需要解释 `kevent` 过滤器的措辞

Question

我在 man 中有以下 kevent 行：

  EVFILT_TIMER        Establishes an arbitrary timer identified by ident.
                     When adding a timer, data specifies the moment to
                     fire the timer (for NOTE_ABSTIME) or the timeout
                     period.  The timer will be periodic unless EV_ONESHOT
                     or NOTE_ABSTIME is specified.  On return, data
                     contains the number of times the timeout has expired
                     since the last call to kevent().  For non-monotonic
                     timers, this filter automatically sets the EV_CLEAR
                     flag internally.

                     The filter accepts the following flags in the fflags
                     argument:

                     NOTE_SECONDS      data is in seconds.

                     NOTE_MSECONDS     data is in milliseconds.

                     NOTE_USECONDS     data is in microseconds.

                     NOTE_NSECONDS     data is in nanoseconds.

                     NOTE_ABSTIME      The specified expiration time is
                                       absolute.

                     If fflags is not set, the default is milliseconds.
                     On return, fflags contains the events which triggered
                     the filter.

现在，我不明白最后一行：

On return, fflags contains the events which triggered the filter.

我的意思是字面意思，我看不懂这句话。根据定义，事件是一个结构。 fflags 如何包含“事件”？

更新

EVFILT_TIMER 的 fflags 的 return 值与传入的值相同。感谢接受答案的 OP。

Answer 1

根据 FreeBSD-13.0-RELEASE-p11 来源：

从我们在/usr/src/sys/kern/kern_event.c中看到的（作为使用示例）：

static int
filt_proc(struct knote *kn, long hint)
{
    struct proc *p;
    u_int event;

    p = kn->kn_ptr.p_proc;
    if (p == NULL) /* already activated, from attach filter */
        return (0);

    /* Mask off extra data. */
    event = (u_int)hint & NOTE_PCTRLMASK;

    /* If the user is interested in this event, record it. */
    if (kn->kn_sfflags & event)
        kn->kn_fflags |= event;

    /* Process is gone, so flag the event as finished. */
    if (event == NOTE_EXIT) {
        kn->kn_flags |= EV_EOF | EV_ONESHOT;
        kn->kn_ptr.p_proc = NULL;
        if (kn->kn_fflags & NOTE_EXIT)
            kn->kn_data = KW_EXITCODE(p->p_xexit, p->p_xsig);
        if (kn->kn_fflags == 0)
            kn->kn_flags |= EV_DROP;
        return (1);
    }

    return (kn->kn_fflags != 0);
}

上面u_int event的值来自/usr/include/sys/event.h中的一些定义，也是u_int。

因此，fflags 用作 input 和 output 以及在手册页中定义的：

The predefined system filters are listed below. Arguments may be passed to and from the filter via the fflags and data fields in the kevent structure.

您会在 /usr/include/sys/event.h

中找到 fflags 引用的 event

需要解释 `kevent` 过滤器的措辞

Need an explanation on wording of `kevent` filter

c

freebsd

manpage

kqueue

kevent