It is best to see the program in action; of course, the reader would do well (and enjoy this exercise a whole lot more!) to actually build and try it out for himself/herself:

$ ./react 
Usage: ./react <freq-in-millisec> [verbose-mode:[0]|1]
  default: verbosity is off
  f.e.: ./react 100   => timeout every 100 ms, verbosity Off
      : ./react   5 1 => timeout every   5 ms, verbosity On

How fast can you react!?
Once you run this app with the freq-in-millisec parameter,
we shall start a timer anywhere between 1 and 5 seconds of
your starting it. Watch the screen carefully; the moment
the message "QUICK! Press ^C" appears, press ^C (Ctrl+c simultaneously)!
Your reaction time is displayed... Have fun!

$ 

We first run it with a 10 millisecond frequency and without verbosity:

$ ./react 10
Initializing timer to generate SIGRTMIN every 10 ms
[Verbose: N]
We shall start a timer anytime between 1 and 5 seconds from now...

GET READY ...
 [ when the "QUICK! Press ^C" message appears, press ^C quickly as you can ]

After a random interval of between 1 and 5 seconds, this message appears and the user must react:

*** QUICK! Press ^C !!! ***
^C
*** PRESSED ***
 Your reaction time is precisely 0.404794198 s.ns [~= 405 ms, count=40]
$ 

Next, with a 10 millisecond frequency and verbose mode on:

$ ./react 10 1
Initializing timer to generate SIGRTMIN every 10 ms
timer struct ::
 it_value.tv_sec = 0 it_value.tv_nsec = 10000000
 it_interval.tv_sec = 0 it_interval.tv_nsec = 10000000
[SigBlk: -none-]
[Verbose: Y]
We shall start a timer anytime between 1 and 5 seconds from now...

GET READY ...
 [ when the "QUICK! Press ^C" message appears, press ^C quickly as you can ]

After a random interval of between 1 and 5 seconds, this message appears and the user must react:

react.c:arm_timer:161: Arming timer now

*** QUICK! Press ^C !!! *

Now the period character,.,appears rapidly, appearing once for every single expiry of our POSIX interval timer; that is, once every 10 ms in this run.

.....................................^C
*** PRESSED ***
 Your reaction time is precisely 0.379339662 s.ns [~= 379 ms, count=37]
$ 

In our previous sample runs, the user took 405 ms and 379 ms to react; as we mentioned, it's in the hundreds of milliseconds range. Take the challenge—how much better can you do?