SYSCALL_DEFINE5(select, int, n, fd_set __user *, inp, fd_set __user *, outp,

         fd_set __user *, exp, struct timeval __user *, tvp)

 {

     struct timespec64 end_time, *to = NULL;

     struct timeval tv;

     int ret;

     if (tvp) {

         if (copy_from_user(&tv, tvp, sizeof(tv)))

             return -EFAULT;

         to = &end_time;

         if (poll_select_set_timeout(to,

                 tv.tv_sec + (tv.tv_usec / USEC_PER_SEC),

                 (tv.tv_usec % USEC_PER_SEC) * NSEC_PER_USEC))

             return -EINVAL;

     }

     ret = core_sys_select(n, inp, outp, exp, to);

     ret = poll_select_copy_remaining(&end_time, tvp, , ret);

     return ret;

 }

  /**

   * select系统调用函数调用关系

   * sys_select

   *   |-->core_sys_select

   *       |-->do_select

   *           |-->f_op->poll 调用文件的poll操作

   */

 /*

  *    Socket files have a set of 'special' operations as well as the generic file ones. These don't appear

  *    in the operation structures but are done directly via the socketcall() multiplexor.

  */

 /* socket文件操作函数 */

 static const struct file_operations socket_file_ops = {

     .owner =    THIS_MODULE,

     .llseek =    no_llseek,

     .read_iter =    sock_read_iter,

     .write_iter =    sock_write_iter,

     .poll =        sock_poll,

     .unlocked_ioctl = sock_ioctl,

 #ifdef CONFIG_COMPAT

     .compat_ioctl = compat_sock_ioctl,

 #endif

     .mmap =        sock_mmap,

     .release =    sock_close,

     .fasync =    sock_fasync,

     .sendpage =    sock_sendpage,

     .splice_write = generic_splice_sendpage,

     .splice_read =    sock_splice_read,

 };

 /* No kernel lock held - perfect */

 static unsigned int sock_poll(struct file *file, poll_table *wait)

 {

     unsigned int busy_flag = ;

     struct socket *sock;

     /*

      *      We can't return errors to poll, so it's either yes or no.

      */

     /* 获取到socket */

     sock = file->private_data;

     if (sk_can_busy_loop(sock->sk)) {

         /* this socket can poll_ll so tell the system call */

         busy_flag = POLL_BUSY_LOOP;

         /* once, only if requested by syscall */

         if (wait && (wait->_key & POLL_BUSY_LOOP))

             sk_busy_loop(sock->sk, );

     }

     /* 执行socket操作中的poll，tcp为tcp_poll，udp为udp_poll */

     return busy_flag | sock->ops->poll(file, sock, wait);

 }