how to call a function automatically at regular intervals? - c

Question

Hi I am writing a C program to interface a serial device which gives data at regular intervals, i need to look for the inputs at the serial port at regular intervals. this can be done by a ' read' function . but i dont know how to call it frequently at fixed time intervals ?

Answer 1

This sort of behavior short-circuits the lovely machinery built in to most OSes to do just this, failing that something like cron would seem to be a lovely option. Failing all of that (if you're just looking for a quick hacky option) busy wait is not super awesome, the system isn't bright enough to hyperthread around that so your program winds up eating up a core doing nothing for the duration of your program, so while it's largely a matter of taste, I'm a nanosleep man myself.
on nix/nux systems:
#include <time.h>
int main(void)
{
struct timespec sleepytime;
sleepytime.tv_sec = seconds_you_want_to_sleep
sleepytime.tv_nsec = nanoseconds_you_want_to_sleep
while( !done)
{
nanosleep(&sleepytime, NULL);
//do your stuff here
}
return 0;
}
if you're worried about getting interrupted, the second parameter should be another timespec struct, in which will be stored the amount of time remaining, check if == 0,
then keep on trucking.
in windows apparently it is a little easier.
#include <windows.h>
int main(void)
{
while( !done)
{
Sleep(milliseconds_you_want_to_sleep);
//do your stuff here
}
return 0;
}
Unfortunately I don't run windows so I haven't been able to test the second code sample.

Answer 2

If you really need to read at regular intervals ( and not just poll for data to be available ) , you can do something like this :
#include <signal.h>
#include <stdio.h>
#include <string.h>
#include <sys/time.h>
void timer_handler (int signum)
{
static int count = 0;
printf ("timer expired %d times\n", ++count);
}
int main ()
{
struct sigaction sa;
struct itimerval timer;
/* Install 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 250 msec... */
timer.it_value.tv_sec = 0;
timer.it_value.tv_usec = 250000;
/* ... and every 250 msec after that. */
timer.it_interval.tv_sec = 0;
timer.it_interval.tv_usec = 250000;
/* Start a virtual timer. It counts down whenever this process is
executing. */
setitimer (ITIMER_REAL, &timer, NULL);
/* Do busy work. */
while (1);
}
I copied this from http://www.informit.com/articles/article.aspx?p=23618&seqNum=14 and changed the timer type, effectively you are setting up an interval timer and handling the signal when the timer runs out.