Strange behaviour of `timer_settime()` - c

Question

I'm trying to set Linux timer so that I get signal each 0.1 seconds (infinitely). I have problem with timer_settime() call. Surprisingly this code seems to work:
#define Msec 1e6
#define Cycle 100 * Msec
//...
#define SIGNO SIGRTMIN
struct sigaction sa = { .sa_handler = &each_cycle };
sigemptyset (&sa.sa_mask);
timer_t timer;
struct sigevent te = {
.sigev_notify = SIGEV_SIGNAL,
.sigev_signo = SIGNO,
.sigev_value.sival_ptr = &timer
};
/* I don't know why this setting works. */
struct itimerspec it = {
.it_interval.tv_nsec = Cycle,
.it_value.tv_nsec = Cycle
};
if (sigaction (SIGNO, &sa, NULL) < 0 ||
timer_create (CLOCK_REALTIME, &te, &timer) != 0 ||
timer_settime (timer, 0, &it, 0) != 0
) {
fprintf (stderr, "Unable to register timer.");
return 1;
}
Surprising because according to my understaning it should cause expiring of timer after one time. Before I was trying .it_value.tv_sec = INT32_MAX — this seems to be most reasonable, because .it_value = {0,0} disarms timer.
Unfortunatelly setting tv_sec to any value results in no signal. I'm running Arch Linux on Raspberry Pi 3 (GCC7), I've tried to remove optimization, no change.

Answer 1

/* I don't know why this setting works. */
struct itimerspec it = {
.it_interval.tv_nsec = Cycle,
.it_value.tv_nsec = Cycle
};
....
[A]ccording to my understanding it should cause expiring of timer after one time.
Setting it_interval arms the timer for intervalic, recurring expiry after the first expiry (scheduled by it_value). That's why you see repeated invocations.