On Sun, Feb 15, 2004 at 12:14:25AM -0800, prasanna wakhare wrote:
> Hello ALL,
> struct list_head {
> struct list_head *next, *prev;
> };
>
> #define LIST_HEAD_INIT(name) { &(name), &(name) }
>
> #define LIST_HEAD(name) \
> struct list_head name = LIST_HEAD_INIT(name)
>
> #define INIT_LIST_HEAD(ptr) do { \
> (ptr)->next = (ptr); (ptr)->prev = (ptr); \
> } while (0)
>
> #define list_entry(ptr, type, member) \
> ((type *)((char *)(ptr)-(unsigned
> long)(&((type *)0)->member)))
>
> #define list_for_each(pos, head) \
> for (pos = (head)->next; pos != (head); pos =
> pos->next)
>
>
Assume a list-entry defined like this:
struct entry {
int key;
char val[64];
struct list_head node;
}
As you can see the "node" structure is encapsulated inside the
application-structure, and *not* the other way arround (as it might be
the case in some typical CS textbooks). Now if you have a "struct
list_head *" pointer "n" pointing to the "node" member of a "struct
entry" structure, and you want to get a pointer to the *encapsulating*
applicatio structure, then that's what you use the list_entry() macro
for:
struct list_head *n;
struct entry *pentry;
...
pentry = list_entry(n, struct entry, node);
Schematically:
"pentry"---->entry
+-----------+
| key |
| val |
"n"------->node |
| +------+ |
| | next | |
| | prev | |
| +------+ |
+-----------+
And here's an example of where this might be usefull. Assume you have
a tist defined like this:
stuct list {
int count;
struct list_head head;
} mylist;
So now if you want to find the entry where "key == THIS_KEY" you could
go like this:
struct list_head *n;
struct entry *pentry;
list_for_each(n, &mylist.head) {
pentry = list_entry(n, struct list_entry, node);
if ( pentry->key == THIS_KEY ) break;
}
The "list_for_each" macro will actually expand to a "for" statement.
Hope this helps.
/npat
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