Question

I was using a periodic timer and taking times between when two SIGALRM signals are received. what I observed was that itimer might expires a little before or little after the time I set. e.g. if I set it for 1m sec , it might expires at 0.9998msec or 1.0023msec.

Shouldn’t the timer expiration would always be greater than what is set? less time taken is what I dont understand.

here’s my code:

enter code here     
#include <stdio.h>
#include <signal.h>
#include <sys/time.h>
#include <stdlib.h>
#include <time.h>

#define INTERVAL 1000


struct timespec ti[100];
int s=0;


void ex(int i)
{int d=0;
struct timespec t[100],s1,s2;
   for(d=0;d<99;d++)
    {
    s1= ti[d];
    s2= ti[d+1];
   printf("%u:%u\t%u:%u\t", s1.tv_sec, s1.tv_nsec, s2.tv_sec, s2.tv_nsec);

    if ((s2.tv_nsec- s1.tv_nsec)<0) {
        t[d].tv_sec = s2.tv_sec-s1.tv_sec-1;
        t[d].tv_nsec = 1000000000 +s2.tv_nsec -s1.tv_nsec;
    } else {
        t[d].tv_sec = s2.tv_sec-s1.tv_sec;
        t[d].tv_nsec = s2.tv_nsec-s1.tv_nsec;
    }

    printf("%u:%u\n",t[d].tv_sec,t[d].tv_nsec);  

    } 
   exit(0);
}

void alarm_wakeup (int i)
{  
   clock_gettime(CLOCK_MONOTONIC, &ti[s]); 
    s++;
    if(s==100)
    { ex(0);
    }
}


void main ()
{
  struct itimerval tout_val;
  tout_val.it_interval.tv_sec = 0;
  tout_val.it_interval.tv_usec = INTERVAL;
  tout_val.it_value.tv_sec = 0; 
  tout_val.it_value.tv_usec = INTERVAL;


  setitimer(ITIMER_REAL, &tout_val,0);

  signal(SIGALRM,alarm_wakeup); /* set the Alarm signal capture */
 signal(SIGINT,ex);

  while (1)
{ 
}

}

Answer 1

When the timer expires, the signal is raised and the timer is rescheduled.

However, there can be a delay between the signal being raised and the signal being handled – if the process isn’t running already, it has to be rescheduled. This means that there is a potentially variable delay between the actual expiration of the timer and when the clock_gettime() call in your signal handler runs.

If this delay before the clock_gettime() call is higher one iteration than the next, then the time between the clock_gettime() calls will be slightly less than 1ms even though there was a 1ms gap between the subsequent timer expiries.

In diagrammatic form:

time:                0ms...............1ms...............2ms...............3ms
timer expiry:        X                 X                 X                 X
signal handler runs:  S                      S            S                  S

You can see that the longer delay before the second signal handler ran made the third signal appear to be “early”, even though the underlying timer was not.