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TIMERFD_CREATE(2) Linux Programmer's Manual TIMERFD_CREATE(2)
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 *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
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.
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 new open
file description. 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
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 val‐
ues:
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_REAL‐
TIME_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_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 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 follow‐
ing 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 inte‐
gers 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 sup‐
plied 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 argu‐
ment; the poll(2) POLLIN flag) if one or more timer expira‐
tions 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 check‐
point/restore. This operation is available only if the
kernel was configured with the CONFIG_CHECK‐
POINT_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 expi‐
rations 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.
timerfd_create() can fail with the following errors:
EINVAL The clockid argument is neither CLOCK_MONOTONIC nor
CLOCK_REALTIME;
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.
timerfd_settime() and timerfd_gettime() can fail with the following
errors:
EBADF fd is not a valid file descriptor.
EFAULT new_value, old_value, or curr_value is not valid a pointer.
EINVAL fd is not a valid timerfd file descriptor.
timerfd_settime() can also fail with the following errors:
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.
These system calls are available on Linux since kernel 2.6.25.
Library support is provided by glibc since version 2.8.
These system calls are Linux-specific.
Currently, timerfd_create() supports fewer types of clock IDs than
timer_create(2).
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 <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);
}
eventfd(2), poll(2), read(2), select(2), setitimer(2), signalfd(2),
timer_create(2), timer_gettime(2), timer_settime(2), epoll(7),
time(7)
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 TIMERFD_CREATE(2)
Pages that refer to this page: alarm(2), eventfd(2), getitimer(2), ioctl_list(2), read(2), signalfd(2), syscalls(2), timer_create(2), sd-event(3), sd_event_add_time(3), proc(5), time(7)
Copyright and license for this manual page
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