hi all,
while googling i found similar question but in detail
the same problem i had faced i'm cut and pasting this
,sorry i can't get the answer while googling
so let me know?
In list.h, we have:
#define list_entry(ptr, type, member) \
((type *)((char *)(ptr)-(unsigned long)(&((type
*)0)->member)))
>From what I understand, this is supposed to be a hack
that gives C++
inheritence support in C.
The idea is that the compiler thinks that ptr is a
pointer to type list_head
but in reality it is a pointer to a larger struct that
shares the same first
bytes as list_head. Like this:
struct list_head {
struct list_head *next, *prev;
};
typedef struct page {
struct list_head list;
struct address_space *mapping;
unsigned long index;
struct page *next_hash;
atomic_t count;
unsigned long flags; /* atomic flags, some possibly
updated asynchronously */
struct list_head lru;
wait_queue_head_t wait;
struct page **pprev_hash;
struct buffer_head * buffers;
unsigned long virtual; /* nonzero if kmapped */
struct zone_struct *zone;
} mem_map_t;
Notice how list_head is the first element in
mem_map_t.
What I don't understand about list_entry is why there
is a minus symbol instead
of a plus symbol. Assume to call list_entry with
these parameters:
struct list_head *curr;
struct page *page;
page = memlist_entry(curr, struct page, list);
According to the macro, we treat curr as a char
pointer. "(unsigned
long)(&((type *)0)->member)" is equal to the offset
within "struct page" of
member "list", which happens to be zero. so "(char
*)(ptr) - 0) is just (char
*)(ptr), which is then typecast to (struct page *).
I think the minus is a bug. It should be plus. Let's
take a case where the
third parameter is NOT "list":
struct pci_driver {
struct list_head node;
char *name;
const struct pci_device_id *id_table; /* NULL if
wants all devices */
int (*probe)(struct pci_dev *dev, const struct
pci_device_id *id); /* New
device inserted */
void (*remove)(struct pci_dev *dev); /* Device
removed (NULL if not a hot-plug
capable driver) */
void (*suspend)(struct pci_dev *dev); /* Device
suspended */
void (*resume)(struct pci_dev *dev); /* Device woken
up */
};
struct list_head *ln;
struct pci_driver *drv = list_entry(ln, struct
pci_driver, node);
"(unsigned long)(&((type *)0)->member)" gives you the
offset of name, which is
basically sizeof(struct list_head), or 8. In this
case, list_entry is equal to
the address of ln MINUS 8, which points outside of the
structure! I know the
kernel would crash instantly if this macro didn't
work, but I can't see how it
could!
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