Re: question about linked list implementation in kernel.h
On Tue, Sep 14, 2010 at 1:27 AM, Jan Ceuleers jan.ceule...@computer.orgwrote:
On 11/09/10 17:49, Bond wrote:
(type *)( (char *)__mptr - offsetof(type,member) );})
there a subtraction has been done why is this subtraction done?
Not having read any of the documentation, what comes to mind is this:
if __mptr is a pointer to the member of a structure, the above expression
returns a pointer to the first byte of the containing structure, typecast to
the type of the structure.
Example:
typedef struct {
int a;
char b;
} test_t;
test_t test;
Now assume that __mptr is a pointer to test.b, then the above expression
returns a pointer to test, i.e. a pointer to the structure that b is a
member of.
Jan
Hi,
Jan no problem I am clear with the original question that I had asked with
Manohar.
Re: question about linked list implementation in kernel.h
On Sat, Sep 11, 2010 at 10:09 PM, Manish Katiyar mkati...@gmail.com wrote: If you want some more details about this macro. http://www.spinics.net/lists/linux-usb-devel/msg11766.html Thanks but Manohars explanation was perfect for newbies like me.
Re: question about linked list implementation in kernel.h
On 11/09/2010 14:33, Bond wrote:
I read
I was going through include/linux/kernel.h
encountered following code
#define container_of(ptr, type, member) ({ \
const typeof( ((type *)0)-member ) *__mptr = (ptr);\
in above code I am not clear with (type *)0
how is it working any link?
First link on google:
http://www.kroah.com/log/linux/container_of.html
--
Carlo
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Re: question about linked list implementation in kernel.h
On Sat, Sep 11, 2010 at 6:22 PM, Carlo Caione carlo.cai...@gmail.comwrote: First link on google: Its not that I had not searched it did search but I had searched typedef and type and I kept reading links related to that only. What to be searched is not that obvious when some one is not clear. http://www.kroah.com/log/linux/container_of.html Any how thanks for link.
Re: question about linked list implementation in kernel.h
On Sat, Sep 11, 2010 at 6:22 PM, Carlo Caione carlo.cai...@gmail.comwrote: http://www.kroah.com/log/linux/container_of.html I am not able to understand a single bit of the explanaition on the above link. They have assumed I programmed PCI buses which I have not.
Re: question about linked list implementation in kernel.h
On Sat, 11 Sep 2010, Bond wrote: On Sat, Sep 11, 2010 at 6:22 PM, Carlo Caione carlo.cai...@gmail.com wrote: http://www.kroah.com/log/linux/container_of.html I am not able to understand a single bit of the explanaition on the above link. They have assumed I programmed PCI buses which I have not. you're making this too difficult -- all container_of() does is, given: * a structure definition, * the name of a member field within that structure, and * the address of that internal member field in an instance what you get back is the address of the enclosing structure. this allows you to work with member fields of a structure but, when you need to, back up to the structure they're contained in. kernel linked lists are built on this idea. http://crashcourse.ca/introduction-linux-kernel-programming/intermission-lets-talk-about-linked-lists-and-containerof-free rday -- Robert P. J. Day Waterloo, Ontario, CANADA Top-notch, inexpensive online Linux/OSS/kernel courses http://crashcourse.ca Twitter: http://twitter.com/rpjday LinkedIn: http://ca.linkedin.com/in/rpjday
Re: question about linked list implementation in kernel.h
I just made a small example (with comments) to play around with :-)
Let me know if you feel something is wrong in my explanations (I'm still
learning as well!)
Hope it helps!
On Sat, Sep 11, 2010 at 6:03 PM, Bond jamesbond.2...@gmail.com wrote:
I read
I was going through include/linux/kernel.h
encountered following code
#define container_of(ptr, type, member) ({ \
const typeof( ((type *)0)-member ) *__mptr = (ptr);\
in above code I am not clear with (type *)0
how is it working any link?
--
/manohar
#include stdio.h
#include stdlib.h
#define offsetof(TYPE, MEMBER) ((size_t) ((TYPE *)0)-MEMBER)
/*
* The first line gets the type of the member inside the structure. It does
* this by casting a NULL pointer to the structure type and using the GCC
* typeof() extension.
*
* The second line evaluates to a value of the address of the containing
* structure. This is done using the value of the member pointer and
* subtracting the offset of the member from its own address.
*
* eg. if a member is at address 10 and is at an offset of 4 bytes into
* the struture, the containing structure's pointer is at address (10 - 4) = 6
*/
#define container_of(ptr, type, member) ({ \
const typeof( ((type *)0)-member ) *__mptr = (ptr);\
(type *)( (char *)__mptr - offsetof(type,member) );})
struct test {
int a;
};
/*
* Given the pointer to a member inside a structure, retreive its containing
* structure pointer
*/
void test_func(int *ptr)
{
/* You can see the output of this macro using: gcc -E cont.c */
struct test *container = container_of(ptr, struct test, a);
printf(Retreived pointer: %x\n, (unsigned int)container);
printf(Value: %d\n, container-a);
}
int main()
{
struct test *t = malloc(sizeof(struct test));
t-a = 5;
printf(Structure pointer: %x\n, (unsigned int)t);
printf(Value: %d\n, t-a);
test_func(t-a);
free(t);
/*
* This is how a block is evaluated to a value (explains how the macro
* works. See output from gcc -E cont.c.
*/
int val = ({5 + 5;});
printf(Block value: %d\n, val);
}
Re: question about linked list implementation in kernel.h
On Sat, Sep 11, 2010 at 9:12 PM, Bond jamesbond.2...@gmail.com wrote: http://crashcourse.ca/introduction-linux-kernel-programming/intermission-lets-talk-about-linked-lists-and-containerof-free Robert (type *)( (char *)__mptr - offsetof(type,member) );}) there a subtraction has been done why is this subtraction done? Manohar I am going through your doc.
Re: question about linked list implementation in kernel.h
On 11/09/2010 17:45, Manohar Vanga wrote: I just made a small example (with comments) to play around with :-) Let me know if you feel something is wrong in my explanations (I'm still learning as well!) Hope it helps! It is correct. Good explanation. -- Carlo -- To unsubscribe from this list: send an email with unsubscribe kernelnewbies to ecar...@nl.linux.org Please read the FAQ at http://kernelnewbies.org/FAQ
Re: question about linked list implementation in kernel.h
On Sat, Sep 11, 2010 at 9:15 PM, Manohar Vanga manohar.va...@gmail.comwrote:
I just made a small example (with comments) to play around with :-)
Let me know if you feel something is wrong in my explanations (I'm still
learning as well!)
Hope it helps!
Manohar I just happen to understand great great work man.It is clear
completely.
Awesome explanation I am copy pasting from the file you gave I hope you
don't mind.
#include stdio.h
#include stdlib.h
#define offsetof(TYPE, MEMBER) ((size_t) ((TYPE *)0)-MEMBER)
/*
* The first line gets the type of the member inside the structure. It does
* this by casting a NULL pointer to the structure type and using the GCC
* typeof() extension.
*
* The second line evaluates to a value of the address of the containing
* structure. This is done using the value of the member pointer and
* subtracting the offset of the member from its own address.
*
* eg. if a member is at address 10 and is at an offset of 4 bytes into
* the struture, the containing structure's pointer is at address (10 - 4) =
6
*/
#define container_of(ptr, type, member) ({ \
const typeof( ((type *)0)-member ) *__mptr = (ptr);\
(type *)( (char *)__mptr - offsetof(type,member) );})
struct test {
int a;
};
/*
* Given the pointer to a member inside a structure, retreive its containing
* structure pointer
*/
void test_func(int *ptr)
{
/* You can see the output of this macro using: gcc -E cont.c */
struct test *container = container_of(ptr, struct test, a);
printf(Retreived pointer: %x\n, (unsigned int)container);
printf(Value: %d\n, container-a);
}
int main()
{
struct test *t = malloc(sizeof(struct test));
t-a = 5;
printf(Structure pointer: %x\n, (unsigned int)t);
printf(Value: %d\n, t-a);
test_func(t-a);
free(t);
/*
* This is how a block is evaluated to a value (explains how the macro
* works. See output from gcc -E cont.c.
*/
int val = ({5 + 5;});
printf(Block value: %d\n, val);
}
Re: question about linked list implementation in kernel.h
On Sat, Sep 11, 2010 at 5:33 AM, Bond jamesbond.2...@gmail.com wrote:
I read
I was going through include/linux/kernel.h
encountered following code
#define container_of(ptr, type, member) ({ \
const typeof( ((type *)0)-member ) *__mptr = (ptr); \
in above code I am not clear with (type *)0
If you want some more details about this macro.
http://www.spinics.net/lists/linux-usb-devel/msg11766.html
how is it working any link?
--
Thanks -
Manish
==
[$\*.^ -- I miss being one of them
==
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Please read the FAQ at http://kernelnewbies.org/FAQ
