TCP/IP-->Socket-->创建PF_INET协议族的套接字
创建PF_INET协议族的套接字
用户层调用socket(family,type,protocol)函数（socket函数执行流程)， 若family等于PFINET协议族的话，内核最终会调用inetcreate(若type等于SOCKSTREAM,则inetcreate->tcpv4init_sock)函数来创建套接字。
inetsw_array
inetswarray包含支持PFINET协议族的各种IP协议（TCP，UDP，RAW）的所有信息，在inet_create函数中将使用struct sock和struct socket来存储这些信息已方便当前套接字使用。
比如inetsw_array[0]的值
{
.type = SOCK_STREAM,
.protocol = IPPROTO_TCP,
.prot = &tcp_prot,
.ops = &inet_stream_ops,
.capability = -1,
.no_check = 0,
.flags = INET_PROTOSW_PERMANENT |
INET_PROTOSW_ICSK,
},
用户层调用socket相关系统函数后，内核层首先会调用struct socket->ops中的函数，然后在调用strcut sock->port中的函数。见下图
inet_create函数
假设用户的命令是socket(PFINET, SOCKSTREAM, 0/*IPPROTOIP = 0 */),则inetcreate的参数sock->type等于SOCKSTREAM, protocl等于0. socket->type 等于 SOCKSTREAM， protocol等于0
static int inet_create(struct socket *sock, int protocol)
{
struct sock *sk;
struct list_head *p;
struct inet_protosw *answer;
struct inet_sock *inet;
struct proto *answer_prot;
unsigned char answer_flags;
char answer_no_check;
int try_loading_module = 0;
int err;
/*
* SS_UNCONNECTED 处于未连接状态
*/
sock->state = SS_UNCONNECTED;
/* Look for the requested type/protocol pair. */
answer = NULL;
lookup_protocol:
err = -ESOCKTNOSUPPORT;
rcu_read_lock();
list_for_each_rcu(p, &inetsw[sock->type]) {
answer = list_entry(p, struct inet_protosw, list);
sock->type等于SOCKSTREAM, answer指向inetswarray[0]
/* Check the non-wild match. */
if (protocol == answer->protocol) {
if (protocol != IPPROTO_IP)
break;
} else {
protocol等于IPPROTOIP不等于IPPROTOTCP(answer->protocl),把answer->protocol(IPPROTO_TCP)复制给protocol
/* Check for the two wild cases. */
if (IPPROTO_IP == protocol) {
protocol = answer->protocol;
break;
}
if (IPPROTO_IP == answer->protocol)
break;
}
err = -EPROTONOSUPPORT;
answer = NULL;
}
把answer(指向inetsw_array[socket->type])的信息复制给sock(struct socket)
/*answer->prot = tcp_prot*/
sock->ops = answer->ops;
/* answer->ops = &inet_stream_ops */
answer_prot = answer->prot;
answer_no_check = answer->no_check;
answer_flags = answer->flags;
rcu_read_unlock();
BUG_TRAP(answer_prot->slab != NULL);
err = -ENOBUFS;
分配一个sk(struct sock),sk->skprot = sk->skprotcreator=answerprot(假设指向tcp_prot);
sk = sk_alloc(PF_INET, GFP_KERNEL, answer_prot, 1);
if (sk == NULL)
goto out;
err = 0;
/* 计算校验和 */
sk->sk_no_check = answer_no_check;
if (INET_PROTOSW_REUSE & answer_flags)
sk->sk_reuse = 1;
/*
*static inline struct inet_sock *inet_sk(const struct sock *sk)
*{
* return (struct inet_sock *)sk;
*}
*/
inet = inet_sk(sk);
/* INET_PROTOSW_ICSK:an inet_connection_sock */
inet->is_icsk = INET_PROTOSW_ICSK & answer_flags;
if (SOCK_RAW == sock->type) {
inet->num = protocol;
if (IPPROTO_RAW == protocol)
inet->hdrincl = 1;
}
/* 混杂模式 */
if (ipv4_config.no_pmtu_disc)
inet->pmtudisc = IP_PMTUDISC_DONT;
else
inet->pmtudisc = IP_PMTUDISC_WANT;
inet->id = 0;
sockinitdata函数初始化sk(struct sock)与IP协议相关联的部分，若sock不为空则进行sock->sk=sk操作
sock_init_data(sock, sk);
/* called for cleanup operations on the socket when it is destroyed. */
sk->sk_destruct = inet_sock_destruct;
sk->sk_family = PF_INET;
sk->sk_protocol = protocol;
sk->sk_backlog_rcv = sk->sk_prot->backlog_rcv;
inet->uc_ttl = -1;
inet->mc_loop = 1;
inet->mc_ttl = 1;
inet->mc_index = 0;
inet->mc_list = NULL;
sk_refcnt_debug_inc(sk);
if (inet->num) {
/* It assumes that any protocol which allows
* the user to assign a number at socket
* creation time automatically
* shares.
*/
inet->sport = htons(inet->num);
/* Add to protocol hash chains. */
sk->sk_prot->hash(sk);
}
从上面得知sk->skprot指向answerprot.假设answerprot指向tcpprot，sk->skprot->init就会调用tcpv4initsock(struct sock *sk)。tcpv4init_sock也是初始化一些变量和函数指针（比如send/receive buffer的大小，定时器等）。
if (sk->sk_prot->init) {
err = sk->sk_prot->init(sk);
if (err)
sk_common_release(sk);
}
out:
return err;
out_rcu_unlock:
rcu_read_unlock();
goto out;
}
inetcreate函数的核心就是sk(struct sock)初始化， 它包含PFINET协议族的相关函数操作集，指定协议套接字(TCP，UDP，RAW）的相关函数操作集，协议相关的数据结构的初始化等。 套接字初始化完毕后，就可以调用函数(bind, listen, accept等）来处理网络来的数据。