Messages in this thread |  | | Date | Sat, 12 Mar 2022 11:24:40 +0100 | From | Michał Mirosław <> | Subject | Re: [PATCH 2/6] list: add new MACROs to make iterator invisiable |
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On Thu, Mar 10, 2022 at 04:46:33PM -0800, Linus Torvalds wrote: > On Thu, Mar 10, 2022 at 3:54 PM Michał Mirosław <mirq-linux@rere.qmqm.pl> wrote: > > > > If the macro implementation doesn't have to be pretty, maybe it could go > > a step further and remember the list_head's offset? That would look > > something like following (expanding on your patch; not compile tested): > > Oh, I thought of it. > > It gets complicated. [...]
It seems that it's not that bad if we don't require checking whether a list_head of an entry is only ever used with a single list parent. The source type is not needed for the macros, and it turns out that pre-declaring the offset type is also not needed.
I compile-tested the code below on godbolt.org with -std=c11:
struct list_head { struct list_head *prev, *next; };
#define offsetof __builtin_offsetof #define typeof __typeof
#define list_traversal_head(name,type,target_member) \ union { \ struct list_head name; \ type *name##_traversal_type; \ char (*name##_list_head_offset)[offsetof(type, target_member)]; \ }
#define self_list_ref_offset_type(type,target_member) \ type##__##target_member##__offset__
#define define_self_list_ref_offset(type,target_member) \ self_list_ref_offset_type(type,target_member) \ { char ignoreme__[offsetof(type, target_member)]; }
#define self_list_traversal_head(name,type,target_member) \ union { \ struct list_head name; \ type *name##_traversal_type; \ self_list_ref_offset_type(type,target_member) *name##_list_head_offset; \ }
#define list_traversal_entry(ptr, head) \ (typeof(*head##_traversal_type))((void *)ptr - sizeof(**head##_list_head_offset))
#define list_traversal_entry_head(ptr, head) \ ((struct list_head *)((void *)ptr + sizeof(**head##_list_head_offset)))
#define list_traversal_entry_is_head(ptr, head) \ (list_traversal_entry_head(ptr, head) == (head))
#define list_traversal_next_entry(ptr, head) \ list_traversal_entry(list_traversal_entry_head(ptr, head)->next, head)
#define list_traverse(pos, head) \ for (typeof(*head##_traversal_type) pos = list_traversal_entry((head)->next, head); \ !list_traversal_entry_is_head(pos, head); \ pos = list_traversal_next_entry(pos,head))
struct entry { self_list_traversal_head(self_list, struct entry, child_head); struct list_head child_head; };
define_self_list_ref_offset(struct entry, child_head);
void bar(struct entry *b);
void foo(struct entry *a) { list_traverse(pos, &a->self_list) { bar(pos); } }
-- Michał Mirosław
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