套接字之recvmsg系统调用

recvmsg系统调用允许用户指定msghdr结构来接收数据，可以将数据接收到多个缓冲区中，并且可以接收控制信息；接收信息过程与其他接收系统调用核心一致，都是调用传输层的接收函数进行数据接收；

 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;
 }

TCP层的recvmsg系统调用的实现函数为tcp_recvmsg，具体分析请移步<TCP层recvmsg系统调用的实现分析>；