timerfd_create(2) - Linux manual page (original) (raw)
timerfdcreate(2) System Calls Manual timerfdcreate(2)
NAME top
timerfd_create, timerfd_settime, timerfd_gettime - timers that
notify via file descriptorsLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <sys/timerfd.h>**
**int timerfd_create(int** _clockid_**, int** _flags_**);**
**int timerfd_settime(int** _fd_**, int** _flags_**,**
**const struct itimerspec ***_newvalue_**,**
**struct itimerspec *_Nullable** _oldvalue_**);**
**int timerfd_gettime(int** _fd_**, struct itimerspec ***_currvalue_**);**DESCRIPTION top
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)](../man2/setitimer.2.html) or
[timer_create(2)](../man2/timer%5Fcreate.2.html), with the advantage that the file descriptor may
be monitored by [select(2)](../man2/select.2.html), [poll(2)](../man2/poll.2.html), and [epoll(7)](../man7/epoll.7.html).
The use of these three system calls is analogous to the use of
[timer_create(2)](../man2/timer%5Fcreate.2.html), [timer_settime(2)](../man2/timer%5Fsettime.2.html), and [timer_gettime(2)](../man2/timer%5Fgettime.2.html). (There
is no analog of [timer_getoverrun(2)](../man2/timer%5Fgetoverrun.2.html), since that functionality is
provided by [read(2)](../man2/read.2.html), 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)](../man2/clock%5Fgetres.2.html) for some further details on the above clocks.
The current value of each of these clocks can be retrieved using
[clock_gettime(2)](../man2/clock%5Fgettime.2.html).
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)](../man2/open.2.html)) referred to by the new file
descriptor. Using this flag saves extra calls to [fcntl(2)](../man2/fcntl.2.html)
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)](../man2/open.2.html) 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 _newvalue_ argument specifies the initial expiration and
interval for the timer. The _itimerspec_ structure used for this
argument is described in [itimerspec(3type)](../man3/itimerspec.3type.html).
_newvalue.itvalue_ specifies the initial expiration of the timer,
in seconds and nanoseconds. Setting either field of
_newvalue.itvalue_ to a nonzero value arms the timer. Setting
both fields of _newvalue.itvalue_ to zero disarms the timer.
Setting one or both fields of _newvalue.itinterval_ to nonzero
values specifies the period, in seconds and nanoseconds, for
repeated timer expirations after the initial expiration. If both
fields of _newvalue.itinterval_ are zero, the timer expires just
once, at the time specified by _newvalue.itvalue_.
By default, the initial expiration time specified in _newvalue_ is
interpreted relative to the current time on the timer's clock at
the time of the call (i.e., _newvalue.itvalue_ 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 _newvalue.itvalue_ 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
_newvalue.itvalue_.
**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)](../man2/settimeofday.2.html), [clock_settime(2)](../man2/clock%5Fsettime.2.html), or similar). When such
changes occur, a current or future [read(2)](../man2/read.2.html) from the file
descriptor will fail with the error **ECANCELED**.
If the _oldvalue_ 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 currvalue, an itimerspec structure that contains the current setting of the timer referred to by the file descriptor fd.
The _itvalue_ 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 _itinterval_ 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 _currvalue.itvalue_.Operating on a timer file descriptor The file descriptor returned by timerfd_create() supports the following additional operations:
[read(2)](../man2/read.2.html)
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)](../man2/read.2.html), then the buffer given to
[read(2)](../man2/read.2.html) returns an unsigned 8-byte integer (_uint64t_)
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)](../man2/read.2.html), 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)](../man2/fcntl.2.html) **F_SETFL** operation to set the **O_NONBLOCK** flag).
A [read(2)](../man2/read.2.html) 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)](../man2/read.2.html)
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)](../man2/clock%5Fsettime.2.html)) may cause [read(2)](../man2/read.2.html) 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)](../man2/read.2.html) on
the file descriptor.
[poll(2)](../man2/poll.2.html)
[select(2)](../man2/select.2.html)
(and similar)
The file descriptor is readable (the [select(2)](../man2/select.2.html) _readfds_
argument; the [poll(2)](../man2/poll.2.html) **POLLIN** flag) if one or more timer
expirations have occurred.
The file descriptor also supports the other file-descriptor
multiplexing APIs: [pselect(2)](../man2/pselect.2.html), [ppoll(2)](../man2/ppoll.2.html), and [epoll(7)](../man7/epoll.7.html).
[ioctl(2)](../man2/ioctl.2.html)
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 (_uint64t_*) 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)](../man2/close.2.html)
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 top
On success, **timerfd_create**() returns a new file descriptor. On
error, -1 is returned and _[errno](../man3/errno.3.html)_ is set to indicate the error.
**timerfd_settime**() and **timerfd_gettime**() return 0 on success; on
error they return -1, and set _[errno](../man3/errno.3.html)_ to indicate the error.ERRORS top
**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** _newvalue_, _oldvalue_, or _currvalue_ 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** _newvalue_ is not properly initialized (one of the _tvnsec_
falls outside the range zero to 999,999,999).
**EINVAL** _flags_ is invalid.STANDARDS top
Linux.HISTORY top
Linux 2.6.25, glibc 2.8.NOTES top
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)](../man2/settimeofday.2.html)) 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)](../man2/read.2.html) on the file descriptor).
In this case the following occurs:
• The **timerfd_settime**() returns -1 with _[errno](../man3/errno.3.html)_ 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.)BUGS top
Currently, **timerfd_create**() supports fewer types of clock IDs than
[timer_create(2)](../man2/timer%5Fcreate.2.html).EXAMPLES top
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 programProgram source
#include <err.h>
#include <inttypes.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/timerfd.h>
#include <sys/types.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);
}SEE ALSO top
[eventfd(2)](../man2/eventfd.2.html), [poll(2)](../man2/poll.2.html), [read(2)](../man2/read.2.html), [select(2)](../man2/select.2.html), [setitimer(2)](../man2/setitimer.2.html),
[signalfd(2)](../man2/signalfd.2.html), [timer_create(2)](../man2/timer%5Fcreate.2.html), [timer_gettime(2)](../man2/timer%5Fgettime.2.html), [timer_settime(2)](../man2/timer%5Fsettime.2.html),
**timespec**(3), [epoll(7)](../man7/epoll.7.html), [time(7)](../man7/time.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 timerfdcreate(2)
Pages that refer to this page:alarm(2), eventfd(2), getitimer(2), read(2), signalfd(2), syscalls(2), timer_create(2), itimerspec(3type), pcap_get_required_select_timeout(3pcap), sd-event(3), sd_event_add_time(3), timespec(3type), proc_pid_fd(5), proc_pid_fdinfo(5), time(7), time_namespaces(7)