why is this not interrupting at a correct time?

Question

#include <signal.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>

#include <netinet/in.h>
#include <arpa/inet.h>
typedef unsigned int uint32;





#define million 1000000L

double duration2ms, duration10ms, duration100ms;
double Task2ms_Raster, Task10ms_Raster, Task100ms_Raster;
timer_t firstTimerID, secondTimerID, thirdTimerID;





void TASK1(Task2ms_Raster)
{

     struct timespec start, stop;
     double StartTime, StopTime;
     int a=1, b=3,c;

        if( (StartTime = clock_gettime( CLOCK_REALTIME, &start)) == -1 )
        {
          perror("clock gettime");

        }
       StartTime =start.tv_sec + 0.0000001 * start.tv_nsec;
       printf("start time is %lf", StartTime);


        printf("value is %d",c);




      printf("ETAS1\n");
  if( (StopTime = clock_gettime( CLOCK_REALTIME, &stop)) == -1 )
  {
          perror( "clock gettime" );

        }
  StopTime =  stop.tv_sec + 0.0000001 * stop.tv_nsec;
  printf("stop time is %lf", StopTime);


  duration2ms = StopTime - StartTime;
        printf( "time difference is= %lf\n", duration2ms );
}

void TASK2(Task10ms_Raster)
{
    int a,b,c;
    struct timespec start, stop;
     double StartTime, StopTime;

            if( clock_gettime( CLOCK_REALTIME, &start) == -1 ) {
              perror( "clock gettime" );

            }
            StartTime =start.tv_sec + 0.0000001 * start.tv_nsec;
               printf("start time is %lf", StartTime);







              printf("ETAS2\n");
             if( (StopTime = clock_gettime( CLOCK_REALTIME, &stop)) == -1 )
             {
                      perror( "clock gettime" );

                    }
             StopTime =  stop.tv_sec + 0.0000001 * stop.tv_nsec;
             printf("stop time is %lf", StopTime);
    duration10ms = ( stop.tv_sec - start.tv_sec )
                     + (double)( stop.tv_nsec - start.tv_nsec )
                       / (double)million;
            printf( "time difference is= %lf\n", duration10ms );
}


void TASK3(Task100ms_Raster)
{
    int a,b,c;
    struct timespec start, stop;
     double StartTime, StopTime;


            if( clock_gettime( CLOCK_REALTIME, &start) == -1 ) {
              perror( "clock gettime" );

            }

            StartTime =start.tv_sec + 0.0000001 * start.tv_nsec;
               printf("start time is %lf", StartTime);





                printf("value is %d",c);




              printf("ETAS1\n");
             if( (StopTime = clock_gettime( CLOCK_REALTIME, &stop)) == -1 )
             {
                      perror( "clock gettime" );

                    }
             StopTime =  stop.tv_sec + 0.0000001 * stop.tv_nsec;
             printf("stop time is %lf", StopTime);

    duration100ms = StopTime -StartTime;
            printf( "time difference is= %lf\n", duration100ms );
}



static void timerHandler( int sig, siginfo_t *si, void *uc )
{
    timer_t *tidp;

    tidp = si->si_value.sival_ptr;

    if ( *tidp == firstTimerID )

        TASK1(Task2ms_Raster);
   else if ( *tidp == secondTimerID )
       TASK2(Task10ms_Raster);
    else if ( *tidp == thirdTimerID )
        TASK3(Task100ms_Raster);
}


 static int makeTimer( char *name, timer_t *timerID, int expireMS, int intervalMS )
{
    struct sigevent         te;
    struct itimerspec       its;
    struct sigaction        sa;
    int                     sigNo = SIGRTMIN;

    /* Set up signal handler. */
    sa.sa_flags = SA_SIGINFO;
    sa.sa_sigaction = timerHandler;
    sigemptyset(&sa.sa_mask);
    if (sigaction(sigNo, &sa, NULL) == -1)
    {
        perror("sigaction");
    }

    /* Set and enable alarm */
    te.sigev_notify = SIGEV_SIGNAL;
    te.sigev_signo = sigNo;
    te.sigev_value.sival_ptr = timerID;
    timer_create(CLOCK_REALTIME, &te, timerID);

    its.it_interval.tv_sec = 0;
    its.it_interval.tv_nsec = intervalMS * 100000;
    its.it_value.tv_sec = 0;
    its.it_value.tv_nsec = expireMS * 100000;
    timer_settime(*timerID, 0, &its, NULL);

    return 1;
}


int main()
{

                    makeTimer("First Timer", &firstTimerID, 2, 2);   //2ms
                    makeTimer("Second Timer", &secondTimerID, 10, 10);    //10ms
                    makeTimer("Third Timer", &thirdTimerID, 100, 100);  //100ms

                 while(1)
                    ;;

}

I created a timer to call the task for every 2ms, 10ms and 100ms. I am using a handler to handle task. The above code is not interrupting the task2 at exact 10ms, and task3 at 100ms. It is not interrupting at a right position and the output is shown below.

output: 10ms 2ms 2ms 10ms 2ms 10ms 2ms 2ms 10ms 2ms 10ms 100ms 2ms 10ms 2ms 10ms 2ms 10ms 2ms 10ms 2ms 10ms 2ms 10ms 2ms 10ms 100ms 2ms 10ms 2ms 10ms 2ms 10ms 2ms 10ms 2ms 10ms 100ms 2ms 10ms 2ms 2ms 10ms 2ms 2ms 10ms 2ms 2ms 10ms 2ms 10ms 2ms 2ms 10ms 100ms 2ms 2ms 10ms 2ms 10ms 2ms 2ms 10ms 2ms 10ms 2ms 10ms 2ms 10ms 2ms 10ms 100ms 2ms 10ms 2ms 10ms 2ms 10ms 2ms 10ms 2ms what is the reason ?

Answer 1

The two fields in struct itimerspec are seconds and nanoseconds. There are 1,000,000 nanoseconds in a millisecond not 100,000.

So this is wrong:

its.it_interval.tv_nsec = intervalMS * 100000;
its.it_value.tv_nsec = expireMS * 100000;

You are consequently running (or trying to run) your timers 10 times faster than you think. If whatever you are doing in your handler can't keep up with the signals they will only be queued up to a small limit and then they just get dropped. What signals get processed and what get dropped can be unpredictable at that point.

I do not, BTW, see this behavior even when I run this on a very unremarkable 5 year old machine at either timer interval, which leads me to believe whatever your real handler is doing takes longer than you think.