669 lines
17 KiB
Groff
669 lines
17 KiB
Groff
.\" $OpenBSD: kqueue.2,v 1.51 2023/08/20 19:52:40 jmc Exp $
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.\"
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.\" Copyright (c) 2000 Jonathan Lemon
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD: src/lib/libc/sys/kqueue.2,v 1.18 2001/02/14 08:48:35 guido Exp $
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.\"
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.Dd $Mdocdate: August 20 2023 $
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.Dt KQUEUE 2
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.Os
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.Sh NAME
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.Nm kqueue ,
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.Nm kqueue1 ,
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.Nm kevent ,
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.Nm EV_SET
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.Nd kernel event notification mechanism
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.Sh SYNOPSIS
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.In sys/types.h
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.In sys/event.h
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.In sys/time.h
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.Ft int
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.Fn kqueue "void"
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.Ft int
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.Fn kevent "int kq" "const struct kevent *changelist" "int nchanges" "struct kevent *eventlist" "int nevents" "const struct timespec *timeout"
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.Fn EV_SET "&kev" ident filter flags fflags data udata
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.In sys/types.h
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.In sys/event.h
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.In sys/time.h
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.In fcntl.h
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.Ft int
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.Fn kqueue1 "int flags"
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.Sh DESCRIPTION
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.Fn kqueue
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provides a generic method of notifying the user when an event
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happens or a condition holds, based on the results of small
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pieces of kernel code termed
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.Dq filters .
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A kevent is identified by the (ident, filter) pair; there may only
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be one unique kevent per kqueue.
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.Pp
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The filter is executed upon the initial registration of a kevent
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in order to detect whether a preexisting condition is present, and is also
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executed whenever an event is passed to the filter for evaluation.
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If the filter determines that the condition should be reported,
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then the kevent is placed on the kqueue for the user to retrieve.
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.Pp
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The filter is also run when the user attempts to retrieve the kevent
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from the kqueue.
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If the filter indicates that the condition that triggered
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the event no longer holds, the kevent is removed from the kqueue and
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is not returned.
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.Pp
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Multiple events which trigger the filter do not result in multiple
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kevents being placed on the kqueue; instead, the filter will aggregate
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the events into a single
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.Vt struct kevent .
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Calling
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.Xr close 2
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on a file descriptor will remove any kevents that reference the descriptor.
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.Pp
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.Fn kqueue
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creates a new kernel event queue and returns a descriptor.
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The queue is not inherited by a child created with
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.Xr fork 2 .
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Similarly, kqueues cannot be passed across UNIX-domain sockets.
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.Pp
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The
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.Fn kqueue1
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function is identical to
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.Fn kqueue
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except that the close-on-exec flag on the new file descriptor
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is determined by the
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.Dv O_CLOEXEC
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flag
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in the
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.Fa flags
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argument.
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.Pp
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.Fn kevent
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is used to register events with the queue, and return any pending
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events to the user.
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.Fa changelist
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is a pointer to an array of
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.Vt kevent
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structures, as defined in
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.In sys/event.h .
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All changes contained in the
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.Fa changelist
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are applied before any pending events are read from the queue.
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.Fa nchanges
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gives the size of
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.Fa changelist .
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.Fa eventlist
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is a pointer to an array of
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.Vt kevent
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structures.
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.Fa nevents
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determines the size of
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.Fa eventlist .
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When
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.Fa nevents
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is zero,
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.Fn kevent
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will return immediately even if there is a
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.Fa timeout
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specified, unlike
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.Xr select 2 .
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If
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.Fa timeout
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is not
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.Dv NULL ,
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it specifies a maximum interval to wait
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for an event, which will be interpreted as a
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.Vt struct timespec .
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If
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.Fa timeout
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is
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.Dv NULL ,
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.Fn kevent
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waits indefinitely.
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To effect a poll, the
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.Fa timeout
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argument should not be
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.Dv NULL ,
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pointing to a zero-valued
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.Vt struct timespec .
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The same array may be used for the
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.Fa changelist
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and
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.Fa eventlist .
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.Pp
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.Fn EV_SET
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is a macro which is provided for ease of initializing a
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.Vt kevent
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structure.
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.Pp
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The
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.Vt kevent
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structure is defined as:
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.Bd -literal
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struct kevent {
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uintptr_t ident; /* identifier for this event */
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short filter; /* filter for event */
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u_short flags; /* action flags for kqueue */
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u_int fflags; /* filter flag value */
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int64_t data; /* filter data value */
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void *udata; /* opaque user data identifier */
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};
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.Ed
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.Pp
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The fields of
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.Vt struct kevent
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are:
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.Bl -tag -width XXXfilter
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.It Fa ident
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Value used to identify this event.
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The exact interpretation is determined by the attached filter,
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but often is a file descriptor.
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.It Fa filter
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Identifies the kernel filter used to process this event.
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The pre-defined system filters are described below.
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.It Fa flags
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Actions to perform on the event.
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.It Fa fflags
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Filter-specific flags.
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.It Fa data
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Filter-specific data value.
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.It Fa udata
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Opaque user-defined value passed through the kernel unchanged.
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.El
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.Pp
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The
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.Fa flags
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field can contain the following values:
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.Bl -tag -width XXXEV_ONESHOT
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.It Dv EV_ADD
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Adds the event to the kqueue.
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Re-adding an existing event will modify the parameters of the original event,
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and not result in a duplicate entry.
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Adding an event automatically enables it, unless overridden by the
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.Dv EV_DISABLE
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flag.
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.It Dv EV_ENABLE
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Permit
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.Fn kevent
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to return the event if it is triggered.
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.It Dv EV_DISABLE
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Disable the event so
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.Fn kevent
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will not return it.
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The filter itself is not disabled.
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.It Dv EV_DISPATCH
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Disable the event source immediately after delivery of an event.
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See
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.Dv EV_DISABLE
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above.
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.It Dv EV_DELETE
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Removes the event from the kqueue.
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Events which are attached to file descriptors are automatically deleted
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on the last close of the descriptor.
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.It Dv EV_RECEIPT
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Causes
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.Fn kevent
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to return with
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.Dv EV_ERROR
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set without draining any pending events after updating events in the kqueue.
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When a filter is successfully added, the
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.Fa data
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field will be zero.
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This flag is useful for making bulk changes to a kqueue.
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.It Dv EV_ONESHOT
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Causes the event to return only the first occurrence of the filter
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being triggered.
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After the user retrieves the event from the kqueue, it is deleted.
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.It Dv EV_CLEAR
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After the event is retrieved by the user, its state is reset.
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This is useful for filters which report state transitions
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instead of the current state.
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Note that some filters may automatically set this flag internally.
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.It Dv EV_EOF
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Filters may set this flag to indicate filter-specific EOF condition.
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.It Dv EV_ERROR
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See
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.Sx RETURN VALUES
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below.
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.El
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.Pp
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The predefined system filters are listed below.
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Arguments may be passed to and from the filter via the
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.Fa fflags
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and
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.Fa data
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fields in the
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.Vt kevent
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structure.
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.Bl -tag -width EVFILT_SIGNAL
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.It Dv EVFILT_READ
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Takes a descriptor as the identifier, and returns whenever
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there is data available to read.
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The behavior of the filter is slightly different depending
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on the descriptor type.
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.Bl -tag -width 2n
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.It Sockets
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Sockets which have previously been passed to
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.Xr listen 2
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return when there is an incoming connection pending.
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.Fa data
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contains the size of the listen backlog.
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.Pp
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Other socket descriptors return when there is data to be read,
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subject to the
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.Dv SO_RCVLOWAT
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value of the socket buffer.
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This may be overridden with a per-filter low water mark at the
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time the filter is added by setting the
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.Dv NOTE_LOWAT
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flag in
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.Fa fflags ,
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and specifying the new low water mark in
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.Fa data .
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On return,
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.Fa data
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contains the number of bytes in the socket buffer.
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.Pp
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If the read direction of the socket has shutdown, then the filter
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also sets
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.Dv EV_EOF
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in
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.Fa flags ,
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and returns the socket error (if any) in
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.Fa fflags .
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It is possible for EOF to be returned (indicating the connection is gone)
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while there is still data pending in the socket buffer.
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.It Vnodes
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Returns when the file pointer is not at the end of file.
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.Fa data
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contains the offset from current position to end of file,
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and may be negative.
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If
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.Dv NOTE_EOF
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is set in
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.Fa fflags ,
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.Fn kevent
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will also return when the file pointer is at the end of file.
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The end of file condition is indicated by the presence of
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.Dv NOTE_EOF
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in
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.Fa fflags
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on return.
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.It "FIFOs, Pipes"
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Returns when there is data to read;
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.Fa data
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contains the number of bytes available.
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.Pp
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When the last writer disconnects, the filter will set
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.Dv EV_EOF
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in
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.Fa flags .
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This may be cleared by passing in
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.Dv EV_CLEAR ,
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at which point the filter will resume waiting for data to become
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available before returning.
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.It "BPF devices"
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Returns when the BPF buffer is full, the BPF timeout has expired, or
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when the BPF has
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.Dq immediate mode
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enabled and there is any data to read;
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.Fa data
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contains the number of bytes available.
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.El
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.It Dv EVFILT_EXCEPT
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Takes a descriptor as the identifier, and returns whenever one of the
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specified exceptional conditions has occurred on the descriptor.
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Conditions are specified in
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.Fa fflags .
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Currently, a filter can monitor the reception of out-of-band data
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on a socket or pseudo terminal with
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.Dv NOTE_OOB .
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.It Dv EVFILT_WRITE
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Takes a descriptor as the identifier, and returns whenever
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it is possible to write to the descriptor.
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For sockets, pipes, and FIFOs,
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.Fa data
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will contain the amount of space remaining in the write buffer.
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The filter will set
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.Dv EV_EOF
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when the reader disconnects, and for the FIFO case,
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this may be cleared by use of
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.Dv EV_CLEAR .
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Note that this filter is not supported for vnodes or BPF devices.
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.Pp
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For sockets, the low water mark and socket error handling is
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identical to the
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.Dv EVFILT_READ
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case.
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.\".It Dv EVFILT_AIO
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.\"The sigevent portion of the AIO request is filled in, with
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.\".Va sigev_notify_kqueue
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.\"containing the descriptor of the kqueue that the event should
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.\"be attached to,
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.\".Va sigev_value
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.\"containing the udata value, and
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.\".Va sigev_notify
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.\"set to
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.\".Dv SIGEV_KEVENT .
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.\"When the aio_* function is called, the event will be registered
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.\"with the specified kqueue, and the
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.\".Va ident
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.\"argument set to the
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.\".Li struct aiocb
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.\"returned by the aio_* function.
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.\"The filter returns under the same conditions as aio_error.
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.\".Pp
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.\"Alternatively, a kevent structure may be initialized, with
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.\".Va ident
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.\"containing the descriptor of the kqueue, and the
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.\"address of the kevent structure placed in the
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.\".Va aio_lio_opcode
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.\"field of the AIO request.
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.\"However, this approach will not work on architectures with 64-bit pointers,
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.\"and should be considered deprecated.
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.It Dv EVFILT_VNODE
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Takes a file descriptor as the identifier and the events to watch for in
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.Fa fflags ,
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and returns when one or more of the requested events occurs on the descriptor.
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The events to monitor are:
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.Bl -tag -width XXNOTE_RENAME
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.It Dv NOTE_DELETE
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.Xr unlink 2
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was called on the file referenced by the descriptor.
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.It Dv NOTE_WRITE
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A write occurred on the file referenced by the descriptor.
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.It Dv NOTE_EXTEND
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The file referenced by the descriptor was extended.
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.It Dv NOTE_TRUNCATE
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The file referenced by the descriptor was truncated.
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.It Dv NOTE_ATTRIB
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The file referenced by the descriptor had its attributes changed.
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.It Dv NOTE_LINK
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The link count on the file changed.
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.It Dv NOTE_RENAME
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The file referenced by the descriptor was renamed.
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.It Dv NOTE_REVOKE
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Access to the file was revoked via
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.Xr revoke 2
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or the underlying file system was unmounted.
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.El
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.Pp
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On return,
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.Fa fflags
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contains the events which triggered the filter.
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.It Dv EVFILT_PROC
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Takes the process ID to monitor as the identifier and the events to watch for
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in
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.Fa fflags ,
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and returns when the process performs one or more of the requested events.
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If a process can normally see another process, it can attach an event to it.
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The events to monitor are:
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.Bl -tag -width XXNOTE_TRACKERR
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.It Dv NOTE_EXIT
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The process has exited.
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The exit status will be stored in
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.Fa data
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in the same format as the status set by
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.Xr wait 2 .
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.It Dv NOTE_FORK
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The process has called
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.Xr fork 2 .
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.It Dv NOTE_EXEC
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The process has executed a new process via
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.Xr execve 2
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or similar call.
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.It Dv NOTE_TRACK
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Follow a process across
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.Xr fork 2
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calls.
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The parent process will return with
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.Dv NOTE_FORK
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set in the
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.Fa fflags
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field, while the child process will return with
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.Dv NOTE_CHILD
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set in
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.Fa fflags
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and the parent PID in
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.Fa data .
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.It Dv NOTE_TRACKERR
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This flag is returned if the system was unable to attach an event to
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the child process, usually due to resource limitations.
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.El
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.Pp
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On return,
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.Fa fflags
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contains the events which triggered the filter.
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.It Dv EVFILT_SIGNAL
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Takes the signal number to monitor as the identifier and returns
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when the given signal is delivered to the process.
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|
This coexists with the
|
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.Xr signal 3
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and
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.Xr sigaction 2
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|
facilities, and has a lower precedence.
|
|
The filter will record all attempts to deliver a signal to a process,
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even if the signal has been marked as
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.Dv SIG_IGN .
|
|
Event notification happens after normal signal delivery processing.
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.Fa data
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returns the number of times the signal has occurred since the last call to
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.Fn kevent .
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This filter automatically sets the
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.Dv EV_CLEAR
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flag internally.
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.It Dv EVFILT_TIMER
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|
Establishes an arbitrary timer identified by
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.Fa ident .
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|
When adding a timer,
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.Fa data
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specifies the timeout period in units described below or, if
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|
.Dv NOTE_ABSTIME
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|
is set in
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.Va fflags ,
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|
the absolute time at which the timer should fire.
|
|
The timer will repeat unless
|
|
.Dv EV_ONESHOT
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|
is set in
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.Va flags
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|
or
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|
.Dv NOTE_ABSTIME
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|
is set in
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|
.Va fflags .
|
|
On return,
|
|
.Fa data
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|
contains the number of times the timeout has expired since the last call to
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|
.Fn kevent .
|
|
This filter automatically sets
|
|
.Dv EV_CLEAR
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|
in
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|
.Va flags
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|
for periodic timers.
|
|
Timers created with
|
|
.Dv NOTE_ABSTIME
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|
remain activated on the kqueue once the absolute time has passed unless
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.Dv EV_CLEAR
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or
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|
.Dv EV_ONESHOT
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|
are also specified.
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|
.Pp
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|
The filter accepts the following flags in the
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.Va fflags
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argument:
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|
.Bl -tag -width NOTE_MSECONDS
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|
.It Dv NOTE_SECONDS
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|
The timer value in
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.Va data
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|
is expressed in seconds.
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.It Dv NOTE_MSECONDS
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|
The timer value in
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.Va data
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|
is expressed in milliseconds.
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.It Dv NOTE_USECONDS
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|
The timer value in
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.Va data
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|
is expressed in microseconds.
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.It Dv NOTE_NSECONDS
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The timer value in
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.Va data
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is expressed in nanoseconds.
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.It Dv NOTE_ABSTIME
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The timer value is an absolute time with
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.Dv CLOCK_REALTIME
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|
as the reference clock.
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.El
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.Pp
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|
Note that
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|
.Dv NOTE_SECONDS ,
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|
.Dv NOTE_MSECONDS ,
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|
.Dv NOTE_USECONDS ,
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and
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|
.Dv NOTE_NSECONDS
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|
are mutually exclusive; behavior is undefined if more than one are specified.
|
|
If a timer value unit is not specified, the default is
|
|
.Dv NOTE_MSECONDS .
|
|
.Pp
|
|
If an existing timer is re-added, the existing timer and related pending events
|
|
will be cancelled.
|
|
The timer will be re-started using the timeout period
|
|
.Fa data .
|
|
.It Dv EVFILT_DEVICE
|
|
Takes a descriptor as the identifier and the events to watch for in
|
|
.Fa fflags ,
|
|
and returns when one or more of the requested events occur on the
|
|
descriptor.
|
|
The events to monitor are:
|
|
.Bl -tag -width XXNOTE_CHANGE
|
|
.It Dv NOTE_CHANGE
|
|
A device change event has occurred,
|
|
e.g. an HDMI cable has been plugged in to a port.
|
|
.El
|
|
.Pp
|
|
On return,
|
|
.Fa fflags
|
|
contains the events which triggered the filter.
|
|
.El
|
|
.Sh RETURN VALUES
|
|
.Fn kqueue
|
|
and
|
|
.Fn kqueue1
|
|
create a new kernel event queue and returns a file descriptor.
|
|
If there was an error creating the kernel event queue, a value of -1 is
|
|
returned and
|
|
.Va errno
|
|
set.
|
|
.Pp
|
|
.Fn kevent
|
|
returns the number of events placed in the
|
|
.Fa eventlist ,
|
|
up to the value given by
|
|
.Fa nevents .
|
|
If an error occurs while processing an element of the
|
|
.Fa changelist
|
|
and there is enough room in the
|
|
.Fa eventlist ,
|
|
then the event will be placed in the
|
|
.Fa eventlist
|
|
with
|
|
.Dv EV_ERROR
|
|
set in
|
|
.Fa flags
|
|
and the system error in
|
|
.Fa data .
|
|
Otherwise, -1 will be returned, and
|
|
.Va errno
|
|
will be set to indicate the error condition.
|
|
If the time limit expires, then
|
|
.Fn kevent
|
|
returns 0.
|
|
.Sh ERRORS
|
|
The
|
|
.Fn kqueue
|
|
and
|
|
.Fn kqueue1
|
|
functions fail if:
|
|
.Bl -tag -width Er
|
|
.It Bq Er ENOMEM
|
|
The kernel failed to allocate enough memory for the kernel queue.
|
|
.It Bq Er EMFILE
|
|
The per-process descriptor table is full.
|
|
.It Bq Er ENFILE
|
|
The system file table is full.
|
|
.El
|
|
.Pp
|
|
In addition,
|
|
.Fn kqueue1
|
|
fails if:
|
|
.Bl -tag -width Er
|
|
.It Bq Er EINVAL
|
|
.Fa flags
|
|
is invalid.
|
|
.El
|
|
.Pp
|
|
The
|
|
.Fn kevent
|
|
function fails if:
|
|
.Bl -tag -width Er
|
|
.It Bq Er EACCES
|
|
The process does not have permission to register a filter.
|
|
.It Bq Er EFAULT
|
|
There was an error reading or writing the
|
|
.Vt kevent
|
|
structure.
|
|
.It Bq Er EBADF
|
|
The specified descriptor is invalid.
|
|
.It Bq Er EINTR
|
|
A signal was delivered before the timeout expired and before any
|
|
events were placed on the kqueue for return.
|
|
.It Bq Er EINVAL
|
|
The specified time limit or filter is invalid.
|
|
.It Bq Er ENOENT
|
|
The event could not be found to be modified or deleted.
|
|
.It Bq Er ENOMEM
|
|
No memory was available to register the event.
|
|
.It Bq Er ESRCH
|
|
The specified process to attach to does not exist.
|
|
.El
|
|
.Sh SEE ALSO
|
|
.Xr clock_gettime 2 ,
|
|
.Xr poll 2 ,
|
|
.Xr read 2 ,
|
|
.Xr select 2 ,
|
|
.Xr sigaction 2 ,
|
|
.Xr wait 2 ,
|
|
.Xr write 2 ,
|
|
.Xr signal 3
|
|
.Sh HISTORY
|
|
The
|
|
.Fn kqueue
|
|
and
|
|
.Fn kevent
|
|
functions first appeared in
|
|
.Fx 4.1
|
|
and have been available since
|
|
.Ox 2.9 .
|
|
.Sh AUTHORS
|
|
The
|
|
.Fn kqueue
|
|
system and this manual page were written by
|
|
.An Jonathan Lemon Aq Mt jlemon@FreeBSD.org .
|