Timerfd

Linux 22 Sep 2026 5 min read

TFD_TIMER_CANCEL_ON_SET Reports Realtime Clock Discontinuities

A timerfd armed against wall-clock time can cross a discontinuous clock adjustment before its deadline. With TFD_TIMER_CANCEL_ON_SET, Linux exposes that event through the descriptor: a subsequent read() fails with ECANCELED rather than presenting the clock jump as an ordinary timer expiration. The flag is deliberately narrow. It applies only when timerfd_settime() arms an absolute timer on CLOCK_REALTIME or CLOCK_REALTIME_ALARM, and it changes the handling of discontinuous changes to those clocks.

Linux timerfd

Linux 18 Sep 2026 4 min read

timerfd Counts Expirations Through Descriptor I/O

A periodic timerfd does not require one userspace wakeup for every timer expiration. If several expirations occur before the descriptor is read, Linux accumulates them and returns the count in one 8-byte integer. That behavior makes timer state fit the same readiness model used for sockets, pipes, and other descriptors. It also gives delayed event loops explicit information about missed periods rather than collapsing several expirations into one notification. Expiration state becomes readable descriptor data timerfd_create() creates a timer object and returns a file descriptor referring to it. The selected clock defines the timer’s time base. Common choices include CLOCK_MONOTONIC, CLOCK_REALTIME, and CLOCK_BOOTTIME.

Linux timerfd