timerfd_create(2) System Calls Manual timerfd_create(2)
NAME
timerfd_create, timerfd_settime, timerfd_gettime - timers that notify
via file descriptors
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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);
DESCRIPTION
These system calls create and operate on a timer that delivers timer ex-
piration notifications via a file descriptor. They provide an alterna-
tive 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 descrip-
tor 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 af-
ter 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_RE-
ALTIME 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 de-
scription (see open(2)) referred to by the new file de-
scriptor. 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 ex-
pirations 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 inter-
preted 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 cur-
rent 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 under-
goes 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 ECAN-
CELED.
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_get-
time() 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 de-
scriptor 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, re-
gardless of whether the TFD_TIMER_ABSTIME flag was specified when set-
ting 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 set-
tings 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_REAL-
TIME_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 al-
lows the reading application to discover such discontinuous
changes to the clock.)
If the associated clock is either CLOCK_REALTIME or CLOCK_REAL-
TIME_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 mul-
tiplexing 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 cre-
ated by timerfd_create(). The file descriptor refers to the same under-
lying 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 ex-
ecve(2), and continues to generate timer expirations if the timer was
armed.
RETURN VALUE
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.
ERRORS
timerfd_create() can fail with the following errors:
EINVAL The clockid is not valid.
EINVAL flags is invalid; or, in Linux 2.6.26 or earlier, flags is
nonzero.
EMFILE The per-process limit on the number of open file descriptors has
been reached.
ENFILE The system-wide limit on the total number of open files has been
reached.
ENODEV Could not mount (internal) anonymous inode device.
ENOMEM There was insufficient kernel memory to create the timer.
EPERM clockid was CLOCK_REALTIME_ALARM or CLOCK_BOOTTIME_ALARM but the
caller did not have the CAP_WAKE_ALARM capability.
timerfd_settime() and timerfd_gettime() can fail with the following er-
rors:
EBADF fd is not a valid file descriptor.
EFAULT new_value, old_value, or curr_value is not a valid pointer.
EINVAL fd is not a valid timerfd file descriptor.
timerfd_settime() can also fail with the following errors:
ECANCELED
See NOTES.
EINVAL new_value is not properly initialized (one of the tv_nsec falls
outside the range zero to 999,999,999).
EINVAL flags is invalid.
STANDARDS
Linux.
HISTORY
Linux 2.6.25, glibc 2.8.
NOTES
Suppose the following scenario for CLOCK_REALTIME or CLOCK_REAL-
TIME_ALARM timer that was created with timerfd_create():
(1) The timer has been started (timerfd_settime()) with the
TFD_TIMER_ABSTIME and TFD_TIMER_CANCEL_ON_SET flags;
(2) A discontinuous change (e.g., settimeofday(2)) is subsequently made
to the CLOCK_REALTIME clock; and
(3) the caller once more calls timerfd_settime() to rearm the timer
(without first doing a read(2) on the file descriptor).
In this case the following occurs:
• The timerfd_settime() returns -1 with errno set to ECANCELED. (This
enables the caller to know that the previous timer was affected by a
discontinuous change to the clock.)
• The timer is successfully rearmed with the settings provided in the
second 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.)
BUGS
Currently, timerfd_create() supports fewer types of clock IDs than
timer_create(2).
EXAMPLES
The following program creates a timer and then monitors its progress.
The program accepts up to three command-line arguments. The first argu-
ment 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 <sys/types.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);
}
SEE ALSO
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)
Linux man-pages 6.9.1 2024-06-15 timerfd_create(2)
Generated by dwww version 1.16 on Tue Dec 16 03:57:28 CET 2025.