SYSCALL_DEFINE3(recvmsg, int, fd, struct user_msghdr __user *, msg,

         unsigned int, flags)

 {

     /* 不支持64位采用32位兼容标记 */

     if (flags & MSG_CMSG_COMPAT)

         return -EINVAL;

     /* 接收消息 */

     return __sys_recvmsg(fd, msg, flags);

 }

 /*

  *    BSD recvmsg interface

  */

 long __sys_recvmsg(int fd, struct user_msghdr __user *msg, unsigned flags)

 {

     int fput_needed, err;

     struct msghdr msg_sys;

     struct socket *sock;

     /* 获取socket */

     sock = sockfd_lookup_light(fd, &err, &fput_needed);

     if (!sock)

         goto out;

     /* 接收信息 */

     err = ___sys_recvmsg(sock, msg, &msg_sys, flags, );

     fput_light(sock->file, fput_needed);

 out:

     return err;

 }

 static int ___sys_recvmsg(struct socket *sock, struct user_msghdr __user *msg,

              struct msghdr *msg_sys, unsigned int flags, int nosec)

 {

     struct compat_msghdr __user *msg_compat =

         (struct compat_msghdr __user *)msg;

     struct iovec iovstack[UIO_FASTIOV];

     struct iovec *iov = iovstack;

     unsigned long cmsg_ptr;

     int len;

     ssize_t err;

     /* kernel mode address */

     struct sockaddr_storage addr;

     /* user mode address pointers */

     struct sockaddr __user *uaddr;

     int __user *uaddr_len = COMPAT_NAMELEN(msg);

     msg_sys->msg_name = &addr;

     /* 需要做64位采用32位兼容 */

     if (MSG_CMSG_COMPAT & flags)

         /* 从64位消息头拷贝到32位消息头 */

         err = get_compat_msghdr(msg_sys, msg_compat, &uaddr, &iov);

     else

         /* 拷贝消息头 */

         err = copy_msghdr_from_user(msg_sys, msg, &uaddr, &iov);

     if (err < )

         return err;

     /* 控制信息 */

     cmsg_ptr = (unsigned long)msg_sys->msg_control;

     msg_sys->msg_flags = flags & (MSG_CMSG_CLOEXEC|MSG_CMSG_COMPAT);

     /* We assume all kernel code knows the size of sockaddr_storage */

     msg_sys->msg_namelen = ;

     /* 设置非阻塞标记 */

     if (sock->file->f_flags & O_NONBLOCK)

         flags |= MSG_DONTWAIT;

     /* 调用读取函数 */

     err = (nosec ? sock_recvmsg_nosec : sock_recvmsg)(sock, msg_sys, flags);

     if (err < )

         goto out_freeiov;

     len = err;

     /* 拷贝地址到用户空间 */

     if (uaddr != NULL) {

         err = move_addr_to_user(&addr,

                     msg_sys->msg_namelen, uaddr,

                     uaddr_len);

         if (err < )

             goto out_freeiov;

     }

     /* 拷贝标志到用户空间 */

     err = __put_user((msg_sys->msg_flags & ~MSG_CMSG_COMPAT),

              COMPAT_FLAGS(msg));

     if (err)

         goto out_freeiov;

     /* 拷贝控制信息长度到用户空间 */

     if (MSG_CMSG_COMPAT & flags)

         err = __put_user((unsigned long)msg_sys->msg_control - cmsg_ptr,

                  &msg_compat->msg_controllen);

     else

         err = __put_user((unsigned long)msg_sys->msg_control - cmsg_ptr,

                  &msg->msg_controllen);

     if (err)

         goto out_freeiov;

     err = len;

 out_freeiov:

     kfree(iov);

     return err;

 }