timerfd_create (2) - Linux Manuals
NAME
timerfd_create, timerfd_settime, timerfd_gettime - timers that notify via file descriptors
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 *old_value);
int timerfd_gettime(int fd, struct itimerspec *curr_value);
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 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 contains two fields, each of which is in turn a structure of type timespec:
struct timespec {
struct itimerspec {
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:
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.
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.
timerfd_settime()
and
timerfd_gettime()
return 0 on success;
on error they return -1, and set
errno
to indicate the error.
timerfd_settime()
and
timerfd_gettime()
can fail with the following errors:
timerfd_settime()
can also fail with the following errors:
In this case the following occurs:
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
haZ # 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
haC # type control-C to suspend the program
#define handle_error(msg) \
static void
print_elapsed_time(void)
{
int
main(int argc, char *argv[])
{
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.
Operating on a timer file descriptor
The file descriptor returned by
timerfd_create()
supports the following additional operations:
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.
RETURN VALUE
On success,
timerfd_create()
returns a new file descriptor.
On error, -1 is returned and
errno
is set to indicate the error.
ERRORS
timerfd_create()
can fail with the following errors:
VERSIONS
These system calls are available on Linux since kernel 2.6.25.
Library support is provided by glibc since version 2.8.
CONFORMING TO
These system calls are Linux-specific.
NOTES
Suppose the following scenario for
CLOCK_REALTIME
or
CLOCK_REALTIME_ALARM
timer that was created with
timerfd_create():
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 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.
Program source
#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 */