timerfd_create, timerfd_settime, timerfd_gettime - timers that notify via file
descriptors
Standard C library (
libc,
-lc)
#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 *_Nullable 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() 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() 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 is described
in
itimerspec(3type).
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:
- 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_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() 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.
The file descriptor returned by
timerfd_create() supports the following
additional 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 the O_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 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.)
- 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 not specified when calling timerfd_settime(), then a
discontinuous negative change to the clock (e.g., clock_settime(2))
may cause read(2) to unblock, but return a value of 0 (i.e., no
bytes read), if the clock change occurs after the time expired, but before
the read(2) on the file descriptor.
-
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">fork(2)_semantics">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">execve(2)_semantics">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 is not valid.
- 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.
- EPERM
-
clockid was CLOCK_REALTIME_ALARM or
CLOCK_BOOTTIME_ALARM but the caller did not have the
CAP_WAKE_ALARM capability.
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 a valid pointer.
- EINVAL
-
fd is not a valid timerfd file descriptor.
timerfd_settime() can also fail with the following errors:
- ECANCELED
- See NOTES.
- 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 since Linux 2.6.25. Library support is provided
since glibc 2.8.
These system calls are Linux-specific.
Suppose the following scenario for
CLOCK_REALTIME or
CLOCK_REALTIME_ALARM timer that was created with
timerfd_create():
- (1)
- The timer has been started (timerfd_settime()) with
the TFD_TIMER_ABSTIME and TFD_TIMER_CANCEL_ON_SET
flags;
- (2)
- A discontinuous change (e.g., settimeofday(2)) is
subsequently made to the CLOCK_REALTIME clock; and
- (3)
- the caller once more calls timerfd_settime() to
rearm the timer (without first doing a read(2) on the file
descriptor).
In this case the following occurs:
- •
- The timerfd_settime() returns -1 with errno
set to ECANCELED. (This enables the caller to know that the
previous timer was affected by a discontinuous change to the clock.)
- •
- The timer is successfully rearmed with the settings
provided in the second timerfd_settime() call. (This was probably
an implementation accident, but won't be fixed now, in case there are
applications that depend on this behaviour.)
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
#include <err.h>
#include <inttypes.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/timerfd.h>
#include <time.h>
#include <unistd.h>
static void
print_elapsed_time(void)
{
int secs, nsecs;
static int first_call = 1;
struct timespec curr;
static struct timespec start;
if (first_call) {
first_call = 0;
if (clock_gettime(CLOCK_MONOTONIC, &start) == -1)
err(EXIT_FAILURE, "clock_gettime");
}
if (clock_gettime(CLOCK_MONOTONIC, &curr) == -1)
err(EXIT_FAILURE, "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[])
{
int fd;
ssize_t s;
uint64_t exp, tot_exp, max_exp;
struct timespec now;
struct itimerspec new_value;
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)
err(EXIT_FAILURE, "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)
err(EXIT_FAILURE, "timerfd_create");
if (timerfd_settime(fd, TFD_TIMER_ABSTIME, &new_value, NULL) == -1)
err(EXIT_FAILURE, "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))
err(EXIT_FAILURE, "read");
tot_exp += exp;
print_elapsed_time();
printf("read: %" PRIu64 "; total=%" PRIu64 "\n", exp, 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),
timespec(3),
epoll(7),
time(7)