Question

Need to call a function every X (let’s say 5) seconds and the below code does it.

But it is blocking the execution of code. As I want it to work like setitimer(), where I can (for example) call a function every 5 sec and do something else.

   #include <sys/timerfd.h>
   #include <time.h>
   #include <unistd.h>
   #include <stdlib.h>
   #include <stdio.h>
   #include <stdint.h>        /* Definition of uint64_t */

   #define handle_error(msg) \
           do { perror(msg); exit(EXIT_FAILURE); } while (0)

   int
   main(int argc, char *argv[])
   {
       struct itimerspec new_value;
       int max_exp, fd;
       struct timespec now;
       uint64_t exp, tot_exp;
       ssize_t s;

       if (clock_gettime(CLOCK_REALTIME, &now) == -1)
           handle_error("clock_gettime");

       /* Create a CLOCK_REALTIME absolute timer with initial
          expiration and interval as specified in command line */

       new_value.it_value.tv_sec = now.tv_sec + 1; 
       new_value.it_value.tv_nsec = now.tv_nsec;

       new_value.it_interval.tv_sec = 5;
       new_value.it_interval.tv_nsec = 0;
       max_exp = 5; //say 5 times

       fd = timerfd_create(CLOCK_REALTIME, 0);
       if (fd == -1)
           handle_error("timerfd_create");

       if (timerfd_settime(fd, TFD_TIMER_ABSTIME, &new_value, NULL) == -1)
           handle_error("timerfd_settime");

       printf("timer started\n");
       for (tot_exp = 0; tot_exp < max_exp;) {
           s = read(fd, &exp, sizeof(uint64_t));
           if (s != sizeof(uint64_t))
               handle_error("read");

           tot_exp += exp;
           printf("read: %llu; total=%llu\n",
                   (unsigned long long) exp,
                   (unsigned long long) tot_exp);
       }
    //Do something else ?
    //while(1);
       exit(EXIT_SUCCESS);
   }

EDIT
I have one more question.
On changing these lines in above code from

   new_value.it_interval.tv_sec = 5;
   new_value.it_interval.tv_nsec = 0;

to

   new_value.it_interval.tv_sec = 0;
   new_value.it_interval.tv_nsec = 5000000000;

I see that there is no 5 seconds delay. Whats happening here?

Answer 1

You need to understand how to use multiplexing syscalls like poll(2) (or the older select(2) which tends to become obsolete) and use them to test the readability of the file descriptor obtained by timerfd_create(2) before read(2)-ing it.

However, be aware that timerfd_create works only when that read call succeeded. So only when the poll says you that the fd is not readable can you do something else. That something else should be quick (last less than 5 seconds).

You might want to investigate event loop libraries, like e.g. libevent (wrapping poll). If you are coding a graphical application (using Qt or Gtk) it does already have its own event loop. If clever enough, you could do your 5-second period without any timerfd_create, just thru your event loop (by carefully setting the timeout given to poll, etc.).

Addenda:

the tv_nsec field should always be non-negative and less than 1000000000 (the number of nanoseconds in a second).