Re: question about linked list implementation in kernel.h
On Tue, Sep 14, 2010 at 1:27 AM, Jan Ceuleers wrote:
> 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 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
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Re: question about linked list implementation in kernel.h
On Sat, Sep 11, 2010 at 10:09 PM, Manish Katiyar 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 Sat, Sep 11, 2010 at 5:33 AM, 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
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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Re: question about linked list implementation in kernel.h
Hey Mohit,
This is a feature of GCC (I don't know if it is a part of the standard).
Here's the link from the GCC manual:
http://gcc.gnu.org/onlinedocs/gcc-4.4.4/gcc/Statement-Exprs.html#Statement-Exprs
On Sat, Sep 11, 2010 at 9:49 PM, mohit verma wrote:
> thanks a lot manohar ,
> but please can u elaborate on
>
>
> int val = ({5 + 5;});
> printf("Block value: %d\n", val);
>
> i mean we can set val =( 5+5) or val=5+5
> then what is the need of all this ({5+5;""});
> the quotes i gave ,i dint get that place completely.
>
>
--
/manohar
Re: question about linked list implementation in kernel.h
On Sat, Sep 11, 2010 at 9:15 PM, 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!
>
> 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
#include
#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 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 9:12 PM, Bond 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
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 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
#include
#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 8:37 PM, Robert P. J. Day wrote: > > 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 > > Yeah thanks Robert you saved my day.
Re: question about linked list implementation in kernel.h
On 11/09/2010 16:52, Bond wrote: On Sat, Sep 11, 2010 at 6:22 PM, Carlo Caione mailto: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. Are you kidding me? That is just an example to show how container_of() is used. Do you think that it is better starting from something simpler than kernel programming like for example simple C programming? -- 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, 11 Sep 2010, Bond wrote: > > > On Sat, Sep 11, 2010 at 6:22 PM, Carlo Caione > 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
On Sat, Sep 11, 2010 at 6:22 PM, Carlo Caione 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.
Re: question about linked list implementation in kernel.h
On Sat, Sep 11, 2010 at 6:22 PM, Carlo Caione wrote: > > 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 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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question about linked list implementation in kernel.h
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?
