Interval Timer not firing signal at specified time interval - c

Question

I want to call timer_handler function at every 2 seconds regardless of execution time of timer_handler function here is my code
#include <signal.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/time.h>
void timer_handler (int signum)
{
static int count = 0;
sleep(1);
printf ("timer expired %d times %d signum \n", ++count, signum);
}
int main ()
{
struct sigaction sa;
struct itimerval timer;
/* timer_handler as the signal handler for SIGVTALRM. */
memset (&sa, 0, sizeof (sa));
sa.sa_handler = &timer_handler;
sigaction (SIGVTALRM, &sa, NULL);
/* Configure the timer to expire after 2000 msec... */
timer.it_value.tv_sec = 2;
timer.it_value.tv_usec = 0;
/* ... and every 2000 msec after that. */
timer.it_interval.tv_sec = 2;
timer.it_interval.tv_usec = 0;
/* Start a virtual timer. It counts down whenever this process is
executing. */
setitimer (ITIMER_VIRTUAL, &timer, NULL);
/* Do busy work. */
while (1);
}
As per above code it should print timer expired 1 times 26 signum at every two second but its prints on every 3 seconds which includes sleep time so i want to call that function on every 2 seconds.
I don't know where i am doing wrong
If any other library is able to do this please let me know
Thank you

Answer 1

Why not use wall-clock time?
To do so
install the signal handler for SIGALRM instead of SIGVTALRM and
specify ITIMER_REAL instead of ITIMER_VIRTUAL.
Unrelated but important: Signal handlers may only call async-signal-safe functions. printf() is not one of those. For a list of the latter click here and scroll down.

Answer 2

the call, in the signal handler: sleep(1) is adding an extra second to the processing of the signal. This extra second is not part of the execution time of the process.
remove from the signal handler:
sleep(1);
regarding:
setitimer (ITIMER_VIRTUAL, &timer, NULL);
since you want to see the signal handler executed every 2 seconds, the proper timer to use is: ITIMER_REAL not ITIMER_VIRTUAL. This will result in the 'clock on the wall' time being measured rather than the 'process run' time being measured.
Strongly suggest having the signal handler ONLY set a flag. Then the 'do nothing' loop in the main function be checking that flag, reset the flag, then call printf()` To properly accomplish this, with out an 'race' conditions, use a mutex/ Both the main and the signal handler would lock the mutex, modify the flag, then unlock the mutex.