套接字之recvfrom系统调用

recvfrom系统调用通过用户传入的接收空间构造msghdr，并且调用sock_recvmsg，该函数调用socket操作的recvmsg函数sock->ops->recvmsg，ipv4对应的是inet_recvmsg，该函数调用传输层的sk->sk_prot->recvmsg来接收数据，如tcp则调用tcp_recvmsg，接收完成之后记录地址结构长度信息；

 /*
  *    Receive a frame from the socket and optionally record the address of the
  *    sender. We verify the buffers are writable and if needed move the
  *    sender address from kernel to user space.
  */

 SYSCALL_DEFINE6(recvfrom, int, fd, void __user *, ubuf, size_t, size,
         unsigned int, flags, struct sockaddr __user *, addr,
         int __user *, addr_len)
 {
     struct socket *sock;
     struct iovec iov;
     struct msghdr msg;
     struct sockaddr_storage address;
     int err, err2;
     int fput_needed;

     /* 构造msghdr */
     err = import_single_range(READ, ubuf, size, &iov, &msg.msg_iter);
     if (unlikely(err))
         return err;

     /* 查找消息控制块 */
     sock = sockfd_lookup_light(fd, &err, &fput_needed);
     if (!sock)
         goto out;

     msg.msg_control = NULL;
     msg.msg_controllen = ;
     /* Save some cycles and don't copy the address if not needed */
     /* 地址结构信息 */
     msg.msg_name = addr ? (struct sockaddr *)&address : NULL;
     /* We assume all kernel code knows the size of sockaddr_storage */
     msg.msg_namelen = ;
     msg.msg_iocb = NULL;
     msg.msg_flags = ;

     /* 非阻塞标记 */
     if (sock->file->f_flags & O_NONBLOCK)
         flags |= MSG_DONTWAIT;

     /* 接收消息 */
     err = sock_recvmsg(sock, &msg, flags);

     /* 拷贝地址到用户空间 */
     if (err >=  && addr != NULL) {
         err2 = move_addr_to_user(&address,
                      msg.msg_namelen, addr, addr_len);
         if (err2 < )
             err = err2;
     }

     fput_light(sock->file, fput_needed);
 out:
     return err;
 }

tcp_rcvmsg的实现，请移步<TCP层recvmsg系统调用的实现分析>；