timerfd_create()¶

CLOCK_REALTIME

A settable system-wide real-time clock.

CLOCK_MONOTONIC

A nonsettable monotonically increasing clock that measures time from some unspecified point in the past that does not change after system startup.

CLOCK_BOOTTIME (Since Linux 3.15)

Like CLOCK_MONOTONIC, this is a monotonically increasing clock. However, whereas the CLOCK_MONOTONIC clock does not measure the time while a system is suspended, the CLOCK_BOOTTIME clock does include the time during which the system is suspended. This is useful for applications that need to be suspend-aware. CLOCK_REALTIME is not suitable for such applications, since that clock is affected by discontinuous changes to the system clock.

CLOCK_REALTIME_ALARM (since Linux 3.11)

This clock is like CLOCK_REALTIME, but will wake the system if it is suspended. The caller must have the CAP_WAKE_ALARM capability in order to set a timer against this clock.

CLOCK_BOOTTIME_ALARM (since Linux 3.11)

This clock is like CLOCK_BOOTTIME, but will wake the system if it is suspended. The caller must have the CAP_WAKE_ALARM capability in order to set a timer against this clock.

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 open file description (see open(2)) referred to by the new file descriptor. 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()¶

The new_value argument specifies the initial expiration and interval for the timer. The itimerspec 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 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.

By default, the initial expiration time specified in new_value is interpreted relative to the current time on the timer's clock at the time of the call (i.e., new_value.it_value specifies a time relative to the current value of the clock specified by clockid). An absolute timeout can be selected via the flags argument.

The flags argument is a bit mask that can include the following values:

TFD_TIMER_ABSTIME

Interpret new_value.it_value as an absolute value on the timer's clock. The timer will expire when the value of the timer's clock reaches the value specified in new_value.it_value.

TFD_TIMER_CANCEL_ON_SET

If this flag is specified along with TFD_TIMER_ABSTIME and the clock for this timer is CLOCK_REALTIME or CLOCK_REALTIME_ALARM, then mark this timer as cancelable if the real-time clock undergoes a discontinuous change (settimeofday(2), clock_settime(2), or similar). When such changes occur, a current or future read(2) from the file descriptor will fail with the error ECANCELED.

If the old_value argument is not NULL, then the itimerspec structure that it points to is used to return the setting of the timer that was current at the time of the call; see the description of timerfd_gettime() following.

timerfd_gettime()¶

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¶

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—that is, 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 nonblocking (via the use of the fcntl(2) F_SETFL operation to set the O_NONBLOCK flag).

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

If the associated clock is either CLOCK_REALTIME or CLOCK_REALTIME_ALARM, the timer is absolute (TFD_TIMER_ABSTIME), and the flag TFD_TIMER_CANCEL_ON_SET was specified when calling timerfd_settime(), then read(2) fails with the error ECANCELED if the real-time clock undergoes a discontinuous change. (This allows the reading application to discover such discontinuous changes to the clock.)

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).

ioctl(2)

The following timerfd-specific command is supported:

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¶

execve(2) semantics¶

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);
}