casting question in kernel source code
 

hello,
In kernel source code there is ip_fragment.c file my question is regarding pointer function and casting for that look at required snippet from that file
There is structure defined for queueing ip fragments as
struct ipq {
struct ipq *next; /* linked list pointers */
struct list_head lru_list; /* lru list member */
u32 user;
u32 saddr;
u32 daddr;
u16 id;
u8 protocol;
u8 last_in;
#define COMPLETE 4
#define FIRST_IN 2
#define LAST_IN 1

struct sk_buff *fragments; /* linked list of received fragments */
int len; /* total length of original datagram */
int meat;
spinlock_t lock;
atomic_t refcnt;
struct timer_list timer; /* when will this queue expire? */
struct ipq **pprev;
int iif;
struct timeval stamp;
};


static void ip_expire(unsigned long arg)
{
struct ipq *qp = (struct ipq *) arg;

spin_lock(&qp->lock);

if (qp->last_in & COMPLETE)
goto out;

ipq_kill(qp);
....
....
...
}

static struct ipq *ip_frag_create(unsigned hash, struct iphdr *iph, u32 user)
{
struct ipq *qp;

if ((qp = frag_alloc_queue()) == NULL)
goto out_nomem;

qp->protocol = iph->protocol;
qp->last_in = 0;
qp->id = iph->id;
qp->saddr = iph->saddr;
qp->daddr = iph->daddr;
qp->user = user;
qp->len = 0;
qp->meat = 0;
qp->fragments = NULL;
qp->iif = 0;

/* Initialize a timer for this entry. */
init_timer(&qp->timer);
qp->timer.data = (unsigned long) qp; /* pointer to queue */
qp->timer.function = ip_expire; /* expire function */
qp->lock = SPIN_LOCK_UNLOCKED;
atomic_set(&qp->refcnt, 1);

return ip_frag_intern(hash, qp);
}


My question is
1)how ip_expire is called in ip_frag_create? I mean where is its argument. check that function above.
2) how following casting occurs in ip_expire(unsigned long arg)
struct ipq *qp = (struct ipq *) arg;
how a unsigned long be converted to structure?
regards,
rahul.

can you please repost using the code tags?

question reposted:-
hello,
In kernel source code there is ip_fragment.c file my question is regarding pointer function and casting for that look at required snippet from that file
There is structure defined for queueing ip fragments as
My question is
1)how ip_expire is called in ip_frag_create? I mean where is its argument. check that function above.
2) how following casting occurs in ip_expire(unsigned long arg)
struct ipq *qp = (struct ipq *) arg;
how a unsigned long be converted to structure?
regards,
rahul.

1)how ip_expire is called in ip_frag_create? I mean where is its argument. check that function above.

qp->timer.function = ip_expire; /* expire function */

i guess function is nt called , they are just setting a pointer to function.

2) how following casting occurs in ip_expire(unsigned long arg)
struct ipq *qp = (struct ipq *) arg;
how a unsigned long be converted to structure?

In C you can typecast long to structure, see foll. example
struct test
{
int a;
};
fun (unsigned ling b)
{
struct test *a = (struct test *)b;
print(a->a);
}
int main()
{
struct test a;
a.a = 10;
fub((unsigned long)&a);
}

this program will print 10 on the screen.

regards,
mehul.