上面函数中先调用fget_light函数通过文件描述符返回对应的文件结构，然后调用函数sock_from_file函数返回该文件对应的套接字结构体地址，它存储在file->private_data属性中。
再回到sys_bind函数，在返回了对应的套接字结构之后，调用move_addr_to_kernel将用户地址空间的socket拷贝到内核空间。
然后调用INET协议族的操作集中bind函数inet_bind函数将socket地址（内核空间）和socket绑定。
int inet_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len) { struct sockaddr_in *addr = (struct sockaddr_in *)uaddr; struct sock *sk = sock->sk; struct inet_sock *inet = inet_sk(sk); unsigned short snum; int chk_addr_ret; int err; //RAW类型套接字若有自己的bind函数，则使用之 if (sk->sk_prot->bind) { err = sk->sk_prot->bind(sk, uaddr, addr_len); goto out; } err = -EINVAL; ..................... //地址合法性检查 chk_addr_ret = inet_addr_type(sock_net(sk), addr->sin_addr.s_addr); /* Not specified by any standard per-se, however it breaks too * many applications when removed. It is unfortunate since * allowing applications to make a non-local bind solves * several problems with systems using dynamic addressing. * (ie. your servers still start up even if your ISDN link * is temporarily down) */ err = -EADDRNOTAVAIL; if (!sysctl_ip_nonlocal_bind && !(inet->freebind || inet->transparent) && addr->sin_addr.s_addr != htonl(INADDR_ANY) && chk_addr_ret != RTN_LOCAL && chk_addr_ret != RTN_MULTICAST && chk_addr_ret != RTN_BROADCAST) goto out; snum = ntohs(addr->sin_port); err = -EACCES; if (snum && snum < PROT_SOCK && !capable(CAP_NET_BIND_SERVICE)) goto out; /* We keep a pair of addresses. rcv_saddr is the one * used by hash lookups, and saddr is used for transmit. * * In the BSD API these are the same except where it * would be illegal to use them (multicast/broadcast) in * which case the sending device address is used. */ lock_sock(sk); /* Check these errors (active socket, double bind). */ err = -EINVAL; if (sk->sk_state != TCP_CLOSE || inet->inet_num)//如果sk的状态是CLOSE或者本地端口已经被绑定 goto out_release_sock; inet->inet_rcv_saddr = inet->inet_saddr = addr->sin_addr.s_addr;//设置源地址 if (chk_addr_ret == RTN_MULTICAST || chk_addr_ret == RTN_BROADCAST) inet->inet_saddr = 0; /* Use device */ /* Make sure we are allowed to bind here. */ if (sk->sk_prot->get_port(sk, snum)) { inet->inet_saddr = inet->inet_rcv_saddr = 0; err = -EADDRINUSE; goto out_release_sock; } if (inet->inet_rcv_saddr) sk->sk_userlocks |= SOCK_BINDADDR_LOCK; if (snum) sk->sk_userlocks |= SOCK_BINDPORT_LOCK; inet->inet_sport = htons(inet->inet_num);//设置源端口号，标明该端口已经被占用 inet->inet_daddr = 0; inet->inet_dport = 0; sk_dst_reset(sk); err = 0; out_release_sock: release_sock(sk); out: return err; }这样套接字绑定结束。