timerfd_create, timerfd_settime, timerfd_gettime - timers that notify via file descriptors

#include <sys/timerfd.h>
int timerfd_create(int clockid, int flags);
int timerfd_settime(int fd, int flags,
 const struct itimerspec *new_value,
 struct itimerspec *_Nullable old_value);
int timerfd_gettime(int fd, struct itimerspec *curr_value);

These system calls create and operate on a timer that delivers timer expiration notifications via a file descriptor. They provide an alternative to the use of setitimer(2) or timer_create(2), with the advantage that the file descriptor may be monitored by select(2), poll(2), and epoll(7).

The use of these three system calls is analogous to the use of timer_create(2), timer_settime(2), and timer_gettime(2). (There is no analog of timer_getoverrun(2), since that functionality is provided by read(2), as described below.)

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 one of the following:

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.

See clock_getres(2) for some further details on the above clocks.

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()

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 itimerspec structure used for this argument is described in itimerspec(3type).

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()

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 additional 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—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.)

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 not specified when calling timerfd_settime(), then a discontinuous negative change to the clock (e.g., clock_settime(2)) may cause read(2) to unblock, but return a value of 0 (i.e., no bytes read), if the clock change occurs after the time expired, but before the read(2) on the file descriptor.

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:

TFD_IOC_SET_TICKS (since Linux 3.17)

Adjust the number of timer expirations that have occurred. The argument is a pointer to a nonzero 8-byte integer (uint64_t*) containing the new number of expirations. Once the number is set, any waiter on the timer is woken up. The only purpose of this command is to restore the expirations for the purpose of checkpoint/restore. This operation is available only if the kernel was configured with the CONFIG_CHECKPOINT_RESTORE option.

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.

On success, timerfd_create() returns a new file descriptor. On error, -1 is returned and errno is set to indicate the error.

timerfd_settime() and timerfd_gettime() return 0 on success; on error they return -1, and set errno to indicate the error.

The following program creates a timer and then monitors its progress. The program accepts up to three command-line arguments. The first argument specifies the number of seconds for the initial expiration of the timer. The second argument specifies the interval for the timer, in seconds. The third argument specifies the number of times the program should allow the timer to expire before terminating. The second and third command-line arguments are optional.

The following shell session demonstrates the use of the program:

$ a.out 3 1 100
0.000: timer started
3.000: read: 1; total=1
4.000: read: 1; total=2
^Z  # type control-Z to suspend the program
[1]+  Stopped                 ./timerfd3_demo 3 1 100
$ fg # Resume execution after a few seconds
a.out 3 1 100
9.660: read: 5; total=7
10.000: read: 1; total=8
11.000: read: 1; total=9
^C  # type control-C to suspend the program

Program source

#include <err.h>
#include <inttypes.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/timerfd.h>
#include <time.h>
#include <unistd.h>
static void
print_elapsed_time(void)
{
    int                     secs, nsecs;
    static int              first_call = 1;
    struct timespec         curr;
    static struct timespec  start;
    if (first_call) {
        first_call = 0;
        if (clock_gettime(CLOCK_MONOTONIC, &start) == -1)
            err(EXIT_FAILURE, "clock_gettime");
    }
    if (clock_gettime(CLOCK_MONOTONIC, &curr) == -1)
        err(EXIT_FAILURE, "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[])
{
    int                fd;
    ssize_t            s;
    uint64_t           exp, tot_exp, max_exp;
    struct timespec    now;
    struct itimerspec  new_value;
    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)
        err(EXIT_FAILURE, "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)
        err(EXIT_FAILURE, "timerfd_create");
    if (timerfd_settime(fd, TFD_TIMER_ABSTIME, &new_value, NULL) == -1)
        err(EXIT_FAILURE, "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))
            err(EXIT_FAILURE, "read");
        tot_exp += exp;
        print_elapsed_time();
        printf("read: %" PRIu64 "; total=%" PRIu64 "\n", exp, tot_exp);
    }
    exit(EXIT_SUCCESS);
}

eventfd(2), poll(2), read(2), select(2), setitimer(2), signalfd(2), timer_create(2), timer_gettime(2), timer_settime(2), timespec(3), epoll(7), time(7)