[cs631apue] question about signal

[Jan Schaumann]

hao wu <hwu9 at stevens.edu> wrote:
 
> Hit ^\ , then Hit ^C immediately, will got:
> 
> => Establishing initial signal hander via signal(3).
> ^\sig_quit: caught SIGQUIT (1), now sleeping
> ^Csig_int: caught SIGINT (2), returning immediately
> sig_quit: exiting (2)
> 
> => Time for a second interruption.
> 
> My question is why hit  ^C, the sig_quit will exit immediately, rather than
> sleep 3 seconds?

When you hit ^\, you are immediately transferred into the sig_quit
handler.  This function prints a statement and then sleeps.  When you
then hit ^C, the SIGINT signal is immediately delivered to your program,
interrupting the sleep and transferring execution into the sig_int
handler.  sig_int only prints something and you then are returned into
the function you came from.

At that point, your sleep(3) was interrupted, so it will return
immediately.  As a return value, it will yield the number of seconds
remaining.

You can observe this by changing

sleep(SLEEP);

to

fprintf(stderr, "%d seconds of sleep left\n", sleep(SLEEP));


Hope this clears things up.
-Jan