Use sigwait to block a specific signal without blocking SIGINT - c

Question

I have a program that runs a loop, each time at the end of the loop, the process should sleep for some seconds(the number of seconds is not constant and is calculated at each loop) or until the process receives SIGINT, I used alarm() and sigwait() to do this but it's blocking the ctrl+c signal(i.e SIGINT) which I don't want, I want SIGINT to be received and acted upon normally, sample code below (note that somefunction() below is just an example, in the original code it does real calculation instead of using rand())
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <errno.h>
#include <sys/time.h>
#include <unistd.h>
sigset_t sigs;
void setup_alarm()
{
printf("setting up signals\n");
sigemptyset(&sigs);
sigaddset(&sigs, SIGALRM);
sigprocmask(SIG_BLOCK, &sigs, NULL);
}
void wait_for_alarm(int interval)
{
printf("setting up alarm for %d seconds\n", interval);
alarm(interval);
printf("waiting for signal\n");
int sig_num = sigwait(&sigs, NULL);
// sigwaitinfo()
if (sig_num == 0)
{
printf("I received the alarm signal, breaking the wait\n");
}
else if (sig_num == EINVAL)
{
printf("some other error occurred");
perror("signal wait failed unexpectedly");
exit(1);
}
}
int somefunction()
{
srand(time(NULL));
return (rand() % 4) + 1;
}
int main(int argc, char *argv[])
{
int alarm_wait = 0;
setup_alarm();
while (1)
{
// do somework here
alarm_wait = somefunction();
// sleep for $alarm_wait or untill we receive SIGALARM
wait_for_alarm(alarm_wait);
}
return 0;
}
The results I'm getting is that when the execution reaches sigwait and I send the SIGINT signal(through ctrl-c) the program is not interrupted instead it keeps waiting until $alarm_wait has elapsed or until I send SIGALRM, what I want to do is have the logic only handles SIGALRM and every other signal should be handled normally(i.e SIGINT should interrupt the program even while it's waiting for SIGALRM signal)

Answer 1

Thanks to #Shawn for pointing out the second argument of sigwait I was able to solve my issue by also blocking SIGINT and using the second argument of sigwait to determine if the signal is SIGINT then execute exit(0)
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <errno.h>
#include <sys/time.h>
#include <unistd.h>
sigset_t sigs;
void setup_alarm()
{
printf("setting up signals\n");
sigemptyset(&sigs);
sigaddset(&sigs, SIGALRM);
sigaddset(&sigs, SIGINT);
sigprocmask(SIG_BLOCK, &sigs, NULL);
}
void wait_for_alarm(int interval)
{
printf("setting up alarm for %d seconds\n", interval);
alarm(interval);
printf("waiting for signal\n");
int sig;
int sig_num = sigwait(&sigs, &sig);
if(sig == SIGINT)
exit(0);
if (sig_num == 0)
{
printf("I received the alarm signal, breaking the wait\n");
}
else if (sig_num == EINVAL)
{
printf("some other error occurred");
perror("signal wait failed unexpectedly");
exit(1);
}
}
int somefunction()
{
srand(time(NULL));
return (rand() % 4) + 1;
}
int main(int argc, char *argv[])
{
int alarm_wait = 0;
setup_alarm();
while (1)
{
// do somework here
alarm_wait = somefunction();
// sleep for $alarm_wait or untill we receive SIGALARM
wait_for_alarm(alarm_wait);
}
return 0;
}
The code now works as i would expect, i'm not sure if this is the best solution since i'm only handling two signals and don't know how the rest of the signals are being handled(maybe some of these signals are important for the init system for example).
I will leave my answer unaccepted for sometime incase someone has a better solution.