timerfd_create()¶

timerfd_create() creates a new timer object, and returns a file descriptor that refers to that timer. The clockid argument specifies the clock that is used to mark the progress of the timer, and must be either CLOCK_REALTIME or CLOCK_MONOTONIC. CLOCK_REALTIME is a settable system-wide clock. CLOCK_MONOTONIC is a non-settable clock that is not affected by discontinuous changes in the system clock (e.g., manual changes to system time). The current value of each of these clocks can be retrieved using clock_gettime(2).

Starting with Linux 2.6.27, the following values may be bitwise ORed in flags to change the behavior of timerfd_create():

TFD_NONBLOCK

Set the O_NONBLOCK file status flag on the new open file description. Using this flag saves extra calls to fcntl(2) to achieve the same result.

TFD_CLOEXEC

Set the close-on-exec (FD_CLOEXEC) flag on the new file descriptor. See the description of the O_CLOEXEC flag in open(2) for reasons why this may be useful.

In Linux versions up to and including 2.6.26, flags must be specified as zero.

timerfd_settime()¶

timerfd_settime() arms (starts) or disarms (stops) the timer referred to by the file descriptor fd.

The new_value argument specifies the initial expiration and interval for the timer. The itimer structure used for this argument contains two fields, each of which is in turn a structure of type timespec:

struct timespec {


    time_t tv_sec;                /* Seconds */


    long   tv_nsec;               /* Nanoseconds */
};
struct itimerspec {


    struct timespec it_interval;  /* Interval for periodic timer */


    struct timespec it_value;     /* Initial expiration */
};

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 non-zero 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 non-zero 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 flags argument is either 0, to start a relative timer (new_value.it_interval specifies a time relative to the current value of the clock specified by clockid), or TFD_TIMER_ABSTIME, to start an absolute timer (new_value.it_value specifies an absolute time for the clock specified by clockid; that is, the timer will expire when the value of that clock reaches the value specified in new_value.it_value).

The old_value argument returns a structure containing the setting of the timer that was current at the time of the call; see the description of timerfd_gettime() following.

timerfd_gettime()¶

timerfd_gettime() returns, in curr_value, an itimerspec structure that contains the current setting of the timer referred to by the file descriptor fd.

The it_value field returns the amount of time until the timer will next expire. If both fields of this structure are zero, then the timer is currently disarmed. This field always contains a relative value, regardless of whether the TFD_TIMER_ABSTIME flag was specified when setting the timer.

The it_interval field returns the interval of the timer. If both fields of this structure are zero, then the timer is set to expire just once, at the time specified by curr_value.it_value.

Operating on a timer file descriptor¶

The file descriptor returned by timerfd_create() supports the following operations:

read(2)

If the timer has already expired one or more times since its settings were last modified using timerfd_settime(), or since the last successful read(2), then the buffer given to read(2) returns an unsigned 8-byte integer (uint64_t) containing the number of expirations that have occurred. (The returned value is in host byte order, i.e., the native byte order for integers on the host machine.)

If no timer expirations have occurred at the time of the read(2), then the call either blocks until the next timer expiration, or fails with the error EAGAIN if the file descriptor has been made non-blocking (via the use of the fcntl(2) F_SETFL operation to set the O_NONBLOCK flag).

A read(2) will fail with the error EINVAL if the size of the supplied buffer is less than 8 bytes.

poll(2), select(2) (and similar)

The file descriptor is readable (the select(2) readfds argument; the poll(2) POLLIN flag) if one or more timer expirations have occurred.

The file descriptor also supports the other file-descriptor multiplexing APIs: pselect(2), ppoll(2), and epoll(7).

close(2)

When the file descriptor is no longer required it should be closed. When all file descriptors associated with the same timer object have been closed, the timer is disarmed and its resources are freed by the kernel.

fork(2) semantics¶

After a fork(2), the child inherits a copy of the file descriptor created by timerfd_create(). The file descriptor refers to the same underlying timer object as the corresponding file descriptor in the parent, and read(2)s in the child will return information about expirations of the timer.

execve(2) semantics¶

A file descriptor created by timerfd_create() is preserved across execve(2), and continues to generate timer expirations if the timer was armed.

Program source¶

#include <sys/timerfd.h>
#include <time.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>        /* Definition of uint64_t */
#define handle_error(msg) \


        do { perror(msg); exit(EXIT_FAILURE); } while (0)
static void
print_elapsed_time(void)
{


    static struct timespec start;


    struct timespec curr;


    static int first_call = 1;


    int secs, nsecs;


    if (first_call) {


        first_call = 0;


        if (clock_gettime(CLOCK_MONOTONIC, &start) == -1)


            handle_error("clock_gettime");


    }


    if (clock_gettime(CLOCK_MONOTONIC, &curr) == -1)


        handle_error("clock_gettime");


    secs = curr.tv_sec - start.tv_sec;


    nsecs = curr.tv_nsec - start.tv_nsec;


    if (nsecs < 0) {


        secs--;


        nsecs += 1000000000;


    }


    printf("%d.%03d: ", secs, (nsecs + 500000) / 1000000);
}
int
main(int argc, char *argv[])
{


    struct itimerspec new_value;


    int max_exp, fd;


    struct timespec now;


    uint64_t exp, tot_exp;


    ssize_t s;


    if ((argc != 2) && (argc != 4)) {


        fprintf(stderr, "%s init-secs [interval-secs max-exp]\n",


                argv[0]);


        exit(EXIT_FAILURE);


    }


    if (clock_gettime(CLOCK_REALTIME, &now) == -1)


        handle_error("clock_gettime");


    /* Create a CLOCK_REALTIME absolute timer with initial


       expiration and interval as specified in command line */


    new_value.it_value.tv_sec = now.tv_sec + atoi(argv[1]);


    new_value.it_value.tv_nsec = now.tv_nsec;


    if (argc == 2) {


        new_value.it_interval.tv_sec = 0;


        max_exp = 1;


    } else {


        new_value.it_interval.tv_sec = atoi(argv[2]);


        max_exp = atoi(argv[3]);


    }


    new_value.it_interval.tv_nsec = 0;


    fd = timerfd_create(CLOCK_REALTIME, 0);


    if (fd == -1)


        handle_error("timerfd_create");


    if (timerfd_settime(fd, TFD_TIMER_ABSTIME, &new_value, NULL) == -1)


        handle_error("timerfd_settime");


    print_elapsed_time();


    printf("timer started\n");


    for (tot_exp = 0; tot_exp < max_exp;) {


        s = read(fd, &exp, sizeof(uint64_t));


        if (s != sizeof(uint64_t))


            handle_error("read");


        tot_exp += exp;


        print_elapsed_time();


        printf("read: %llu; total=%llu\n",


                (unsigned long long) exp,


                (unsigned long long) tot_exp);


    }


    exit(EXIT_SUCCESS);
}