Dear,
This code snippet comes from libevent-2.0.13. The list operation is
the same. I think it does not have any bugs.
yrs,
/* insert this nameserver into the list of them */
if (!base->server_head) {
ns->next = ns->prev = ns;
base->server_head = ns;
} else {
ns->next = base->server_head->next;
ns->prev = base->server_head;
base->server_head->next = ns;
if (base->server_head->prev == base->server_head) {
base->server_head->prev = ns;
}
}
On 12/22/11, Nick Mathewson <ni...@freehaven.net> wrote:
> On Wed, Dec 21, 2011 at 7:53 AM, wang qin <scpl0...@gmail.com> wrote:
>>
>> Dear,
>>
>> I do not understand of the list operation of the
>> _evdns_nameserver_add_impl.
>>
>> How to understand this snippet?
>>
>> Best is figures and notes.
>>
>>
>>
>> Thank you very much!
>>
>>
>> /* insert this nameserver into the list of them */
>> if (!server_head) {
>> ns->next = ns->prev = ns;
>> server_head = ns;
>> } else {
>> ns->next = server_head->next;
>> ns->prev = server_head;
>> server_head->next = ns;
>> if (server_head->prev == server_head) {
>> server_head->prev = ns;
>> }
>> }
>
> It looks like it's supposed to be a circular doubly linked list to me.
> If the list is empty, then the new nameserver is inserted as the sole
> node, and its prev and next pointers are set to itself (to make the
> list circular). But if the list is notempty, we insert the new
> nameserver after the current head (server_head).
>
> It looks like there's a possible bug there; I don't see anything that
> points ns->next->prev at ns. If that's so, the best solution here is
> probably to replace the whole thing evdns_base.server_head thing with
> a CIRCLEQ or such.
>
> (FWIW, this looks like really old code; Libevent 2.0 is the version
> folks should be using. Libevent 1.4 is not under active development.)
>
> yrs,
> --
> Nick
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