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 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()
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'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()
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 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
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.
