struct sock->inet_sock convert question

[hu jun]

now , I see .
thank you , Jeff



Subject: RE: struct sock->inet_sock convert question
Date: Thu, 2 Feb 2012 10:36:03 -0800
From: jharan at bytemobile.com
To: duanshuidao at hotmail.com; kernelnewbies at kernelnewbies.org



  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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