.\" Copyright (C) 2008 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
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.\" the Free Software Foundation; either version 2 of the License, or
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.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
.\" GNU General Public License for more details.
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.\" You should have received a copy of the GNU General Public License
.\" along with this program; if not, write to the Free Software
.\" Foundation, Inc., 59 Temple Place, Suite 330, Boston,
.\" MA  02111-1307  USA
.\"
.TH TIMERFD_CREATE 2 2009-03-10 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>
.sp
.BI "int timerfd_create(int " clockid ", int " flags );
.sp
.BI "int timerfd_settime(int " fd ", int " flags ,
.BI "                    const struct itimerspec *" new_value ,
.BI "                    struct itimerspec *" old_value );
.sp
.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).

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 either
.B CLOCK_REALTIME
or
.BR CLOCK_MONOTONIC .
.B CLOCK_REALTIME
is a settable system-wide clock.
.B CLOCK_MONOTONIC
is a nonsettable 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
.BR clock_gettime (2).

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 new open file description.
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 .

The
.I new_value
argument specifies the initial expiration and interval for the timer.
The
.I itimer
structure used for this argument contains two fields,
each of which is in turn a structure of type
.IR timespec :
.in +4n
.nf

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 */
};
.fi
.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.

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 .

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

The
.I 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
.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 .

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.

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 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,
i.e., 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)
will fail with the error
.B EINVAL
if the size of the supplied buffer is less than 8 bytes.
.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 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.

.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
argument is neither
.B CLOCK_MONOTONIC
nor
.BR CLOCK_REALTIME ;
.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 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.
.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 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 EXAMPLE
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:
.in +4n
.nf

.RB "$" " a.out 3 1 100"
0.000: timer started
3.000: read: 1; total=1
4.000: read: 1; total=2
.BR "^Z " "                 # type control-Z to suspend the program"
[1]+  Stopped                 ./timerfd3_demo 3 1 100
.RB "$ " "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 "^C " "                 # type control-C to suspend the program"
.fi
.in
.SS Program source
\&
.nf
.\" 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 <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);
}
.fi
.SH BUGS
Currently,
.\" 2.6.29
.BR timerfd_create ()
supports fewer types of clock IDs than
.BR timer_create (2).
.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)
.SH COLOPHON
This page is part of release 3.27 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.