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.br read (2)
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fails with the error
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.b einval
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if the size of the supplied buffer is less than 8 bytes.
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.ip
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if the associated clock is either
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.br clock_realtime
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or
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.br clock_realtime_alarm ,
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the timer is absolute
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.rb ( tfd_timer_abstime ),
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and the flag
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.br tfd_timer_cancel_on_set
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was specified when calling
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.br timerfd_settime (),
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then
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.br read (2)
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fails with the error
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.br ecanceled
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if the real-time clock undergoes a discontinuous change.
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(this allows the reading application to discover
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such discontinuous changes to the clock.)
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.ip
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if the associated clock is either
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.br clock_realtime
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or
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.br clock_realtime_alarm ,
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the timer is absolute
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.rb ( tfd_timer_abstime ),
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and the flag
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.br tfd_timer_cancel_on_set
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was
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.i not
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specified when calling
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.br timerfd_settime (),
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then a discontinuous negative change to the clock (e.g.,
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.br clock_settime (2))
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may cause
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.br read (2)
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to unblock, but return a value of 0 (i.e., no bytes read),
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if the clock change occurs after the time expired,
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but before the
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.br read (2)
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on the file descriptor.
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.tp
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.br poll "(2), " select "(2) (and similar)"
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the file descriptor is readable
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(the
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.br select (2)
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.i readfds
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argument; the
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.br poll (2)
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.b pollin
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flag)
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if one or more timer expirations have occurred.
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.ip
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the file descriptor also supports the other file-descriptor
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multiplexing apis:
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.br pselect (2),
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.br ppoll (2),
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and
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.br epoll (7).
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.tp
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.br ioctl (2)
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the following timerfd-specific command is supported:
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.rs
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.tp
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.br tfd_ioc_set_ticks " (since linux 3.17)"
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.\" commit 5442e9fbd7c23172a1c9bc736629cd123a9923f0
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adjust the number of timer expirations that have occurred.
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the argument is a pointer to a nonzero 8-byte integer
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.ri ( uint64_t *)
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containing the new number of expirations.
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once the number is set, any waiter on the timer is woken up.
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the only purpose of this command is to restore the expirations
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for the purpose of checkpoint/restore.
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this operation is available only if the kernel was configured with the
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.br config_checkpoint_restore
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option.
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.re
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.tp
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.br close (2)
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when the file descriptor is no longer required it should be closed.
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when all file descriptors associated with the same timer object
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have been closed,
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the timer is disarmed and its resources are freed by the kernel.
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.\"
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.ss fork(2) semantics
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after a
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.br fork (2),
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the child inherits a copy of the file descriptor created by
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.br timerfd_create ().
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the file descriptor refers to the same underlying
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timer object as the corresponding file descriptor in the parent,
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and
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.br read (2)s
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in the child will return information about
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expirations of the timer.
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.\"
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.ss execve(2) semantics
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