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download PDF document: timerfd_gettime.2.pdf

TIMERFD(2) FreeBSD System Calls Manual TIMERFD(2)
NAME timerfd, timerfd_create, timerfd_gettime, timerfd_settime - timers with file descriptor semantics
LIBRARY Standard C Library (libc, -lc)
SYNOPSIS #include <sys/timerfd.h>
int timerfd_create(int clockid, int flags);
int timerfd_gettime(int fd, struct itimerspec *curr_value);
int timerfd_settime(int fd, int flags, const struct itimerspec *new_value, struct itimerspec *old_value);
DESCRIPTION The timerfd system calls operate on timers, identified by special timerfd file descriptors. These calls are analogous to timer_create(), timer_gettime(), and timer_settime() per-process timer functions, but use a timerfd descriptor in place of timerid.
All timerfd descriptors possess traditional file descriptor semantics; they may be passed to other processes, preserved across fork(2), and monitored via kevent(2), poll(2), or select(2). When a timerfd descriptor is no longer needed, it may be disposed of using close(2).
timerfd_create() Initialize a timerfd object and return its file descriptor. The clockid argument specifies the clock used as a timing base and may be:
CLOCK_REALTIME Increments as a wall clock should. CLOCK_MONOTONIC Increments monotonically in SI seconds.
The flags argument may contain the result of or'ing the following values:
TFD_CLOEXEC The newly generated file descriptor will close-on-exec. TFD_NONBLOCK Do not block on read/write operations.
timerfd_gettime() Retrieve the current state of the timer denoted by fd. The result is stored in curr_value as a struct itimerspec. The it_value and it_interval members of curr_value represent the relative time until the next expiration and the interval reload value last set by timerfd_settime(), respectively.
timerfd_settime() Update the timer denoted by fd with the struct itimerspec in new_value. The it_value member of
The flags argument may contain the result of or'ing the following values:
TFD_TIMER_ABSTIME Expiration will occur at the absolute time provided in new_value. Normally, new_value represents a relative time compared to the timer's clockid clock. TFD_TIMER_CANCEL_ON_SET If clockid has been set to CLOCK_REALTIME and the realtime clock has experienced a discontinuous jump, then the timer will be canceled and the next read(2) will fail with ECANCELED.
File operations have the following semantics:
read(2) Transfer the number of timer expirations that have occurred since the last successful read(2) or timerfd_settime() into the output buffer of size uint64_t. If the expiration counter is zero, read(2) blocks until a timer expiration occurs unless TFD_NONBLOCK is set, where EAGAIN is returned.
poll(2) The file descriptor is readable when its timer expiration counter is greater than zero.
ioctl(2)
FIOASYNC int A non-zero input will set the FASYNC flag. A zero input will clear the FASYNC flag.
FIONBIO int A non-zero input will set the FNONBLOCK flag. A zero input will clear the FNONBLOCK flag.
RETURN VALUES The timerfd_create() system call creates a timerfd object and returns its file descriptor. If an error occurs, -1 is returned and the global variable errno is set to indicate the error.
The timerfd_gettime() and timerfd_settime() system calls return 0 on success. If an error occurs, -1 is returned and the global variable errno is set to indicate the error.
ERRORS The timerfd_create() system call fails if:
[EINVAL] The specified clockid is not supported.
[EINVAL] The provided flags are invalid.
[EMFILE] The per-process descriptor table is full.
[EBADF] The provided fd is invalid.
[EFAULT] The addresses provided by curr_value, new_value, or old_value are invalid.
[EINVAL] The provided fd is valid, but was not generated by timerfd_create().
The following errors only apply to timerfd_settime():
[EINVAL] The provided flags are invalid.
[EINVAL] A nanosecond field in the new_value argument specified a value less than zero, or greater than or equal to 10^9.
[ECANCELED] The timer was created with the clock ID CLOCK_REALTIME, was configured with the TFD_TIMER_CANCEL_ON_SET flag, and the system realtime clock experienced a discontinuous change without being read.
A read from a timerfd object fails if:
[EAGAIN] The timer's expiration counter is zero and the timerfd object is is set for non-blocking I/O.
[ECANCELED] The timer was created with the clock ID CLOCK_REALTIME, was configured with the TFD_TIMER_CANCEL_ON_SET flag, and the system realtime clock experienced a discontinuous change.
[EINVAL] The size of the read buffer is not large enough to hold the uint64_t sized timer expiration counter.
SEE ALSO eventfd(2), kqueue(2), poll(2), read(2), timer_create(2), timer_gettime(2), timer_settime(2)
STANDARDS The timerfd system calls originated from Linux and are non-standard.
HISTORY The timerfd facility was originally ported to FreeBSD's Linux compatibility layer by Dmitry Chagin <dchagin@FreeBSD.org> in FreeBSD 12.0. It was revised and adapted to be native by Jake Freeland <jfree@FreeBSD.org> in FreeBSD 14.0.
FreeBSD 14.0-RELEASE-p11 May 21, 2023 FreeBSD 14.0-RELEASE-p11