timerfd won't be ready to read when using epoll - c

Question

I'm intending to use epoll to check out timerfd and fire some actions.
Code is blow:
#include <time.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/timerfd.h>
#include <stdint.h>
#include <unistd.h>
#include <sys/epoll.h>
int main(int argc, char const *argv[])
{
struct timespec now;
clock_gettime(CLOCK_MONOTONIC, &now);
int timerfd;
timerfd = timerfd_create(CLOCK_MONOTONIC, 0);
struct itimerspec new_value;
new_value.it_value.tv_sec = 1;
new_value.it_interval.tv_sec = 1;
timerfd_settime(timerfd, 0, &new_value, NULL);
// uint64_t buff;
// while(true) {
// read(timerfd, &buff, sizeof(uint64_t));
// printf("%s\n", "ding");
// }
// code above works fine.
struct epoll_event ev, events[10];
int epollfd;
epollfd = epoll_create1(0);
if (epollfd == -1) {
perror("epoll_create1");
exit(EXIT_FAILURE);
}
ev.events = EPOLLIN;
ev.data.fd = timerfd;
if (epoll_ctl(epollfd, EPOLL_CTL_ADD, timerfd, &ev) == -1) {
perror("epoll_ctl: timerfd");
exit(EXIT_FAILURE);
}
int num;
printf("start\n");
while(true) {
num = epoll_wait(epollfd, events, 10, -1);
printf("%d\n", num);
uint64_t buff;
read(timerfd, &buff, sizeof(uint64_t));
printf("%s\n", "ding");
}
return 0;
}
When using timerfd seperately, it works fine. Every second will print "ding". But when adding epoll to observe timerfd, progrom will block at epoll_wait for ever.
I'v tryed using EPOLLET, but noting changed. What's wrong with this code?

Answer 1

Your itimerspec is not properly initialized, so depending on what particular garbage values it contains, timerfd_settime() might fail. To detect that, do error checking:
if (timerfd_settime(timerfd, 0, &new_value, NULL) != 0) {
perror("settime");
exit(-1);
}
Another way to debug this is to run your program under the strace, program, and you will see which, if any, system calls that fails.
The relevant structs looks like this:
struct timespec {
time_t tv_sec;
long tv_nsec;
};
struct itimerspec {
struct timespec it_interval;
struct timespec it_value;
};
You have to initialize both these members completely, and your program will work reliably:
new_value.it_value.tv_sec = 1;
new_value.it_value.tv_nsec = 0;
new_value.it_interval.tv_sec = 1;
new_value.it_interval.tv_nsec = 0;