Signal Handler to stop Timer in C - c

Question

I am trying to have a signal handler stop a timer without exiting my program. How should I go about. I want StopTimer to handle the signal to stop the timer
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <sys/time.h>
#include <signal.h>
#include <unistd.h>
#define INTERVAL 2 // number of seconds to go off
int main(int argc, char* argv[]) {
TimerSet(INTERVAL);
while(1)
{
// do stuff
}
return 0;
}
void TimerSet(int interval)
{
printf("starting timer\n");
struct itimerval it_val;
// interval value
it_val.it_value.tv_sec = interval;
it_val.it_interval = it_val.it_value;
// on SIGALRM, close window
if (signal(SIGALRM, TimerStop) == SIG_ERR)
{
perror("Unable to catch SIGALRM");
exit(1);
}
// set interval timer, returns SIGALRM on expiration
if (setitimer(ITIMER_REAL, &it_val, NULL) == -1)
{
perror("error calling setitimer()");
exit(1);
}
}
void TimerStop(int signum)
{
printf("Timer ran out! Stopping timer\n");
exit(signum);
}
I tried to set the setitimer interval to 0, but I am not sure how to use the same timer within the TimerStop signal handler function

Answer 1

Just set it_interval to zero, and you'll get a one-shot timer. You don't need to do anything with it in your handler.
For instance, with this:
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <sys/time.h>
#include <signal.h>
#include <unistd.h>
#define INTERVAL 2 // number of seconds to go off
void TimerStop(int signum) {
printf("Timer ran out! Stopping timer\n");
}
void TimerSet(int interval) {
printf("starting timer\n");
struct itimerval it_val;
it_val.it_value.tv_sec = interval;
it_val.it_value.tv_usec = 0;
it_val.it_interval.tv_sec = 0;
it_val.it_interval.tv_usec = 0;
if (signal(SIGALRM, TimerStop) == SIG_ERR) {
perror("Unable to catch SIGALRM");
exit(1);
}
if (setitimer(ITIMER_REAL, &it_val, NULL) == -1) {
perror("error calling setitimer()");
exit(1);
}
}
int main(int argc, char *argv[]) {
TimerSet(INTERVAL);
while (1) {
// do stuff
}
return 0;
}
the message "Timer ran out! Stopping timer" will appear only once, and your timer will stop without you doing anything.
Note that you need to fill in the tv_usec members of your struct itimerval, which your current code does not do. If you don't, it_interval is highly unlikely to be zero, and your timer will never stop.
printf(), along with the other standard IO functions, is not really safe to call from a signal handler, although in this particular case it won't cause you any problems, since the main code is just sitting in a loop and not doing anything.
Also, presume you're calling signal() on purpose - sigaction() is the recommended way for setting handlers. setitimer() is also obsolete, now, and timer_settime() is recommended.

Answer 2

According to manual:
Timers decrement from it_value to zero, generate a signal, and reset to
it_interval. A timer which is set to zero (it_value is zero or the timer
expires and it_interval is zero) stops.
Both tv_sec and tv_usec are significant in determining the duration of a
timer.
So timer can be set to run only once if interval is set to zero before setitimer() call (thanks to Duck's comment).