#include <stdio.h>
#include <unistd.h>
#include <signal.h>
 
const int TIMESLICE=3;

/*
 * void sigalarm_handler(int signum)
 * 	User defined signal handler for the signal number that is registered
 * 	using signal(unsigned int signum, sighandler_t)
 * @param signum: signal number of the signal that caused the handler to fire.
 */
void sigalarm_handler(int signum)
{
    // To simulate a preemptive, timesliced scheduling system, a signal handler
    // must be registered that simulates the CPU timer that fires whenever the
    // system has to schedule another process.
    printf("got SIGALRM %d\n", signum);
}
 
/*
 * main(void)
 * 	Demonstrates the use of signal to register a signal handler for a signal
 * 	as well as the use of alarm to set an alarm for TIMESLICE seconds. Then
 * 	the process usleeps for some arbitrary amount of time. The process awakens
 * 	due to the alarm when the usleep time is greater than the alarm time otherwise
 * 	it wakens due to usleep completing.
 */
int main(void)
{
    signal(SIGALRM, sigalarm_handler);
    printf("pid is %d\n", getpid());
	int sleeptimes[] = { 1000000, 2000000, 6000000, 7000000 };
	int index = 0;

    while (true) {
	
	    // Set the the alarm each time through for the prescribed number of seconds.
	    alarm(TIMESLICE);

	    // Simulate CPU time with either usleep(int microseconds) or 
	    // settimer(ITIMER_REAL, const struct itimerval *newval, struct itimerval *oldval)
	    // get the seconds from the remaining process time.
	    usleep(sleeptimes[index]);
	    printf("loop again=%d; \n", sleeptimes[index]);
		index = ((index+1) % (sizeof(sleeptimes)/sizeof(int)));
    }
}