How to use timerfd properly?

Question

I use timerfd with zmq.

How can I use timerfd_create and timerfd_set to wait one second for the timer (https://man7.org/linux/man-pages/man2/timerfd_create.2.html)?

I have looked through the link but I still do not get how I can initilize a timer that waits one second per tick with create and set. This is exactly my task:

We start a timer with timerfd_create(), which is 1 / sec. ticking. When setting a timer with timer_set_(..) a counter is simply incremented, which is decremented with every tick. When the counter reaches 0, the timer has expired.

In this project we have a function timer _ set _(), where the timer is set with the function timerfd_create and timerfd_settimer(). I hope you can help me.

This is my progress (part of my code):

    struct itimerspec timerValue;

    g_items[n].socket = nullptr; 
    g_items[n].events = ZMQ_POLLIN;

    g_items[n].fd = timerfd_create(CLOCK_REALTIME, 0);
    if(g_items[n].fd == -1 ){
        printf("timerfd_create() failed: errno=%d\n", errno);
        return -1;
    }  

    timerValue.it_value.tv_sec = 1;
    timerValue.it_value.tv_nsec = 0;
    timerValue.it_interval.tv_sec = 1;
    timerValue.it_interval.tv_nsec = 0;

    timerfd_settime(g_items[n].fd,  0, &timerValue, NULL); 

Answer 1

The question appears about setting correctly the timeouts of the timer.

With the settings

timerValue.it_value.tv_sec = 1;
timerValue.it_value.tv_nsec = 0;
timerValue.it_interval.tv_sec = 1;
timerValue.it_interval.tv_nsec = 0;

You are correctly setting the initial timeout to 1s (field timerValue.it_value). But you are also setting a periodic interval of 1s, and you didn't mention the will to do it.

---

About the timeouts

This behavior is described by the following passage of the manual:

int timerfd_create(int clockid, int flags);

new_value.it_value specifies the initial expiration of the timer, in seconds and nanoseconds. Setting either field of new_value.it_value to a nonzero value arms the timer.
Setting both fields of new_value.it_value to zero disarms the timer.

Setting one or both fields of new_value.it_interval to nonzero values specifies the period, in seconds and nanoseconds, for repeated timer expirations after the initial expiration. If both fields of new_value.it_interval are zero, the timer expires just once, at the time specified by new_value.it_value.

The emphasis on the last paragraph is mine, as it shows what to do in order to have a single-shot timer.

---

The benefits of timerrfd. How to detect timer expiration?

The main advantage provided by timerfd is that the timer is associated to a file descriptor, and this means that it

may be monitored by select(2), poll(2), and epoll(7).

The information contained in the other answer about read() is valid as well: let's just say that, even using functions such as select(), read() function will be required in order to consume data in the file descriptor.

---

A complete example

In the following demonstrative program, a timeout of 4 seconds is set; after that a periodic interval of 5 seconds is set.

The good old select() is used in order to wait for timer expiration, and read() is used to consume data (that is the number of expired timeouts; we will ignore it).

#include <stdio.h>
#include <sys/timerfd.h>
#include <sys/select.h>
#include <time.h>

int main()
{
    int tfd = timerfd_create(CLOCK_REALTIME,  0);
    
    printf("Starting at (%d)...\n", (int)time(NULL));
    
    if(tfd > 0)
    {
        char dummybuf[8];
        struct itimerspec spec =
        {
            { 5, 0 }, // Set to {0, 0} if you need a one-shot timer
            { 4, 0 }
        };
        timerfd_settime(tfd, 0, &spec, NULL);

        /* Wait */
        fd_set rfds;
        int retval;

        /* Watch timefd file descriptor */
        FD_ZERO(&rfds);
        FD_SET(0, &rfds);
        FD_SET(tfd, &rfds);

        /* Let's wait for initial timer expiration */
        retval = select(tfd+1, &rfds, NULL, NULL, NULL); /* Last parameter = NULL --> wait forever */
        printf("Expired at %d! (%d) (%d)\n", (int)time(NULL), retval, read(tfd, dummybuf, 8) );
        
        /* Let's wait (twice) for periodic timer expiration */
        retval = select(tfd+1, &rfds, NULL, NULL, NULL);
        printf("Expired at %d! (%d) (%d)\n", (int)time(NULL), retval, read(tfd, dummybuf, 8) );

        retval = select(tfd+1, &rfds, NULL, NULL, NULL);
        printf("Expired at %d! (%d) (%d)\n", (int)time(NULL), retval, read(tfd, dummybuf, 8) );
    }
    
    return 0;
}

And here it is the output. Every row contains also the timestamp, so that the actual elapsed time can be checked:

Starting at (1596547762)...
Expired at 1596547766! (1) (8)
Expired at 1596547771! (1) (8)
Expired at 1596547776! (1) (8)

Please note:

• We just performed 3 reads, for test
• The intervals are 4s + 5s + 5s (initial timeout + two interval timeouts)
• 8 bytes are returned by read(). We ignored them, but they contained the number of the expired timeouts