On 04/12/2015 12:16 AM, nick wrote:
> This finds the next node in the red black tree for sched_enities.
> Basically rb_next finds the next node in the tree. The argument is
> the rb_node structure embedded in the structure using a red black
> tree.
The problem is this
struct mystruc {
int a;
char * string;
} wonderful;
char * string2 = wonderful.string;
There is no way to know that string2 comes from wonderful.
That is the problem I have with the nodes
The node is an instance of sched_entity. But unless there is a field
that says what task_entity it is embedded in, in of itself, there is no
way to know. There is no 'this' C.
Obviously if does know. I'll look at chapter 6
I'm also looking at this
static struct sched_entity *__pick_next_entity(struct sched_entity *se)
{
struct rb_node *next = rb_next(&se->run_node);
if (!next)
return NULL;
return rb_entry(next, struct sched_entity, run_node); <<==macro
}
The macro is: I think:
http://lxr.free-electrons.com/source/include/linux/rbtree.h#L50
50 #define rb_entry(ptr, type, member) container_of(ptr, type, member)
container_of is
http://lxr.free-electrons.com/source/include/linux/kernel.h#L798
791 /**
792 * container_of - cast a member of a structure out to the containing
structure
793 * @ptr: the pointer to the member.
794 * @type: the type of the container struct this is embedded in.
795 * @member: the name of the member within the struct.
796 *
797 */
798 #define container_of(ptr, type, member) ({ \
799 const typeof( ((type *)0)->member ) *__mptr = (ptr); \
800 (type *)( (char *)__mptr - offsetof(type,member) );})
Now I have embedded macros which I thought you couldn't even do...
In trying to understand this I stumbled on
http://linuxkernel51.blogspot.com/2011/02/how-containerof-macro-works-example.html
How "container_of" macro works, & an Example
Here iam giving a small code snippet that gives and idea about working
of "container_of", this posed me little difficulty in understanding,
after google-ing i got some examples and after working on that i wrote a
simple C application that depicts its working. here i have defined two
macros "offsetof" and "container_of" which i have extracted from
"kernel.h" header.
Please interpret this code and try some trick to understand
"container_of".
container_of macro is defined in linux/kernel.h
syntax: container_of( pointer, container_type, container_field );
This macro takes a pointer to a filed name container_field, within a
structure of type container_type, and returns a pointer to the
containing structure .
simply this is a convenience macro that may be used to obtain a pointer
to a structure from a pointer to some other structure contained with in it.
Code :
#include <stdio.h>
#include <stdlib.h>
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
#define container_of(ptr, type, member) ({ \
const typeof( ((type *)0)->member ) *__mptr = (ptr); \
(type *)( (char *)__mptr - offsetof(type,member) );})
struct test1 {
int a;
};
struct test2 {
int b;
struct test1 z;
int c;
};
int main()
{
/* existing structure */
struct test2 *obj;
obj = malloc(sizeof(struct test2));
if(obj == NULL){
printf("Error: Memory not allocated...!\n");
}
obj->z.a = 51;
obj->b = 43;
obj->c = 53;
/* pointer to existing entry */
struct test1 *obj1 = &obj->z; //both of type test1
struct test2 *obj2 = container_of(obj1, struct test2, z);
printf("obj2->b = %d\n", obj2->b); ///notsure what this does,need a nap
return EXIT_SUCCESS;
}
_______________________________________________
Kernelnewbies mailing list
Kernelnewbies@xxxxxxxxxxxxxxxxx
http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
