existing syntax and code for all the nice cases that did everything inside the loop anyway
(b) gives us a nice warning for any normal use-after-loop case (unless you explicitly initialized it like that sgx_mmu_notifier_release() function did for no good reason
(c) also guarantees that even if you don't get a warning, non-converted (or newly written) bad code won't actually _work_
so you end up getting the new rules without any ambiguity or mistaken
I presume the goal is that we can do this without changing existing code? Otherwise actually moving the iterator into the loop body would be an option, by creating a different hidden variable:
#define list_iter(head) \ for (struct list head *_l = (head)->next; _l != (head); _l = _l->next)
#define list_iter_entry(var, member) \ list_entry(_l, typeof(*var), member)
list_iter(&p->a_head) { struct entry *e = list_iter_entry(e, a_member);
/* use e->... */ }
Or we can slide into soft insanity and exploit one of Kees'es tricks to encode the type of the entries "next to" the head:
#define LIST_HEAD_MEM(name, type) \ union { \ struct list_head name; \ type *name ## _entry; \ }
struct entry { struct list_head a_member; };
struct parent { LIST_HEAD_MEM(a_head, struct entry); };
#define list_for_each_magic(pos, head, member) \ for (typeof(**(head ## _entry)) *pos = list_first_entry(head, typeof(**(head ## _entry)), member); \ &pos->member != (head); \ pos = list_next_entry(pos, member))
list_for_each_magic(e, &p->a_head, a_member) { /* use e->... */ }
I'll show myself out...