TIMERFD(2) - FreeBSD 시스템 호출 설명서

 timerfd, timerfd_create, timerfd_gettime, timerfd_settime – timers with
 file descriptor semantics

 Standard C Library (libc, -lc)

 #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);

 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      벽시계가 해야하는 것처럼 증가합니다.
                     CLOCK_MONOTONIC     SI 초 단위로 단조롭게 증가합니다.

                     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()   fd로 표시된 타이머를 new_value의 struct itimerspec으로 업데이트합니다.
                       new_value의 it_value 멤버에는 타이머가 만료되기 전의 시간이 포함되어야 하며,
                     만약 타이머가 해제되어야 한다면 0이 포함되어야 합니다.  만약 간격 타이머가 필요하면
                     it_interval 멤버에는 리로드 시간이 포함되어야 합니다.

                     The previous timer state will be stored in old_value
                     given old_value is not NULL.

                     The flags argument may contain the result of or'ing
                     the following values:

                     TFD_TIMER_ABSTIME        만료는 new_value에 제공된 절대 시간에
                                              발생합니다.  보통, 일반적으로 new_value는
                                              타이머의 clockid 시계와 비교한 상대 시간을
                                              나타냅니다.
                     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.

 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.

 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.

 [ENFILE]           The system file table is full.

 [ENOMEM]           The kernel failed to allocate enough memory for the
                    timer.

 Both timerfd_gettime() and timerfd_settime() system calls fail if:

 [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.

 eventfd(2), kqueue(2), poll(2), read(2), timer_create(2),
 timer_gettime(2), timer_settime(2)

 The timerfd system calls originated from Linux and are non-standard.

 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.