TIMERFD_CREATE(2) Linux Programmer's Manual TIMERFD_CREATE(2)
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 alter-
native to the use of setitimer(2) or timer_create(2), with the advan-
tage 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_value 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).
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 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 expi-
ration, 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 mul-
tiplexing 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 cre-
ated 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 expira-
tions 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_REAL-
TIME;
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.
BUGS
Currently, timerfd_create() supports fewer types of clock IDs than
timer_create(2).
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 pro-
gram 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);
}
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 3.53 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/.
Linux 2011-09-14 TIMERFD_CREATE(2)
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