Section (2) timerfd_create
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 theCLOCK_MONOTONIC
clock does not measure the time while a system is suspended, theCLOCK_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 theCAP_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 theCAP_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 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 theO_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_zsingle_quotesz_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:
TFD_TIMER_ABSTIME
-
Interpret
new_value.it_value
as an absolute value on the timer_zsingle_quotesz_s clock. The timer will expire when the value of the timer_zsingle_quotesz_s clock reaches the value specified innew_value.it_value
. TFD_TIMER_CANCEL_ON_SET
-
If this flag is specified along with
TFD_TIMER_ABSTIME
and the clock for this timer isCLOCK_REALTIME
orCLOCK_REALTIME_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_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—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 theO_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
orCLOCK_REALTIME_ALARM
, the timer is absolute (TFD_TIMER_ABSTIME
), and the flagTFD_TIMER_CANCEL_ON_SET
was specified when callingtimerfd_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
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).
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 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 neitherCLOCK_MONOTONIC
norCLOCK_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
, orcurr_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 thetv_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.
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] , 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 ); 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 , (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 5.04 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/.
Copyright (C) 2008 Michael Kerrisk <mtk.manpagesgmail.com> %%%LICENSE_START(GPLv2+_SW_3_PARA) This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this manual; if not, see <http://www.gnu.org/licenses/>. %%%LICENSE_END FIXME Linux 3.0: timerfd_settime() adds a TFD_TIMER_CANCEL_ON_SET flag; This flag needs to documented. |