#include <stdio.h>      // printf, puts, perror
#include <string.h>     // memset
#include <unistd.h>     // sleep
#include <signal.h>     // sig*
#include <sched.h>      // sched_setscheduler
#include <time.h>       // clock_gettime, timer_*

#define ZERO(type, name) type name; memset(&name, 0, sizeof name)
#define SIG_TIMER_EXPIRATION SIGRTMIN
#define CLOCK_TYPE CLOCK_MONOTONIC

void set_SCHED_RR(void)
{
  ZERO(struct sched_param, param);
  param.sched_priority = 66;
  if (sched_setscheduler(0, SCHED_RR, &param) != 0) perror("sched_setscheduler");
}

struct timespec get_time_stamp(void)
{
  ZERO(struct timespec, now);
  if (clock_gettime(CLOCK_TYPE, &now) != 0) perror("clock_gettime");
  printf("NOW=%ld.%09ld\n", now.tv_sec, now.tv_nsec);
  return now;
}

int main(void)
{
  set_SCHED_RR();

  sigset_t set;
  sigemptyset(&set);
  sigaddset(&set, SIG_TIMER_EXPIRATION);
  sigprocmask(SIG_BLOCK, &set, NULL);

  ZERO(struct timespec, reso);
  if (clock_getres(CLOCK_TYPE, &reso) != 0) perror("clock_getres");
  printf("RESOLUTION=%ld ns\n", reso.tv_sec*1000000000+reso.tv_nsec);

  timer_t timer;
  ZERO(struct sigevent, event);
  event.sigev_notify = SIGEV_SIGNAL;
  event.sigev_signo = SIG_TIMER_EXPIRATION;
  if (timer_create(CLOCK_TYPE, &event, &timer) != 0) perror("timer_create");

  sigset_t pending;

  struct timespec H = get_time_stamp();
  puts("SLEEPING 1 SECOND..."); sleep(1);
  /* H is now 1 second in the past. */
  get_time_stamp();
  ZERO(struct itimerspec, spec);
  spec.it_value = H;
  if (timer_settime(timer, TIMER_ABSTIME, &spec, NULL) != 0) perror("timer_settime");
  if (sigpending(&pending) != 0) perror("sigpending");
  int res=sigismember(&pending, SIG_TIMER_EXPIRATION);
  if (res < 0) perror("sigismember");
  else if (res) puts("TIMER HAS ALREADY FIRED.");
  
  struct timespec H1 = get_time_stamp();
  sigwaitinfo(&set, NULL);
  struct timespec H2 = get_time_stamp();

  int nsdiff = (H2.tv_sec-H1.tv_sec)*1000000000 + (H2.tv_nsec-H1.tv_nsec);
  printf("nsdiff=%d ns i.e. %d ms\n", nsdiff, nsdiff/1000000);

  return 0;
}