man2/timerfd_create.2

.\" Copyright (C) 2008 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
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.\"
.TH TIMERFD_CREATE 2 2021-03-22 Linux "Linux Programmer's Manual"
.SH NAME
timerfd_create, timerfd_settime, timerfd_gettime \-
timers that notify via file descriptors
.SH SYNOPSIS
.nf
.B #include <sys/timerfd.h>
.PP
.BI "int timerfd_create(int " clockid ", int " flags );
.PP
.BI "int timerfd_settime(int " fd ", int " flags ,
.BI "                    const struct itimerspec *" new_value ,
.BI "                    struct itimerspec *" old_value );
.BI "int timerfd_gettime(int " fd ", struct itimerspec *" curr_value );
.fi
.SH DESCRIPTION
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
.BR setitimer (2)
or
.BR timer_create (2),
with the advantage that the file descriptor may be monitored by
.BR select (2),
.BR poll (2),
and
.BR epoll (7).
.PP
The use of these three system calls is analogous to the use of
.BR timer_create (2),
.BR timer_settime (2),
and
.BR timer_gettime (2).
(There is no analog of
.BR timer_getoverrun (2),
since that functionality is provided by
.BR read (2),
as described below.)
.\"
.SS timerfd_create()
.BR timerfd_create ()
creates a new timer object,
and returns a file descriptor that refers to that timer.
The
.I clockid
argument specifies the clock that is used to mark the progress
of the timer, and must be one of the following:
.TP
.B CLOCK_REALTIME
A settable system-wide real-time clock.
.TP
.B CLOCK_MONOTONIC
A nonsettable monotonically increasing clock that measures time
from some unspecified point in the past that does not change
after system startup.
.TP
.BR CLOCK_BOOTTIME " (Since Linux 3.15)"
.\"    commit 4a2378a943f09907fb1ae35c15de917f60289c14
Like
.BR CLOCK_MONOTONIC ,
this is a monotonically increasing clock.
However, whereas the
.BR CLOCK_MONOTONIC
clock does not measure the time while a system is suspended, the
.BR CLOCK_BOOTTIME
clock does include the time during which the system is suspended.
This is useful for applications that need to be suspend-aware.
.BR CLOCK_REALTIME
is not suitable for such applications, since that clock is affected
by discontinuous changes to the system clock.
.TP
.BR CLOCK_REALTIME_ALARM " (since Linux 3.11)"
.\" commit 11ffa9d6065f344a9bd769a2452f26f2f671e5f8
This clock is like
.BR CLOCK_REALTIME ,
but will wake the system if it is suspended.
The caller must have the
.B CAP_WAKE_ALARM
capability in order to set a timer against this clock.
.TP
.BR CLOCK_BOOTTIME_ALARM " (since Linux 3.11)"
.\" commit 11ffa9d6065f344a9bd769a2452f26f2f671e5f8
This clock is like
.BR CLOCK_BOOTTIME ,
but will wake the system if it is suspended.
The caller must have the
.B CAP_WAKE_ALARM
capability in order to set a timer against this clock.
.PP
See
.BR clock_getres (2)
for some further details on the above clocks.
.PP
The current value of each of these clocks can be retrieved using
.BR clock_gettime (2).
.PP
Starting with Linux 2.6.27, the following values may be bitwise ORed in
.IR flags
to change the behavior of
.BR timerfd_create ():
.TP 14
.B TFD_NONBLOCK
Set the
.BR O_NONBLOCK
file status flag on the open file description (see
.BR open (2))
referred to by the new file descriptor.
Using this flag saves extra calls to
.BR fcntl (2)
to achieve the same result.
.TP
.B TFD_CLOEXEC
Set the close-on-exec
.RB ( FD_CLOEXEC )
flag on the new file descriptor.
See the description of the
.B O_CLOEXEC
flag in
.BR open (2)
for reasons why this may be useful.
.PP
In Linux versions up to and including 2.6.26,
.I flags
must be specified as zero.
.SS timerfd_settime()
.BR timerfd_settime ()
arms (starts) or disarms (stops)
the timer referred to by the file descriptor
.IR fd .
.PP
The
.I new_value
argument specifies the initial expiration and interval for the timer.
The
.I itimerspec
structure used for this argument contains two fields,
each of which is in turn a structure of type
.IR timespec :
.PP
.in +4n
.EX
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 */
};
.EE
.in
.PP
.I new_value.it_value
specifies the initial expiration of the timer,
in seconds and nanoseconds.
Setting either field of
.I new_value.it_value
to a nonzero value arms the timer.
Setting both fields of
.I new_value.it_value
to zero disarms the timer.
.PP
Setting one or both fields of
.I 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
.I new_value.it_interval
are zero, the timer expires just once, at the time specified by
.IR new_value.it_value .
.PP
By default,
the initial expiration time specified in
.I new_value
is interpreted relative to the current time
on the timer's clock at the time of the call (i.e.,
.I new_value.it_value
specifies a time relative to the current value of the clock specified by
.IR clockid ).
An absolute timeout can be selected via the
.I flags
argument.
.PP
The
.I flags
argument is a bit mask that can include the following values:
.TP
.B TFD_TIMER_ABSTIME
Interpret
.I 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
.IR new_value.it_value .
.TP
.BR TFD_TIMER_CANCEL_ON_SET
If this flag is specified along with
.B TFD_TIMER_ABSTIME
and the clock for this timer is
.BR CLOCK_REALTIME
or
.BR CLOCK_REALTIME_ALARM ,
then mark this timer as cancelable if the real-time clock
undergoes a discontinuous change
.RB ( settimeofday (2),
.BR clock_settime (2),
or similar).
When such changes occur, a current or future
.BR read (2)
from the file descriptor will fail with the error
.BR ECANCELED .
.PP
If the
.I old_value
argument is not NULL, then the
.I 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
.BR timerfd_gettime ()
following.
.\"
.SS timerfd_gettime()
.BR timerfd_gettime ()
returns, in
.IR curr_value ,
an
.IR itimerspec
structure that contains the current setting of the timer
referred to by the file descriptor
.IR fd .
.PP
The
.I 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
.BR TFD_TIMER_ABSTIME
flag was specified when setting the timer.
.PP
The
.I 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
.IR curr_value.it_value .
.SS Operating on a timer file descriptor
The file descriptor returned by
.BR timerfd_create ()
supports the following additional operations:
.TP
.BR read (2)
If the timer has already expired one or more times since
its settings were last modified using
.BR timerfd_settime (),
or since the last successful
.BR read (2),
then the buffer given to
.BR read (2)
returns an unsigned 8-byte integer
.RI ( uint64_t )
containing the number of expirations that have occurred.
(The returned value is in host byte order\(emthat is,
the native byte order for integers on the host machine.)
.IP
If no timer expirations have occurred at the time of the
.BR read (2),
then the call either blocks until the next timer expiration,
or fails with the error
.B EAGAIN
if the file descriptor has been made nonblocking
(via the use of the
.BR fcntl (2)
.B F_SETFL
operation to set the
.B O_NONBLOCK
flag).
.IP
A
.BR read (2)
fails with the error
.B EINVAL
if the size of the supplied buffer is less than 8 bytes.
.IP
If the associated clock is either
.BR CLOCK_REALTIME
or
.BR CLOCK_REALTIME_ALARM ,
the timer is absolute
.RB ( TFD_TIMER_ABSTIME ),
and the flag
.BR TFD_TIMER_CANCEL_ON_SET
was specified when calling
.BR timerfd_settime (),
then
.BR read (2)
fails with the error
.BR ECANCELED
if the real-time clock undergoes a discontinuous change.
(This allows the reading application to discover
such discontinuous changes to the clock.)
.IP
If the associated clock is either
.BR CLOCK_REALTIME
or
.BR CLOCK_REALTIME_ALARM ,
the timer is absolute
.RB ( TFD_TIMER_ABSTIME ),
and the flag
.BR TFD_TIMER_CANCEL_ON_SET
was
.I not
specified when calling
.BR timerfd_settime (),
then a discontinuous negative change to the clock (e.g.,
.BR clock_settime (2))
may cause
.BR 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
.BR read (2)
on the file descriptor.
.TP
.BR poll "(2), " select "(2) (and similar)"
The file descriptor is readable
(the
.BR select (2)
.I readfds
argument; the
.BR poll (2)
.B POLLIN
flag)
if one or more timer expirations have occurred.
.IP
The file descriptor also supports the other file-descriptor
multiplexing APIs:
.BR pselect (2),
.BR ppoll (2),
and
.BR epoll (7).
.TP
.BR ioctl (2)
The following timerfd-specific command is supported:
.RS
.TP
.BR TFD_IOC_SET_TICKS " (since Linux 3.17)"
.\" commit 5442e9fbd7c23172a1c9bc736629cd123a9923f0
Adjust the number of timer expirations that have occurred.
The argument is a pointer to a nonzero 8-byte integer
.RI ( 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
.BR CONFIG_CHECKPOINT_RESTORE
option.
.RE
.TP
.BR 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.
.\"
.SS fork(2) semantics
After a
.BR fork (2),
the child inherits a copy of the file descriptor created by
.BR timerfd_create ().
The file descriptor refers to the same underlying
timer object as the corresponding file descriptor in the parent,
and
.BR read (2)s
in the child will return information about
expirations of the timer.
.\"
.SS execve(2) semantics
A file descriptor created by
.BR timerfd_create ()
is preserved across
.BR execve (2),
and continues to generate timer expirations if the timer was armed.
.SH RETURN VALUE
On success,
.BR timerfd_create ()
returns a new file descriptor.
On error, \-1 is returned and
.I errno
is set to indicate the error.
.PP
.BR timerfd_settime ()
and
.BR timerfd_gettime ()
return 0 on success;
on error they return \-1, and set
.I errno
to indicate the error.
.SH ERRORS
.BR timerfd_create ()
can fail with the following errors:
.TP
.B EINVAL
The
.I clockid
is not valid.
.TP
.B EINVAL
.I flags
is invalid;
or, in Linux 2.6.26 or earlier,
.I flags
is nonzero.
.TP
.B EMFILE
The per-process limit on the number of open file descriptors has been reached.
.TP
.B ENFILE
The system-wide limit on the total number of open files has been
reached.
.TP
.B ENODEV
Could not mount (internal) anonymous inode device.
.TP
.B ENOMEM
There was insufficient kernel memory to create the timer.
.TP
.B EPERM
.I clockid
was
.BR CLOCK_REALTIME_ALARM
or
.BR CLOCK_BOOTTIME_ALARM
but the caller did not have the
.BR CAP_WAKE_ALARM
capability.
.PP
.BR timerfd_settime ()
and
.BR timerfd_gettime ()
can fail with the following errors:
.TP
.B EBADF
.I fd
is not a valid file descriptor.
.TP
.B EFAULT
.IR new_value ,
.IR old_value ,
or
.I curr_value
is not valid a pointer.
.TP
.B EINVAL
.I fd
is not a valid timerfd file descriptor.
.PP
.BR timerfd_settime ()
can also fail with the following errors:
.TP
.B ECANCELED
See NOTES.
.TP
.B EINVAL
.I new_value
is not properly initialized (one of the
.I tv_nsec
falls outside the range zero to 999,999,999).
.TP
.B EINVAL
.\" This case only checked since 2.6.29, and 2.2.2[78].some-stable-version.
.\" In older kernel versions, no check was made for invalid flags.
.I flags
is invalid.
.SH VERSIONS
These system calls are available on Linux since kernel 2.6.25.
Library support is provided by glibc since version 2.8.
.SH CONFORMING TO
These system calls are Linux-specific.
.SH NOTES
Suppose the following scenario for
.BR CLOCK_REALTIME
or
.BR CLOCK_REALTIME_ALARM
timer that was created with
.BR timerfd_create ():
.IP (a) 4
The timer has been started
.RB ( timerfd_settime ())
with the
.BR TFD_TIMER_ABSTIME
and
.BR TFD_TIMER_CANCEL_ON_SET
flags;
.IP (b)
A discontinuous change (e.g.,
.BR settimeofday (2))
is subsequently made to the
.BR CLOCK_REALTIME
clock; and
.IP (c)
the caller once more calls
.BR timerfd_settime ()
to rearm the timer (without first doing a
.BR read (2)
on the file descriptor).
.PP
In this case the following occurs:
.IP \(bu 2
The
.BR timerfd_settime ()
returns \-1 with
.I errno
set to
.BR ECANCELED .
(This enables the caller to know that the previous timer was affected
by a discontinuous change to the clock.)
.IP \(bu
The timer
.I "is successfully rearmed"
with the settings provided in the second
.BR timerfd_settime ()
call.
(This was probably an implementation accident, but won't be fixed now,
in case there are applications that depend on this behaviour.)
.SH BUGS
Currently,
.\" 2.6.29
.BR timerfd_create ()
supports fewer types of clock IDs than
.BR timer_create (2).
.SH EXAMPLES
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.
.PP
The following shell session demonstrates the use of the program:
.PP
.in +4n
.EX
.RB "$" " a.out 3 1 100"
0.000: timer started
3.000: read: 1; total=1
4.000: read: 1; total=2
.BR "\(haZ " "                 # type control\-Z to suspend the program"
[1]+  Stopped                 ./timerfd3_demo 3 1 100
.RB "$ " "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
.BR "\(haC " "                 # type control\-C to suspend the program"
.EE
.in
.SS Program source
\&
.EX
.\" The commented out code here is what we currently need until
.\" the required stuff is in glibc
.\"
.\"
.\"/* Link with \-lrt */
.\"#define _GNU_SOURCE
.\"#include <sys/syscall.h>
.\"#include <unistd.h>
.\"#include <time.h>
.\"#if defined(__i386__)
.\"#define __NR_timerfd_create 322
.\"#define __NR_timerfd_settime 325
.\"#define __NR_timerfd_gettime 326
.\"#endif
.\"
.\"static int
.\"timerfd_create(int clockid, int flags)
.\"{
.\"    return syscall(__NR_timerfd_create, clockid, flags);
.\"}
.\"
.\"static int
.\"timerfd_settime(int fd, int flags, struct itimerspec *new_value,
.\"        struct itimerspec *curr_value)
.\"{
.\"    return syscall(__NR_timerfd_settime, fd, flags, new_value,
.\"                   curr_value);
.\"}
.\"
.\"static int
.\"timerfd_gettime(int fd, struct itimerspec *curr_value)
.\"{
.\"    return syscall(__NR_timerfd_gettime, fd, curr_value);
.\"}
.\"
.\"#define TFD_TIMER_ABSTIME (1 << 0)
.\"
.\"////////////////////////////////////////////////////////////
#include <sys/timerfd.h>
#include <time.h>
#include <unistd.h>
#include <inttypes.h>      /* Definition of PRIu64 */
#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>        /* Definition of uint64_t */

#define handle_error(msg) \e
        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]\en",
                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\en");

    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: %" PRIu64 "; total=%" PRIu64 "\en", exp, tot_exp);
    }

    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.BR eventfd (2),
.BR poll (2),
.BR read (2),
.BR select (2),
.BR setitimer (2),
.BR signalfd (2),
.BR timer_create (2),
.BR timer_gettime (2),
.BR timer_settime (2),
.BR epoll (7),
.BR time (7)