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);
}
