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.\"

.th timerfd_create 2 2021-03-22 linux "linux programmer's manual"

.sh name

timerfd_create, timerfd_settime, timerfd_gettime \-

timers that notify via file descriptors

.sh synopsis

.nf

.b #include <sys/timerfd.h>

.pp

.bi "int timerfd_create(int " clockid ", int " flags );

.pp

.bi "int timerfd_settime(int " fd ", int " flags ,

.bi " const struct itimerspec *" new_value ,

.bi " struct itimerspec *" old_value );

.bi "int timerfd_gettime(int " fd ", struct itimerspec *" curr_value );

.fi

.sh 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

.br setitimer (2)

or

.br timer_create (2),

with the advantage that the file descriptor may be monitored by

.br select (2),

.br poll (2),

and

.br epoll (7).

.pp

the use of these three system calls is analogous to the use of

.br timer_create (2),

.br timer_settime (2),

and

.br timer_gettime (2).

(there is no analog of

.br timer_getoverrun (2),

since that functionality is provided by

.br read (2),

as described below.)

.\"

.ss timerfd_create()

.br timerfd_create ()

creates a new timer object,

and returns a file descriptor that refers to that timer.

the

.i clockid

argument specifies the clock that is used to mark the progress

of the timer, and must be one of the following:

.tp

.b clock_realtime

a settable system-wide real-time clock.

.tp

.b clock_monotonic

a nonsettable monotonically increasing clock that measures time

from some unspecified point in the past that does not change

after system startup.

.tp

.br clock_boottime " (since linux 3.15)"

.\" commit 4a2378a943f09907fb1ae35c15de917f60289c14

like

.br clock_monotonic ,

this is a monotonically increasing clock.

however, whereas the

.br clock_monotonic

clock does not measure the time while a system is suspended, the

.br clock_boottime

clock does include the time during which the system is suspended.

this is useful for applications that need to be suspend-aware.

.br clock_realtime

is not suitable for such applications, since that clock is affected

by discontinuous changes to the system clock.

.tp

.br clock_realtime_alarm " (since linux 3.11)"

.\" commit 11ffa9d6065f344a9bd769a2452f26f2f671e5f8

this clock is like

.br clock_realtime ,

but will wake the system if it is suspended.

the caller must have the

.b cap_wake_alarm

capability in order to set a timer against this clock.

.tp

.br clock_boottime_alarm " (since linux 3.11)"

.\" commit 11ffa9d6065f344a9bd769a2452f26f2f671e5f8