TIMERFD_CREATE
Section: Linux Programmer's Manual (2)
Updated: 2009-03-10
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 either
CLOCK_REALTIME
or
CLOCK_MONOTONIC.
CLOCK_REALTIME
is a settable system-wide clock.
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
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
itimer
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.
The
flags
argument is either 0, to start a relative timer
(new_value.it_interval
specifies a time relative to the current value of the clock specified by
clockid),
or
TFD_TIMER_ABSTIME,
to start an absolute timer
(new_value.it_value
specifies an absolute time for the clock specified by
clockid;
that is, the timer will expire when the value of that
clock reaches the value specified in
new_value.it_value).
The
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
timerfd_gettime()
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 following 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,
i.e., 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)
will fail with the error
EINVAL
if the size of the supplied buffer is less than 8 bytes.
- 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
multiplexing APIs:
pselect(2),
ppoll(2),
and
epoll(7).
- 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
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.
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
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 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.
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.
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:
$ 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);
}
BUGS
Currently,
timerfd_create()
supports fewer types of clock IDs than
timer_create(2).
SEE ALSO
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)
COLOPHON
This page is part of release 3.27 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- timerfd_create()
-
- timerfd_settime()
-
- timerfd_gettime()
-
- Operating on a timer file descriptor
-
- fork(2) semantics
-
- execve(2) semantics
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- CONFORMING TO
-
- EXAMPLE
-
- Program source
-
- BUGS
-
- SEE ALSO
-
- COLOPHON
-
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Time: 07:34:57 GMT, March 26, 2013