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argument.
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.pp
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the
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.i flags
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argument is a bit mask that can include the following values:
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.tp
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.b tfd_timer_abstime
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interpret
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.i new_value.it_value
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as an absolute value on the timer's clock.
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the timer will expire when the value of the timer's
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clock reaches the value specified in
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.ir new_value.it_value .
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.tp
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.br tfd_timer_cancel_on_set
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if this flag is specified along with
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.b tfd_timer_abstime
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and the clock for this timer is
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.br clock_realtime
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or
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.br clock_realtime_alarm ,
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then mark this timer as cancelable if the real-time clock
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undergoes a discontinuous change
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.rb ( settimeofday (2),
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.br clock_settime (2),
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or similar).
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when such changes occur, a current or future
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.br read (2)
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from the file descriptor will fail with the error
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.br ecanceled .
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.pp
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if the
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.i old_value
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argument is not null, then the
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.i itimerspec
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structure that it points to is used to return the setting of the timer
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that was current at the time of the call;
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see the description of
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.br timerfd_gettime ()
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following.
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.\"
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.ss timerfd_gettime()
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.br timerfd_gettime ()
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returns, in
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.ir curr_value ,
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an
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.ir itimerspec
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structure that contains the current setting of the timer
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referred to by the file descriptor
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.ir fd .
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.pp
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the
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.i it_value
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field returns the amount of time
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until the timer will next expire.
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if both fields of this structure are zero,
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then the timer is currently disarmed.
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this field always contains a relative value, regardless of whether the
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.br tfd_timer_abstime
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flag was specified when setting the timer.
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.pp
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the
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.i it_interval
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field returns the interval of the timer.
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if both fields of this structure are zero,
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then the timer is set to expire just once, at the time specified by
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.ir curr_value.it_value .
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.ss operating on a timer file descriptor
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the file descriptor returned by
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.br timerfd_create ()
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supports the following additional operations:
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.tp
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.br read (2)
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if the timer has already expired one or more times since
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its settings were last modified using
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.br timerfd_settime (),
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or since the last successful
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.br read (2),
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then the buffer given to
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.br read (2)
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returns an unsigned 8-byte integer
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.ri ( uint64_t )
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containing the number of expirations that have occurred.
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(the returned value is in host byte order\(emthat is,
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the native byte order for integers on the host machine.)
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.ip
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if no timer expirations have occurred at the time of the
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.br read (2),
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then the call either blocks until the next timer expiration,
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or fails with the error
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.b eagain
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if the file descriptor has been made nonblocking
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(via the use of the
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.br fcntl (2)
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.b f_setfl
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operation to set the
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.b o_nonblock
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flag).
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.ip
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a
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