* About head of kernel linked list structure @ 2015-05-06 10:39 Huaicheng Li 2015-05-06 19:45 ` Robert P. J. Day 2015-05-07 10:52 ` Mulyadi Santosa 0 siblings, 2 replies; 9+ messages in thread From: Huaicheng Li @ 2015-05-06 10:39 UTC (permalink / raw) To: kernelnewbies In my understanding, the head initialised using LIST_HEAD_INIT or defined by LIST_HEAD corresponds to no *real* data field. But it *does* have its own _next_ and _prev_ pointers. The _next_ pointer points to the first real node in the doubly linked list, and the _prev_ pointer points to the last real node. I'm wondering if the picture shown at the very bottom of http://kernelnewbies.org/FAQ/LinkedLists is wrong about this. Because I believe head's _prev_ should point to the last node, which is at the very right. And the last node's _next_ should point to the head. Just as shown like the red line. Am I right? -- Best Regards Huaicheng Li -------------- next part -------------- An HTML attachment was scrubbed... URL: http://lists.kernelnewbies.org/pipermail/kernelnewbies/attachments/20150506/5cd631c4/attachment-0001.html -------------- next part -------------- A non-text attachment was scrubbed... Name: linked_list_example copy.png Type: image/png Size: 37281 bytes Desc: not available Url : http://lists.kernelnewbies.org/pipermail/kernelnewbies/attachments/20150506/5cd631c4/attachment-0001.png ^ permalink raw reply [flat|nested] 9+ messages in thread
* About head of kernel linked list structure 2015-05-06 10:39 About head of kernel linked list structure Huaicheng Li @ 2015-05-06 19:45 ` Robert P. J. Day 2015-05-07 10:52 ` Huaicheng Li 2015-05-07 10:52 ` Mulyadi Santosa 1 sibling, 1 reply; 9+ messages in thread From: Robert P. J. Day @ 2015-05-06 19:45 UTC (permalink / raw) To: kernelnewbies On Wed, 6 May 2015, Huaicheng Li wrote: > In my understanding, the head initialised using LIST_HEAD_INIT or > defined by LIST_HEAD corresponds to no *real* data field. correct. a better way to describe it would be that it corresponds to no real enclosing payload, so it should never be dereferenced to get to said payload. > But it *does* have its own _next_ and _prev_ pointers. The _next_ > pointer points to the first real node in the doubly linked list, and > the _prev_ pointer points to the last real node. so far, so good -- by "real", you mean list heads that actually correspond to an enclosing payload. > I'm wondering if the picture shown at the very bottom of > http://kernelnewbies.org/FAQ/LinkedLists is wrong about this. > Because > I believe head's _prev_ should point to the last node, which is at > the very right. And the last node's _next_ should point to the head. > Just as shown like the red line. > > Am I right? it looks fine to me, although my standard is to show next pointers going to the right, and prev to the left, but this way looks fine. maybe i'm misunderstanding your objection. i once wrote a tutorial on LLs: http://www.crashcourse.ca/introduction-linux-kernel-programming/intermission-lets-talk-about-linked-lists-and-containerof-free rday -- ======================================================================== Robert P. J. Day Ottawa, Ontario, CANADA http://crashcourse.ca Twitter: http://twitter.com/rpjday LinkedIn: http://ca.linkedin.com/in/rpjday ======================================================================== ^ permalink raw reply [flat|nested] 9+ messages in thread
* About head of kernel linked list structure 2015-05-06 19:45 ` Robert P. J. Day @ 2015-05-07 10:52 ` Huaicheng Li 2015-05-07 11:05 ` Robert P. J. Day 0 siblings, 1 reply; 9+ messages in thread From: Huaicheng Li @ 2015-05-07 10:52 UTC (permalink / raw) To: kernelnewbies Hi Robert, You got my point and I?m sorry for not stating it in a clear way. You are right about the "prev-to-left" and "next-to-right" when drawing the line. At that time, I just wanted to show which node they were pointing at. Anyway, you solved my puzzle. Also thanks for your excellent article. -- Best Regards Huaicheng Li > On May 7, 2015, at 3:45 AM, Robert P. J. Day <rpjday@crashcourse.ca> wrote: > > On Wed, 6 May 2015, Huaicheng Li wrote: > >> In my understanding, the head initialised using LIST_HEAD_INIT or >> defined by LIST_HEAD corresponds to no *real* data field. > > correct. a better way to describe it would be that it corresponds to > no real enclosing payload, so it should never be dereferenced to get > to said payload. > >> But it *does* have its own _next_ and _prev_ pointers. The _next_ >> pointer points to the first real node in the doubly linked list, and >> the _prev_ pointer points to the last real node. > > so far, so good -- by "real", you mean list heads that actually > correspond to an enclosing payload. > >> I'm wondering if the picture shown at the very bottom of >> http://kernelnewbies.org/FAQ/LinkedLists is wrong about this. >> Because > >> I believe head's _prev_ should point to the last node, which is at >> the very right. And the last node's _next_ should point to the head. >> Just as shown like the red line. >> >> Am I right? > > it looks fine to me, although my standard is to show next pointers > going to the right, and prev to the left, but this way looks fine. > maybe i'm misunderstanding your objection. > > i once wrote a tutorial on LLs: > > http://www.crashcourse.ca/introduction-linux-kernel-programming/intermission-lets-talk-about-linked-lists-and-containerof-free > > rday > > -- > > ======================================================================== > Robert P. J. Day Ottawa, Ontario, CANADA > http://crashcourse.ca > > Twitter: http://twitter.com/rpjday > LinkedIn: http://ca.linkedin.com/in/rpjday > ======================================================================== ^ permalink raw reply [flat|nested] 9+ messages in thread
* About head of kernel linked list structure 2015-05-07 10:52 ` Huaicheng Li @ 2015-05-07 11:05 ` Robert P. J. Day 0 siblings, 0 replies; 9+ messages in thread From: Robert P. J. Day @ 2015-05-07 11:05 UTC (permalink / raw) To: kernelnewbies On Thu, 7 May 2015, Huaicheng Li wrote: > Hi Robert, > > You got my point and I?m sorry for not stating it in a clear way. > You are right about the "prev-to-left" and "next-to-right" when > drawing the line. At that time, I just wanted to show which node > they were pointing at. Anyway, you solved my puzzle. Also thanks > for your excellent article. it's a moderately common mistake to not realize that the "head" of a list is represented by just another "struct list_head" like all the other elements in the list, but it's a special list_head in the sense that it does ***not*** represent a valid enclosing data payload, and should never be dereferenced. the way i normally demonstrate this is to show the following macro from include/linux/list.h: /** * list_for_each - iterate over a list * @pos: the &struct list_head to use as a loop cursor. * @head: the head for your list. */ #define list_for_each(pos, head) \ for (pos = (head)->next; pos != (head); pos = pos->next) note well how iteration over a list starts from "(head)->next" and continues until returning back to "(head)" while *not* including that last address to be dereferenced. this also has the (obvious) consequence that you can never forget which element in a list is the head; otherwise, you'll never know which element you're not supposed to dereference. rday -- ======================================================================== Robert P. J. Day Ottawa, Ontario, CANADA http://crashcourse.ca Twitter: http://twitter.com/rpjday LinkedIn: http://ca.linkedin.com/in/rpjday ======================================================================== ^ permalink raw reply [flat|nested] 9+ messages in thread
* About head of kernel linked list structure 2015-05-06 10:39 About head of kernel linked list structure Huaicheng Li 2015-05-06 19:45 ` Robert P. J. Day @ 2015-05-07 10:52 ` Mulyadi Santosa 2015-05-07 10:57 ` Mulyadi Santosa 2015-05-08 23:05 ` Jeff Haran 1 sibling, 2 replies; 9+ messages in thread From: Mulyadi Santosa @ 2015-05-07 10:52 UTC (permalink / raw) To: kernelnewbies On Wed, May 6, 2015 at 5:39 PM, Huaicheng Li <lhcwhu@gmail.com> wrote: > In my understanding, the head initialised using LIST_HEAD_INIT or defined > by LIST_HEAD corresponds to no *real* data field. > But it *does* have its own _next_ and _prev_ pointers. The _next_ pointer > points to the first real node in the doubly linked list, > and the _prev_ pointer points to the last real node. > > I'm wondering if the picture shown at the very bottom of *http://kernelnewbies.org/FAQ/LinkedLists > <http://kernelnewbies.org/FAQ/LinkedLists>* is wrong about this. Because > I believe head's _prev_ should point to the last node, which is at the > very right. And the last node's _next_ should point to the head. > Just as shown like the red line. > > Am I right? > > > > Hi Huachieng AFAIK, if the linked list is not circular one, the last node's next should point to NULL, so does the head prev's. This is done so you know when you hit head i.e if !(head.prev) { // we're at head } > -- > Best Regards > Huaicheng Li > > > > > > > > _______________________________________________ > Kernelnewbies mailing list > Kernelnewbies at kernelnewbies.org > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies > > -- regards, Mulyadi Santosa Freelance Linux trainer and consultant blog: the-hydra.blogspot.com training: mulyaditraining.blogspot.com -------------- next part -------------- An HTML attachment was scrubbed... URL: http://lists.kernelnewbies.org/pipermail/kernelnewbies/attachments/20150507/b43227d9/attachment-0001.html -------------- next part -------------- A non-text attachment was scrubbed... Name: linked_list_example copy.png Type: image/png Size: 37281 bytes Desc: not available Url : http://lists.kernelnewbies.org/pipermail/kernelnewbies/attachments/20150507/b43227d9/attachment-0001.png ^ permalink raw reply [flat|nested] 9+ messages in thread
* About head of kernel linked list structure 2015-05-07 10:52 ` Mulyadi Santosa @ 2015-05-07 10:57 ` Mulyadi Santosa 2015-05-08 1:02 ` Bernd Petrovitsch 2015-05-08 23:05 ` Jeff Haran 1 sibling, 1 reply; 9+ messages in thread From: Mulyadi Santosa @ 2015-05-07 10:57 UTC (permalink / raw) To: kernelnewbies On Wed, May 6, 2015 at 5:39 PM, Huaicheng Li <lhcwhu@gmail.com> wrote: > In my understanding, the head initialised using LIST_HEAD_INIT or defined > by LIST_HEAD corresponds to no *real* data field. > But it *does* have its own _next_ and _prev_ pointers. The _next_ pointer > points to the first real node in the doubly linked list, > and the _prev_ pointer points to the last real node. > > I'm wondering if the picture shown at the very bottom of *http://kernelnewbies.org/FAQ/LinkedLists > <http://kernelnewbies.org/FAQ/LinkedLists>* is wrong about this. Because > I believe head's _prev_ should point to the last node, which is at the > very right. And the last node's _next_ should point to the head. > Just as shown like the red line. > > Am I right? > > > > Hi Huachieng AFAIK, if the linked list is not circular one, the last node's next should point to NULL, so does the head prev's. This is done so you know when you hit head i.e if !(head.prev) { // we're at head } -- regards, Mulyadi Santosa Freelance Linux trainer and consultant blog: the-hydra.blogspot.com training: mulyaditraining.blogspot.com -------------- next part -------------- An HTML attachment was scrubbed... URL: http://lists.kernelnewbies.org/pipermail/kernelnewbies/attachments/20150507/a7d3605c/attachment-0001.html ^ permalink raw reply [flat|nested] 9+ messages in thread
* About head of kernel linked list structure 2015-05-07 10:57 ` Mulyadi Santosa @ 2015-05-08 1:02 ` Bernd Petrovitsch 0 siblings, 0 replies; 9+ messages in thread From: Bernd Petrovitsch @ 2015-05-08 1:02 UTC (permalink / raw) To: kernelnewbies Hi all! On Don, 2015-05-07 at 17:57 +0700, Mulyadi Santosa wrote: > On Wed, May 6, 2015 at 5:39 PM, Huaicheng Li <lhcwhu@gmail.com> wrote: > > In my understanding, the head initialised using LIST_HEAD_INIT or defined > > by LIST_HEAD corresponds to no *real* data field. > > But it *does* have its own _next_ and _prev_ pointers. The _next_ pointer > > points to the first real node in the doubly linked list, > > and the _prev_ pointer points to the last real node. [...] > AFAIK, if the linked list is not circular one, the last node's next should > point to NULL, so does the head prev's. This is done so you know when you > hit head i.e > > if !(head.prev) What programming language? Bernd -- "I dislike type abstraction if it has no real reason. And saving on typing is not a good reason - if your typing speed is the main issue when you're coding, you're doing something seriously wrong." - Linus Torvalds ^ permalink raw reply [flat|nested] 9+ messages in thread
* About head of kernel linked list structure 2015-05-07 10:52 ` Mulyadi Santosa 2015-05-07 10:57 ` Mulyadi Santosa @ 2015-05-08 23:05 ` Jeff Haran 2015-05-11 4:45 ` Mulyadi Santosa 1 sibling, 1 reply; 9+ messages in thread From: Jeff Haran @ 2015-05-08 23:05 UTC (permalink / raw) To: kernelnewbies From: kernelnewbies-bounces@kernelnewbies.org [mailto:kernelnewbies-bounces at kernelnewbies.org] On Behalf Of Mulyadi Santosa Sent: Thursday, May 07, 2015 3:53 AM To: Huaicheng Li Cc: kernelnewbies Subject: Re: About head of kernel linked list structure On Wed, May 6, 2015 at 5:39 PM, Huaicheng Li <lhcwhu at gmail.com<mailto:lhcwhu@gmail.com>> wrote: In my understanding, the head initialised using LIST_HEAD_INIT or defined by LIST_HEAD corresponds to no *real* data field. But it *does* have its own _next_ and _prev_ pointers. The _next_ pointer points to the first real node in the doubly linked list, and the _prev_ pointer points to the last real node. I'm wondering if the picture shown at the very bottom of http://kernelnewbies.org/FAQ/LinkedLists is wrong about this. Because I believe head's _prev_ should point to the last node, which is at the very right. And the last node's _next_ should point to the head. Just as shown like the red line. Am I right? [cid:image002.png at 01D089A8.8896F550] Hi Huachieng AFAIK, if the linked list is not circular one, the last node's next should point to NULL, so does the head prev's. This is done so you know when you hit head i.e if !(head.prev) { // we're at head } Just pointing out that these lists are circular. Back in the old days we used to call them "rings". >From list.h: 183 /** 184 * list_empty - tests whether a list is empty 185 * @head: the list to test. 186 */ 187 static inline int list_empty(const struct list_head *head) 188 { 189 return head->next == head; 190 } Jeff Haran -------------- next part -------------- An HTML attachment was scrubbed... URL: http://lists.kernelnewbies.org/pipermail/kernelnewbies/attachments/20150508/1377f263/attachment.html -------------- next part -------------- A non-text attachment was scrubbed... Name: image002.png Type: image/png Size: 11417 bytes Desc: image002.png Url : http://lists.kernelnewbies.org/pipermail/kernelnewbies/attachments/20150508/1377f263/attachment.png ^ permalink raw reply [flat|nested] 9+ messages in thread
* About head of kernel linked list structure 2015-05-08 23:05 ` Jeff Haran @ 2015-05-11 4:45 ` Mulyadi Santosa 0 siblings, 0 replies; 9+ messages in thread From: Mulyadi Santosa @ 2015-05-11 4:45 UTC (permalink / raw) To: kernelnewbies On Sat, May 9, 2015 at 6:05 AM, Jeff Haran <Jeff.Haran@citrix.com> wrote: > > Just pointing out that these lists are circular. Back in the old days we > used to call them "rings". > > > > From list.h: > > > > 183 /** > > 184 * list_empty - tests whether a list is empty > > 185 * @head: the list to test. > > 186 */ > > 187 static inline int list_empty(const struct list_head *head) > > 188 { > > 189 return head->next == head; > > 190 } > > > > Jeff Haran > > > After re-reading the page about linked list in kernelnewbies, I agree it's circular list. I just didn't realize that even after reading carefully the first post -- regards, Mulyadi Santosa Freelance Linux trainer and consultant blog: the-hydra.blogspot.com training: mulyaditraining.blogspot.com -------------- next part -------------- An HTML attachment was scrubbed... URL: http://lists.kernelnewbies.org/pipermail/kernelnewbies/attachments/20150511/e8abfbe4/attachment.html ^ permalink raw reply [flat|nested] 9+ messages in thread
end of thread, other threads:[~2015-05-11 4:45 UTC | newest] Thread overview: 9+ messages (download: mbox.gz follow: Atom feed -- links below jump to the message on this page -- 2015-05-06 10:39 About head of kernel linked list structure Huaicheng Li 2015-05-06 19:45 ` Robert P. J. Day 2015-05-07 10:52 ` Huaicheng Li 2015-05-07 11:05 ` Robert P. J. Day 2015-05-07 10:52 ` Mulyadi Santosa 2015-05-07 10:57 ` Mulyadi Santosa 2015-05-08 1:02 ` Bernd Petrovitsch 2015-05-08 23:05 ` Jeff Haran 2015-05-11 4:45 ` Mulyadi Santosa
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