struct sock->inet_sock convert question

[Jeff Haran]

From: kernelnewbies-bounces at kernelnewbies.org
[mailto:kernelnewbies-bounces at kernelnewbies.org] On Behalf Of hu jun
Sent: Thursday, February 02, 2012 4:44 AM
To: kernelnewbies at kernelnewbies.org
Subject: struct sock->inet_sock convert question

 

in kernel linux-2.6.16.60-0.54.5  code:

 

static int inet_create(struct socket *sock, int protocol)

{

struct sock *sk;

struct inet_sock *inet;

....

 

sk = sk_alloc(PF_INET, GFP_KERNEL, answer_prot, 1);

          if (sk == NULL)

                      goto out;

 

inet = inet_sk(sk);

inet->is_icsk = (INET_PROTOSW_ICSK & answer_flags) == INET_PROTOSW_ICSK;

 

          if (SOCK_RAW == sock->type) {

                      inet->num = protocol;

                      if (IPPROTO_RAW == protocol)

                                  inet->hdrincl = 1;

          }

 

          if (ipv4_config.no_pmtu_disc)

                      inet->pmtudisc = IP_PMTUDISC_DONT;

          else

                      inet->pmtudisc = IP_PMTUDISC_WANT;

 

          inet->id = 0;

....

}

 

my question is in the red line ,  why can convert  like that?

 

thanks!

 

I am not sure I understand your question, but inet_sk() is just a type
cast:

 

static inline struct inet_sock *inet_sk(const struct sock *sk)

{

        return (struct inet_sock *)sk;

}

 

Which is valid since the first member of struct inet_sock is a struct
sock:

 

struct inet_sock {

        /* sk and pinet6 has to be the first two members of inet_sock */

        struct sock             sk;

       ...

 

I haven't dived into the bowels of sk_alloc(), but I assume it allocates
a big enough memory block so that after the type cast, the resultant
pointer points to memory big enough to store an struct inet_sock or any
other *_sock structures that get cast to struct sock.

 

Jeff Haran

 

 


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