CODEWITHSUNDEEP
clinux-kernel
lilzz
lilzz

Reputation: 5413

Linux's INIT_LIST_HEAD_FIRST,

I have find a lot of references to INIT_LIST_HEAD but I couldn't find anything on INIT_LIST_HEAD_FIRST. what's the differences between the two.

INIT_LIST_HEAD(&orig_event->list);
INIT_LIST_HEAD_FIRST(orig_event->event_list);
INIT_LIST_HEAD_FIRST(orig_event->rt_hist_list);

one pass in the address of thing pointed by a pointer and another pass in just something pointed by a pointer.

INIT_HLIST_HEAD(&bat_priv->gw.list);
    INIT_LIST_HEAD(&bat_priv->tt.changes_list);

Also what are differences between INIT_LIST_HEAD and INIT_HLIST_HEAD?

Upvotes: 0

Views: 505

Answers (1)

Steve
Steve

Reputation: 493

Here are the definitions for each macro, from list-batman.h:

00062 #define INIT_LIST_HEAD(ptr) do { \
00063        (ptr)->next = (ptr); \
00064 } while (0)
00065 
00066 #define INIT_LIST_HEAD_FIRST(ptr) \
00067        ptr.next = (struct list_head *)&ptr; ptr.prev = (struct list_head *)&ptr; \
00068

(Line 68 is blank, so the continuation from line 67 doesn't do anything.)

So, what does it mean? This question explains the peculiar do { . . . } while (0) construction: Why use apparently meaningless do-while and if-else statements in macros? Thus, INIT_LIST_HEAD(ptr) is meant to step forward to the next list element.

The INIT_LIST_HEAD_FIRST(ptr) sets the next and previous pointers of a list_head structure, which is the empty state of a doubly linked list.

I'm puzzled about the names of the macros, because both sound like initializations, but that's a question for the developers who wrote the code.

More:

I initially overlooked the question about INIT_HLIST_HEAD. I looked it up, and linux/list.h defines it as this:

 #define INIT_HLIST_HEAD(ptr) ((ptr)->first = NULL)

It deals with the hlist_node structure, which contains a next pointer and a pprev pointer to pointer. The macro initializes the list.

Upvotes: 1

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