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+.\"	$NetBSD: kqueue.2,v 1.60 2023/07/28 23:41:16 wiz Exp $
+.\"
+.\" Copyright (c) 2000 Jonathan Lemon
+.\" All rights reserved.
+.\"
+.\" Copyright (c) 2001, 2002, 2003 The NetBSD Foundation, Inc.
+.\" All rights reserved.
+.\"
+.\" Portions of this documentation is derived from text contributed by
+.\" Luke Mewburn.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.\" $FreeBSD: src/lib/libc/sys/kqueue.2,v 1.22 2001/06/27 19:55:57 dd Exp $
+.\"
+.Dd June 19, 2023
+.Dt KQUEUE 2
+.Os
+.Sh NAME
+.Nm kqueue ,
+.Nm kqueue1 ,
+.Nm kevent ,
+.Nm EV_SET
+.Nd kernel event notification mechanism
+.Sh LIBRARY
+.Lb libc
+.Sh SYNOPSIS
+.In sys/event.h
+.In sys/time.h
+.Ft int
+.Fn kqueue "void"
+.Ft int
+.Fn kqueue1 "int flags"
+.Ft int
+.Fn kevent "int kq" "const struct kevent *changelist" "size_t nchanges" "struct kevent *eventlist" "size_t nevents" "const struct timespec *timeout"
+.Fn EV_SET "&kev" ident filter flags fflags data udata
+.Sh DESCRIPTION
+The
+.Fn kqueue
+system call
+provides a generic method of notifying the user when an event
+happens or a condition holds, based on the results of small
+pieces of kernel code termed filters.
+A kevent is identified by the (ident, filter) pair; there may only
+be one unique kevent per kqueue.
+.Pp
+The filter is executed upon the initial registration of a kevent
+in order to detect whether a preexisting condition is present, and is also
+executed whenever an event is passed to the filter for evaluation.
+If the filter determines that the condition should be reported,
+then the kevent is placed on the kqueue for the user to retrieve.
+.Pp
+The filter is also run when the user attempts to retrieve the kevent
+from the kqueue.
+If the filter indicates that the condition that triggered
+the event no longer holds, the kevent is removed from the kqueue and
+is not returned.
+.Pp
+Multiple events which trigger the filter do not result in multiple
+kevents being placed on the kqueue; instead, the filter will aggregate
+the events into a single struct kevent.
+Calling
+.Xr close 2
+on a file descriptor will remove any kevents that reference the descriptor.
+.Pp
+The
+.Fn kqueue
+system call
+creates a new kernel event queue and returns a descriptor.
+.Pp
+The
+.Fn kqueue1
+system call also allows to set the following
+.Fa flags
+on the returned file descriptor:
+.Bl -column O_NONBLOCK -offset indent
+.It Dv O_CLOEXEC
+Set the close on exec property.
+.It Dv O_NONBLOCK
+Set non-blocking I/O.
+.It Dv O_NOSIGPIPE
+Return
+.Er EPIPE
+instead of raising
+.Dv SIGPIPE .
+.El
+.Pp
+The queue is not inherited by a child created with
+.Xr fork 2 .
+.\" However, if
+.\" .Xr rfork 2
+.\" is called without the
+.\" .Dv RFFDG
+.\" flag, then the descriptor table is shared,
+.\" which will allow sharing of the kqueue between two processes.
+.Pp
+The
+.Fn kevent
+system call
+is used to register events with the queue, and return any pending
+events to the user.
+The
+.Fa changelist
+argument
+is a pointer to an array of
+.Va kevent
+structures, as defined in
+.In sys/event.h .
+All changes contained in the
+.Fa changelist
+are applied before any pending events are read from the queue.
+The
+.Fa nchanges
+argument
+gives the size of
+.Fa changelist .
+The
+.Fa eventlist
+argument
+is a pointer to an array of kevent structures.
+The
+.Fa nevents
+argument
+determines the size of
+.Fa eventlist .
+When
+.Fa nevents
+is zero,
+.Fn kevent
+will return immediately even if there is a
+.Fa timeout
+specified unlike
+.Xr select 2 .
+If
+.Fa timeout
+is a
+.No non- Ns Dv NULL
+pointer, it specifies a maximum interval to wait
+for an event, which will be interpreted as a
+.Li struct timespec .
+If
+.Fa timeout
+is a
+.Dv NULL
+pointer,
+.Fn kevent
+waits indefinitely.
+To effect a poll, the
+.Fa timeout
+argument should be
+.No non- Ns Dv NULL ,
+pointing to a zero-valued
+.Xr timespec 3
+structure.
+The same array may be used for the
+.Fa changelist
+and
+.Fa eventlist .
+.Pp
+The
+.Fn EV_SET
+macro is provided for ease of initializing a kevent structure.
+This macro does not evaluate its parameters multiple times.
+.Pp
+The
+.Va kevent
+structure is defined as:
+.Bd -literal
+struct kevent {
+	uintptr_t ident;	/* identifier for this event */
+	uint32_t  filter;	/* filter for event */
+	uint32_t  flags;	/* action flags for kqueue */
+	uint32_t  fflags;	/* filter flag value */
+	int64_t   data;		/* filter data value */
+	void     *udata;	/* opaque user data identifier */
+	uint64_t  ext[4];	/* extensions */
+};
+.Ed
+.Pp
+The fields of
+.Fa struct kevent
+are:
+.Bl -tag -width "Fa filter" -offset indent
+.It ident
+Value used to identify this event.
+The exact interpretation is determined by the attached filter,
+but often is a file descriptor.
+.It Fa filter
+Identifies the kernel filter used to process this event.
+There are pre-defined system filters (which are described below), and
+other filters may be added by kernel subsystems as necessary.
+.It Fa flags
+Actions to perform on the event.
+.It Fa fflags
+Filter-specific flags.
+.It Fa data
+Filter-specific data value.
+.It Fa udata
+Opaque user-defined value passed through the kernel unchanged.
+.It Fa ext
+Extended data passed to and from kernel.
+The
+.Fa ext[0]
+and
+.Fa ext[1]
+members use is defined by the filter.
+If the filter does not use them, the members are copied unchanged.
+The
+.Fa ext[2]
+and
+.Fa ext[3]
+members are always passed through the kernel as-is,
+making additional context available to application.
+.El
+.Pp
+The
+.Va flags
+field can contain the following values:
+.Bl -tag -width XXXEV_ONESHOT -offset indent
+.It Dv EV_ADD
+Adds the event to the kqueue.
+Re-adding an existing event will modify the parameters of the original
+event, and not result in a duplicate entry.
+Adding an event automatically enables it,
+unless overridden by the EV_DISABLE flag.
+.It Dv EV_ENABLE
+Permit
+.Fn kevent
+to return the event if it is triggered.
+.It Dv EV_DISABLE
+Disable the event so
+.Fn kevent
+will not return it.
+The filter itself is not disabled.
+.It Dv EV_DISPATCH
+Disable the event source immediately after delivery of an event.
+See
+.Dv EV_DISABLE
+above.
+.It Dv EV_DELETE
+Removes the event from the kqueue.
+Events which are attached to file descriptors are automatically deleted
+on the last close of the descriptor.
+.It Dv EV_RECEIPT
+This flag is useful for making bulk changes to a kqueue without draining
+any pending events.
+When passed as input, it forces
+.Dv EV_ERROR
+to always be returned.
+When a filter is successfully added the
+.Va data
+field will be zero.
+Note that if this flag is encountered and there is no remaining space in
+.Fa eventlist
+to hold the
+.Dv EV_ERROR
+event, then subsequent changes will not get processed.
+.It Dv EV_ONESHOT
+Causes the event to return only the first occurrence of the filter
+being triggered.
+After the user retrieves the event from the kqueue, it is deleted.
+.It Dv EV_CLEAR
+After the event is retrieved by the user, its state is reset.
+This is useful for filters which report state transitions
+instead of the current state.
+Note that some filters may automatically set this flag internally.
+.It Dv EV_EOF
+Filters may set this flag to indicate filter-specific EOF condition.
+.It Dv EV_ERROR
+See
+.Sx RETURN VALUES
+below.
+.El
+.Ss Filters
+Filters are identified by a number.
+There are two types of filters; pre-defined filters which
+are described below, and third-party filters that may be added with
+.Xr kfilter_register 9
+by kernel sub-systems, third-party device drivers, or loadable
+kernel modules.
+.Pp
+As a third-party filter is referenced by a well-known name instead
+of a statically assigned number, two
+.Xr ioctl 2 Ns s
+are supported on the file descriptor returned by
+.Fn kqueue
+to map a filter name to a filter number, and vice-versa (passing
+arguments in a structure described below):
+.Bl -tag -width KFILTER_BYFILTER -offset indent
+.It Dv KFILTER_BYFILTER
+Map
+.Va filter
+to
+.Va name ,
+which is of size
+.Va len .
+.It Dv KFILTER_BYNAME
+Map
+.Va name
+to
+.Va filter .
+.Va len
+is ignored.
+.El
+.Pp
+The following structure is used to pass arguments in and out of the
+.Xr ioctl 2 :
+.Bd -literal -offset indent
+struct kfilter_mapping {
+	char	 *name;		/* name to lookup or return */
+	size_t	 len;		/* length of name */
+	uint32_t filter;	/* filter to lookup or return */
+};
+.Ed
+.Pp
+The predefined system filters are listed below.
+Arguments may be passed to and from the filter via the
+.Va fflags
+and
+.Va data
+fields in the kevent structure.
+.Pp
+The predefined system filters are:
+.Bl -tag -width EVFILT_SIGNAL
+.It Dv EVFILT_READ
+Takes a descriptor as the identifier, and returns whenever
+there is data available to read.
+The behavior of the filter is slightly different depending
+on the descriptor type.
+.Bl -tag -width 2n
+.It Sockets
+Sockets which have previously been passed to
+.Xr listen 2
+return when there is an incoming connection pending.
+.Va data
+contains the size of the listen backlog (i.e., the number of
+connections ready to be accepted with
+.Xr accept 2 . )
+.Pp
+Other socket descriptors return when there is data to be read,
+subject to the
+.Dv SO_RCVLOWAT
+value of the socket buffer.
+This may be overridden with a per-filter low water mark at the
+time the filter is added by setting the
+NOTE_LOWAT
+flag in
+.Va fflags ,
+and specifying the new low water mark in
+.Va data .
+On return,
+.Va data
+contains the number of bytes in the socket buffer.
+.Pp
+If the read direction of the socket has shutdown, then the filter
+also sets EV_EOF in
+.Va flags ,
+and returns the socket error (if any) in
+.Va fflags .
+It is possible for EOF to be returned (indicating the connection is gone)
+while there is still data pending in the socket buffer.
+.It Vnodes
+Returns when the file pointer is not at the end of file.
+.Va data
+contains the offset from current position to end of file,
+and may be negative.
+.\" .Pp
+.\" This behavior is different from
+.\" .Xr poll 2 ,
+.\" where read events are triggered for regular files unconditionally.
+.\" This event can be triggered unconditionally by setting the
+.\" .Dv NOTE_FILE_POLL
+.\" flag in
+.\" .Va fflags .
+.It "Fifos, Pipes"
+Returns when there is data to read;
+.Va data
+contains the number of bytes available.
+.Pp
+When the last writer disconnects, the filter will set EV_EOF in
+.Va flags .
+This may be cleared by passing in EV_CLEAR, at which point the
+filter will resume waiting for data to become available before
+returning.
+.It "BPF devices"
+Returns when the BPF buffer is full, the BPF timeout has expired, or
+when the BPF has
+.Dq immediate mode
+enabled and there is any data to read;
+.Va data
+contains the number of bytes available.
+.El
+.It Dv EVFILT_WRITE
+Takes a descriptor as the identifier, and returns whenever
+it is possible to write to the descriptor.
+For sockets, pipes, fifos, and ttys,
+.Va data
+will contain the amount of space remaining in the write buffer.
+The filter will set EV_EOF when the reader disconnects, and for
+the fifo case, this may be cleared by use of EV_CLEAR.
+Note that this filter is not supported for vnodes.
+.Pp
+For sockets, the low water mark and socket error handling is
+identical to the EVFILT_READ case.
+.It Dv EVFILT_EMPTY
+Takes a descriptor as the identifier, and returns whenever
+there is no remaining data in the write buffer.
+This is currently implemented only for sockets.
+It's primary purpose is to provide visibility to an application that all
+previously written data has been acknowledged by the receiver at the TCP
+layer.
+.It Dv EVFILT_AIO
+This is not implemented in
+.Nx .
+.ig
+The sigevent portion of the AIO request is filled in, with
+.Va sigev_notify_kqueue
+containing the descriptor of the kqueue that the event should
+be attached to,
+.Va sigev_value
+containing the udata value, and
+.Va sigev_notify
+set to SIGEV_EVENT.
+When the aio_* function is called, the event will be registered
+with the specified kqueue, and the
+.Va ident
+argument set to the
+.Fa struct aiocb
+returned by the aio_* function.
+The filter returns under the same conditions as aio_error.
+.Pp
+Alternatively, a kevent structure may be initialized, with
+.Va ident
+containing the descriptor of the kqueue, and the
+address of the kevent structure placed in the
+.Va aio_lio_opcode
+field of the AIO request.
+However, this approach will not work on
+architectures with 64-bit pointers, and should be considered deprecated.
+..
+.It Dv EVFILT_VNODE
+Takes a file descriptor as the identifier and the events to watch for in
+.Va fflags ,
+and returns when one or more of the requested events occurs on the descriptor.
+The events to monitor are:
+.Bl -tag -width NOTE_CLOSE_WRITE
+.It Dv NOTE_ATTRIB
+The file referenced by the descriptor had its attributes changed.
+.It Dv NOTE_CLOSE
+A file descriptor without write access referencing the file was closed.
+.It Dv NOTE_CLOSE_WRITE
+A file descriptor with write access referencing the file was closed.
+.It Dv NOTE_DELETE
+.Xr unlink 2
+was called on the file referenced by the descriptor.
+.It Dv NOTE_EXTEND
+The file referenced by the descriptor was extended.
+.It Dv NOTE_LINK
+The link count on the file changed.
+.It Dv NOTE_OPEN
+The file referenced by the descriptor was opened.
+.It Dv NOTE_READ
+A read occurred on the file referenced by the descriptor.
+.It Dv NOTE_RENAME
+The file referenced by the descriptor was renamed.
+.It Dv NOTE_REVOKE
+Access to the file was revoked via
+.Xr revoke 2
+or the underlying file system was unmounted.
+.It Dv NOTE_WRITE
+A write occurred on the file referenced by the descriptor.
+.El
+.Pp
+On return,
+.Va fflags
+contains the events which triggered the filter.
+.It Dv EVFILT_PROC
+Takes the process ID to monitor as the identifier and the events to watch for
+in
+.Va fflags ,
+and returns when the process performs one or more of the requested events.
+If a process can normally see another process, it can attach an event to it.
+The events to monitor are:
+.Bl -tag -width XXNOTE_TRACKERR
+.It Dv NOTE_EXIT
+The process has exited.
+The exit code of the process is stored in
+.Va data .
+.It Dv NOTE_FORK
+The process has called
+.Xr fork 2 .
+.It Dv NOTE_EXEC
+The process has executed a new process via
+.Xr execve 2
+or similar call.
+.It Dv NOTE_TRACK
+Follow a process across
+.Xr fork 2
+calls.
+The parent process will return with NOTE_TRACK set in the
+.Va fflags
+field, while the child process will return with NOTE_CHILD set in
+.Va fflags
+and the parent PID in
+.Va data .
+.It Dv NOTE_TRACKERR
+This flag is returned if the system was unable to attach an event to
+the child process, usually due to resource limitations.
+.El
+.Pp
+On return,
+.Va fflags
+contains the events which triggered the filter.
+.It Dv EVFILT_SIGNAL
+Takes the signal number to monitor as the identifier and returns
+when the given signal is delivered to the current process.
+This coexists with the
+.Xr signal 3
+and
+.Xr sigaction 2
+facilities, and has a lower precedence.
+The filter will record
+all attempts to deliver a signal to a process, even if the signal has
+been marked as SIG_IGN.
+Event notification happens after normal signal delivery processing.
+.Va data
+returns the number of times the signal has occurred since the last call to
+.Fn kevent .
+This filter automatically sets the EV_CLEAR flag internally.
+.It Dv EVFILT_TIMER
+Establishes an arbitrary timer identified by
+.Va ident .
+When adding a timer,
+.Va data
+specifies the timeout period in units described below, or, if
+.Dv NOTE_ABSTIME
+is set in
+.Va fflags ,
+specifies the absolute time at which the timer should fire.
+The timer will repeat unless
+.Dv EV_ONESHOT
+is set in
+.Va flags
+or
+.Dv NOTE_ABSTIME
+is set in
+.Va fflags .
+On return,
+.Va data
+contains the number of times the timeout has expired since the last call to
+.Fn kevent .
+This filter automatically sets
+.Dv EV_CLEAR
+in
+.Va flags
+for periodic timers.
+Timers created with
+.Dv NOTE_ABSTIME
+remain activated on the kqueue once the absolute time has passed unless
+.Dv EV_CLEAR
+or
+.Dv EV_ONESHOT
+are also specified.
+.Dv CLOCK_REALTIME
+is the reference clock for timers created with
+.Dv NOTE_ABSTIME .
+.Pp
+The filter accepts the following flags in the
+.Va fflags
+argument:
+.Bl -tag -width XXNOTE_TRACKERR
+.It Dv NOTE_SECONDS
+The timer value in
+.Va data
+is expressed in seconds.
+.It Dv NOTE_MSECONDS
+The timer value in
+.Va data
+is expressed in milliseconds.
+.It Dv NOTE_USECONDS
+The timer value in
+.Va data
+is expressed in microseconds.
+.It Dv NOTE_NSECONDS
+The timer value in
+.Va data
+is expressed in nanoseconds.
+.It Dv NOTE_ABSTIME
+The timer value is an absolute time; see discussion above.
+.El
+.Pp
+Note that
+.Dv NOTE_SECONDS ,
+.Dv NOTE_MSECONDS ,
+.Dv NOTE_USECONDS ,
+and
+.Dv NOTE_NSECONDS
+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 .
+.It Dv EVFILT_FS
+Establishes a file system monitor.
+Currently it only monitors file system mount and unmount actions.
+.It Dv EVFILT_USER
+Establishes a user event identified by
+.Va ident
+which is not associated with any kernel mechanism but is triggered by
+user level code.
+The lower 24 bits of the
+.Va fflags
+may be used for user defined flags and manipulated using the following:
+.Bl -tag -width "Dv NOTE_FFLAGSMASK"
+.It Dv NOTE_FFNOP
+Ignore the input
+.Va fflags .
+.It Dv NOTE_FFAND
+Bitwise AND
+.Va fflags .
+.It Dv NOTE_FFOR
+Bitwise OR
+.Va fflags .
+.It Dv NOTE_FFCOPY
+Copy
+.Va fflags .
+.It Dv NOTE_FFCTRLMASK
+Control mask for
+.Va fflags .
+.It Dv NOTE_FFLAGSMASK
+User defined flag mask for
+.Va fflags .
+.El
+.Pp
+A user event is triggered for output with the following:
+.Bl -tag -width "Dv NOTE_FFLAGSMASK"
+.It Dv NOTE_TRIGGER
+Cause the event to be triggered.
+.El
+.Pp
+On return,
+.Va fflags
+contains the users defined flags in the lower 24 bits.
+.El
+.Sh CANCELLATION BEHAVIOUR
+If
+.Fa nevents
+is non-zero, i.e., the function is potentially blocking, the call
+is a cancellation point.
+Otherwise, i.e., if
+.Fa nevents
+is zero, the call is not cancellable.
+Cancellation can only occur before any changes are made to the kqueue,
+or when the call was blocked and no changes to the queue were requested.
+.Sh RETURN VALUES
+The
+.Fn kqueue
+system call
+creates 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
+.Dv errno
+is set.
+.Pp
+The
+.Fn kevent
+system call
+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
+.Va flags
+and the system error in
+.Va data .
+Otherwise,
+.Dv \-1
+will be returned, and
+.Dv errno
+will be set to indicate the error condition.
+If the time limit expires, then
+.Fn kevent
+returns 0.
+.Sh EXAMPLES
+The following example program monitors a file (provided to it as the first
+argument) and prints information about some common events it receives
+notifications for:
+.Bd -literal -offset indent
+#include <sys/types.h>
+#include <sys/event.h>
+#include <sys/time.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <fcntl.h>
+#include <err.h>
+
+int
+main(int argc, char *argv[])
+{
+        int fd, kq, nev;
+        struct kevent ev;
+        static const struct timespec tout = { 1, 0 };
+
+        if ((fd = open(argv[1], O_RDONLY)) == -1)
+                err(1, "Cannot open `%s'", argv[1]);
+
+        if ((kq = kqueue()) == -1)
+                err(1, "Cannot create kqueue");
+
+        EV_SET(&ev, fd, EVFILT_VNODE, EV_ADD | EV_ENABLE | EV_CLEAR,
+            NOTE_DELETE|NOTE_WRITE|NOTE_EXTEND|NOTE_ATTRIB|NOTE_LINK|
+            NOTE_RENAME|NOTE_REVOKE, 0, 0);
+        if (kevent(kq, &ev, 1, NULL, 0, &tout) == -1)
+                err(1, "kevent");
+        for (;;) {
+                nev = kevent(kq, NULL, 0, &ev, 1, &tout);
+                if (nev == -1)
+                        err(1, "kevent");
+                if (nev == 0)
+                        continue;
+                if (ev.fflags & NOTE_DELETE) {
+                        printf("deleted ");
+                        ev.fflags &= ~NOTE_DELETE;
+                }
+                if (ev.fflags & NOTE_WRITE) {
+                        printf("written ");
+                        ev.fflags &= ~NOTE_WRITE;
+                }
+                if (ev.fflags & NOTE_EXTEND) {
+                        printf("extended ");
+                        ev.fflags &= ~NOTE_EXTEND;
+                }
+                if (ev.fflags & NOTE_ATTRIB) {
+                        printf("chmod/chown/utimes ");
+                        ev.fflags &= ~NOTE_ATTRIB;
+                }
+                if (ev.fflags & NOTE_LINK) {
+                        printf("hardlinked ");
+                        ev.fflags &= ~NOTE_LINK;
+                }
+                if (ev.fflags & NOTE_RENAME) {
+                        printf("renamed ");
+                        ev.fflags &= ~NOTE_RENAME;
+                }
+                if (ev.fflags & NOTE_REVOKE) {
+                        printf("revoked ");
+                        ev.fflags &= ~NOTE_REVOKE;
+                }
+                printf("\\n");
+                if (ev.fflags)
+                        warnx("unknown event 0x%x\\n", ev.fflags);
+        }
+}
+.Ed
+.Sh ERRORS
+The
+.Fn kqueue
+function fails if:
+.Bl -tag -width Er
+.It Bq Er EMFILE
+The per-process descriptor table is full.
+.It Bq Er ENFILE
+The system file table is full.
+.It Bq Er ENOMEM
+The kernel failed to allocate enough memory for the kernel queue.
+.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 EBADF
+The specified descriptor is invalid.
+.It Bq Er EFAULT
+There was an error reading or writing the
+.Va kevent
+structure.
+.It Bq Er EINTR
+A signal was delivered before the timeout expired and before any
+events were placed on the kqueue for return.
+All changes contained in the
+.Fa changelist
+are applied before returning this error.
+.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 EOPNOTSUPP
+This type of file descriptor is not supported for
+.Fn kevent
+operations.
+.It Bq Er ESRCH
+The specified process to attach to does not exist.
+.El
+.Sh SEE ALSO
+.\" .Xr aio_error 2 ,
+.\" .Xr aio_read 2 ,
+.\" .Xr aio_return 2 ,
+.Xr fork 2 ,
+.Xr ioctl 2 ,
+.Xr listen 2 ,
+.Xr poll 2 ,
+.Xr read 2 ,
+.Xr select 2 ,
+.Xr sigaction 2 ,
+.Xr unlink 2 ,
+.Xr write 2 ,
+.Xr signal 3 ,
+.Xr timespec 3 ,
+.Xr kfilter_register 9 ,
+.Xr knote 9
+.Rs
+.%A Jonathan Lemon
+.%T "Kqueue: A Generic and Scalable Event Notification Facility"
+.%I USENIX Association
+.%B Proceedings of the FREENIX Track: 2001 USENIX Annual Technical Conference
+.%D June 25-30, 2001
+.%U http://www.usenix.org/event/usenix01/freenix01/full_papers/lemon/lemon.pdf
+.Re
+.Sh HISTORY
+The
+.Fn kqueue
+and
+.Fn kevent
+functions first appeared in
+.Fx 4.1 ,
+and then in
+.Nx 2.0 .
+The
+.Fn kqueue1
+function first appeared in
+.Nx 6.0 .
+.Pp
+The
+.Fn EV_SET
+macro was protected from evaluating multiple times the first argument in
+.Nx 8.0 .
+.Pp
+The
+.Va udata
+type was changed from intptr_t to void * in
+.Nx 10.0 .
+.Pp
+Support for
+.Dv NOTE_SECONDS ,
+.Dv NOTE_MSECONDS ,
+.Dv NOTE_USECONDS ,
+.Dv NOTE_NSECONDS ,
+and
+.Dv NOTE_ABSTIME
+filter flags for
+.Dv EVFILT_TIMER
+was added in
+.Nx 10.0 .
+.Pp
+Support for
+.Dv NOTE_OPEN ,
+.Dv NOTE_CLOSE ,
+.Dv NOTE_CLOSE_WRITE ,
+and
+NOTE_READ
+filter flags for
+.Dv EVFILT_VNODE
+was added in
+.Nx 10.0 .
+.Pp
+Support for
+.Dv EVFILT_EMPTY
+was added in
+.Nx 10.0 .