setitimer signalling seems only to work after fork - c++

Question

I would like to use the following C++ code to wait for a predefined amount of time (in this example always 2 seconds), but still be interruptible by a signal (that's why I don't use sleep):
#include <unistd.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <sys/types.h>
#include <sys/time.h>
#include <signal.h>
#include <iostream>
using namespace std;
int measure() {
itimerval idle;
sigset_t sigset;
int sig;
idle.it_value.tv_sec = 2;
idle.it_value.tv_usec = 0;
setitimer(ITIMER_REAL, &idle, NULL); // TODO: check return value
sigemptyset(&sigset);
sigaddset(&sigset, SIGALRM); // TODO return values
sigaddset(&sigset, SIGUSR1);
sigprocmask(SIG_BLOCK, &sigset, NULL); // TODO return value?
sigwait(&sigset, &sig); // TODO check return value
while(sig != SIGUSR1) {
cout << "Hohoho" << endl;
idle.it_value.tv_sec = 2;
idle.it_value.tv_usec = 0;
setitimer(ITIMER_REAL, &idle, NULL); // TODO: check return value
sigwait(&sigset, &sig); // TODO check return value
}
cout << "Done with measurements." << endl;
return 0;
}
int main(int argc, char **argv) {
//if(fork() != 0) exit(0);
//if(fork() == 0) exit(0);
return measure();
}
I would expect this code to print "Hohoho" every 2 seconds until it receives SIGUSR1. Then it prints "Done with measurements." and exits. The second part works as expected. However, I see no "Hohoho", so it seems to me that the SIGALRM from setitimer somehow is not received. The strange thing is that if I do a fork before, the program works as expected. More specifically, if I uncomment either one of the two fork commands at the end, it works. Hence it does not depend on if it's the parent or child process, but somehow the fork event matters. Can someone explain to me what's going on and how to fix my code?
Thanks a lot,
Lutz

Answer 1

(1) Your setitimer is failing because you haven't set it correctly. Struct itimerval contains two structs of type timeval. You are only setting one and thereby picking up whatever garbage was in local storage when idle was declared.
struct itimerval {
struct timeval it_interval; /* next value */
struct timeval it_value; /* current value */
};
struct timeval {
time_t tv_sec; /* seconds */
suseconds_t tv_usec; /* microseconds */
};
If you want a repeating timer every 2 seconds then set the 2nd set to repeat with the same values.
idle.it_value.tv_sec = 2;
idle.it_value.tv_usec = 0;
idle.it_interval.tv_sec = 2;
idle.it_interval.tv_usec = 0;