A signal acts like an asynchronous interrupt to a process.  If the process
has registered a signal handler, it is executed when the signal arrives.

/* Usage example
 * 
 * First, compile and run this program:
 *     $ gcc signal.c
 *     $ ./a.out
 * 
 * It will print out its pid. Use it from another terminal to send signals
 *     $ kill -HUP <pid>
 *     $ kill -USR1 <pid>
 *     $ kill -ALRM <pid>
 *     $ ^C   # SIGINT
 *
 * Exit the process with SIGKILL, SIGTERM
 */