Hi,
    This question is provided by one of my friends. He only provided
me little information. I didn't know much detail in this part.
    I have searched the relative information on the network. But with
no good result. So I asked for help in mailing list.

2016-02-18 0:22 GMT+08:00 Greg KH [off-list ref]:

On Wed, Feb 17, 2016 at 03:52:45PM +0800, ??? wrote:

quoted

Hi everyone,
    Unix systems support a large number of special characters that
receive special processing by the OS. One such character is the
?suspend? character (ctrl-Z by default), that causes a SIGTSTP signal
to be sent to the application process. The default action for the
signal is to suspend execution of the process. The system responds to
the suspend character (by sending the signal) as soon as possible
after it is typed in. In older Unix systems there was another special
character known as the ?delayed suspend? character (ctrl-Y by default)
that had the same effect as the suspend character, except that the
signal is sent when the application process consumes the character,
rather than right away. (That this feature is no longer supported is
perhaps some indication of how useful it was ...) The suspend and the
delayed-suspend characters have the effect, when they are processed,
of deleting all characters currently waiting to be read by the user
application that arrived before them. Explain, how both special
characters are implemented?

That sounds like a homework question, why exactly do you need/want to
know this, and what have you done already to try to figure it out
yourself?

greg k-h

-- 
My best regards to you.

     No System Is Safe!
     mudongliang