Re: About head of kernel linked list structure
On Sat, May 9, 2015 at 6:05 AM, Jeff Haran jeff.ha...@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
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regards,
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Freelance Linux trainer and consultant
blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com
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RE: About head of kernel linked list structure
From: kernelnewbies-boun...@kernelnewbies.org
[mailto:kernelnewbies-boun...@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
lhc...@gmail.commailto:lhc...@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@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
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Re: About head of kernel linked list structure
On Wed, May 6, 2015 at 5:39 PM, Huaicheng Li lhc...@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
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--
regards,
Mulyadi Santosa
Freelance Linux trainer and consultant
blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com
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Re: About head of kernel linked list structure
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 lhc...@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 ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Re: About head of kernel linked list structure
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 ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Re: About head of kernel linked list structure
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 rpj...@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 ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Re: About head of kernel linked list structure
On Wed, May 6, 2015 at 5:39 PM, Huaicheng Li lhc...@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
___
Kernelnewbies mailing list
Kernelnewbies@kernelnewbies.org
http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
About head of kernel linked list structure
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 ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Re: About head of kernel linked list structure
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 ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
