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KQUEUE(2)		  FreeBSD System Calls Manual		     KQUEUE(2)

     kqueue, kevent -- kernel event notification mechanism

     Standard C	Library	(libc, -lc)

     #include <sys/types.h>
     #include <sys/event.h>
     #include <sys/time.h>


     kevent(int	kq, const struct kevent	*changelist, int nchanges,
	 struct	kevent *eventlist, int nevents,
	 const struct timespec *timeout);

     EV_SET(kev, ident,	filter,	flags, fflags, data, udata);

     The 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.

     The filter	is executed upon the initial registration of a kevent in order
     to	detect whether a preexisting condition is present, and is also exe-
     cuted 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.

     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.

     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 close() on a file
     descriptor	will remove any	kevents	that reference the descriptor.

     The kqueue() system call creates a	new kernel event queue and returns a
     descriptor.  The queue is not inherited by	a child	created	with fork(2).
     However, if rfork(2) is called without the	RFFDG flag, then the descrip-
     tor table is shared, which	will allow sharing of the kqueue between two

     The kevent() system call is used to register events with the queue, and
     return any	pending	events to the user.  The changelist argument is	a
     pointer to	an array of kevent structures, as defined in <sys/event.h>.
     All changes contained in the changelist are applied before	any pending
     events are	read from the queue.  The nchanges argument gives the size of
     changelist.  The eventlist	argument is a pointer to an array of kevent
     structures.  The nevents argument determines the size of eventlist.  When
     nevents is	zero, kevent() will return immediately even if there is	a
     timeout specified unlike select(2).  If timeout is	a non-NULL pointer, it
     specifies a maximum interval to wait for an event,	which will be inter-
     preted as a struct	timespec.  If timeout is a NULL	pointer, kevent()
     waits indefinitely.  To effect a poll, the	timeout	argument should	be
     non-NULL, pointing	to a zero-valued timespec structure.  The same array
     may be used for the changelist and	eventlist.

     The EV_SET() macro	is provided for	ease of	initializing a kevent struc-

     The kevent	structure is defined as:

     struct kevent {
	     uintptr_t ident;	     /*	identifier for this event */
	     short     filter;	     /*	filter for event */
	     u_short   flags;	     /*	action flags for kqueue	*/
	     u_int     fflags;	     /*	filter flag value */
	     intptr_t  data;	     /*	filter data value */
	     void      *udata;	     /*	opaque user data identifier */

     The fields	of struct kevent are:

     ident	Value used to identify this event.  The	exact interpretation
		is determined by the attached filter, but often	is a file

     filter	Identifies the kernel filter used to process this event.  The
		pre-defined system filters are described below.

     flags	Actions	to perform on the event.

     fflags	Filter-specific	flags.

     data	Filter-specific	data value.

     udata	Opaque user-defined value passed through the kernel unchanged.

     The flags field can contain the following values:

     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.

     EV_ENABLE	  Permit kevent() to return the	event if it is triggered.

     EV_DISABLE	  Disable the event so kevent()	will not return	it.  The fil-
		  ter itself is	not disabled.

     EV_DISPATCH  Disable the event source immediately after delivery of an
		  event.  See EV_DISABLE above.

     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.

     EV_RECEIPT	  This flag is useful for making bulk changes to a kqueue
		  without draining any pending events.	When passed as input,
		  it forces EV_ERROR to	always be returned.  When a filter is
		  successfully added the data field will be zero.

     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.

     EV_CLEAR	  After	the event is retrieved by the user, its	state is
		  reset.  This is useful for filters which report state	tran-
		  sitions instead of the current state.	 Note that some	fil-
		  ters may automatically set this flag internally.

     EV_EOF	  Filters may set this flag to indicate	filter-specific	EOF

     EV_ERROR	  See RETURN VALUES below.

     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

     EVFILT_READ	 Takes a descriptor as the identifier, and returns
			 whenever there	is data	available to read.  The	behav-
			 ior of	the filter is slightly different depending on
			 the descriptor	type.

			     Sockets which have	previously been	passed to
			     listen() return when there	is an incoming connec-
			     tion pending.  data contains the size of the lis-
			     ten backlog.

			     Other socket descriptors return when there	is
			     data to be	read, subject to the 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
			     fflags, and specifying the	new low	water mark in
			     data.  On return, data contains the number	of
			     bytes of protocol data available to read.

			     If	the read direction of the socket has shutdown,
			     then the filter also sets EV_EOF in flags,	and
			     returns the socket	error (if any) in fflags.  It
			     is	possible for EOF to be returned	(indicating
			     the connection is gone) while there is still data
			     pending in	the socket buffer.

			     Returns when the file pointer is not at the end
			     of	file.  data contains the offset	from current
			     position to end of	file, and may be negative.

			 Fifos,	Pipes
			     Returns when the there is data to read; data con-
			     tains the number of bytes available.

			     When the last writer disconnects, the filter will
			     set EV_EOF	in 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.

			 BPF devices
			     Returns when the BPF buffer is full, the BPF
			     timeout has expired, or when the BPF has
			     ``immediate mode''	enabled	and there is any data
			     to	read; data contains the	number of bytes	avail-

     EVFILT_WRITE	 Takes a descriptor as the identifier, and returns
			 whenever it is	possible to write to the descriptor.
			 For sockets, pipes and	fifos, 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	or BPF devices.

			 For sockets, the low water mark and socket error han-
			 dling is identical to the EVFILT_READ case.

     EVFILT_AIO		 The sigevent portion of the AIO request is filled in,
			 with sigev_notify_kqueue containing the descriptor of
			 the kqueue that the event should be attached to,
			 sigev_notify_kevent_flags containing the kevent flags
			 which should be EV_ONESHOT, EV_CLEAR or EV_DISPATCH,
			 sigev_value containing	the udata value, and
			 sigev_notify set to SIGEV_KEVENT.  When the aio_*()
			 system	call is	made, the event	will be	registered
			 with the specified kqueue, and	the ident argument set
			 to the	struct aiocb returned by the aio_*() system
			 call.	The filter returns under the same conditions
			 as aio_error().

     EVFILT_VNODE	 Takes a file descriptor as the	identifier and the
			 events	to watch for in	fflags,	and returns when one
			 or more of the	requested events occurs	on the
			 descriptor.  The events to monitor are:

			 NOTE_DELETE	 The unlink() system call was called
					 on the	file referenced	by the

			 NOTE_WRITE	 A write occurred on the file refer-
					 enced by the descriptor.

			 NOTE_EXTEND	 The file referenced by	the descriptor
					 was extended.

			 NOTE_ATTRIB	 The file referenced by	the descriptor
					 had its attributes changed.

			 NOTE_LINK	 The link count	on the file changed.

			 NOTE_RENAME	 The file referenced by	the descriptor
					 was renamed.

			 NOTE_REVOKE	 Access	to the file was	revoked	via
					 revoke(2) or the underlying file sys-
					 tem was unmounted.

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

     EVFILT_PROC	 Takes the process ID to monitor as the	identifier and
			 the events to watch for in 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:

			 NOTE_EXIT	   The process has exited.  The	exit
					   status will be stored in data.

			 NOTE_FORK	   The process has called fork().

			 NOTE_EXEC	   The process has executed a new
					   process via execve(2) or a similar

			 NOTE_TRACK	   Follow a process across fork()
					   calls.  The parent process regis-
					   ters	a new kevent to	monitor	the
					   child process using the same	fflags
					   as the original event.  The child
					   process will	signal an event	with
					   NOTE_CHILD set in fflags and	the
					   parent PID in data.

					   If the parent process fails to reg-
					   ister a new kevent (usually due to
					   resource limitations), it will sig-
					   nal an event	with NOTE_TRACKERR set
					   in fflags, and the child process
					   will	not signal a NOTE_CHILD	event.

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

     EVFILT_SIGNAL	 Takes the signal number to monitor as the identifier
			 and returns when the given signal is delivered	to the
			 process.  This	coexists with the signal() and
			 sigaction() facilities, and has a lower precedence.
			 The filter will record	all attempts to	deliver	a sig-
			 nal to	a process, even	if the signal has been marked
			 as SIG_IGN, except for	the SIGCHLD signal, which, if
			 ignored, won't	be recorded by the filter.  Event
			 notification happens after normal signal delivery
			 processing.  data returns the number of times the
			 signal	has occurred since the last call to kevent().
			 This filter automatically sets	the EV_CLEAR flag

     EVFILT_TIMER	 Establishes an	arbitrary timer	identified by ident.
			 When adding a timer, data specifies the timeout
			 period.  The timer will be periodic unless EV_ONESHOT
			 is specified.	On return, data	contains the number of
			 times the timeout has expired since the last call to
			 kevent().  This filter	automatically sets the
			 EV_CLEAR flag internally.  There is a system wide
			 limit on the number of	timers which is	controlled by
			 the kern.kq_calloutmax	sysctl.

			 NOTE_SECONDS	   data	is in seconds.

			 NOTE_MSECONDS	   data	is in milliseconds.

			 NOTE_USECONDS	   data	is in microseconds.

			 NOTE_NSECONDS	   data	is in nanoseconds.

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

     EVFILT_USER	 Establishes a user event identified by	ident which is
			 not associated	with any kernel	mechanism but is trig-
			 gered by user level code.  The	lower 24 bits of the
			 fflags	may be used for	user defined flags and manipu-
			 lated using the following:

			 NOTE_FFNOP	     Ignore the	input fflags.

			 NOTE_FFAND	     Bitwise AND fflags.

			 NOTE_FFOR	     Bitwise OR	fflags.

			 NOTE_FFCOPY	     Copy fflags.

			 NOTE_FFCTRLMASK     Control mask for fflags.

			 NOTE_FFLAGSMASK     User defined flag mask for

			 A user	event is triggered for output with the follow-

			 NOTE_TRIGGER	     Cause the event to	be triggered.

			 On return, fflags contains the	users defined flags in
			 the lower 24 bits.

     If	nevents	is non-zero, i.e. the function is potentially blocking,	the
     call is a cancellation point.  Otherwise, i.e. if 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.

     The 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 errno set.

     The kevent() system call returns the number of events placed in the
     eventlist,	up to the value	given by nevents.  If an error occurs while
     processing	an element of the changelist and there is enough room in the
     eventlist,	then the event will be placed in the eventlist with EV_ERROR
     set in flags and the system error in data.	 Otherwise, -1 will be
     returned, and errno will be set to	indicate the error condition.  If the
     time limit	expires, then kevent() returns 0.

     The kqueue() system call fails if:

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

     [EMFILE]		The per-process	descriptor table is full.

     [ENFILE]		The system file	table is full.

     The kevent() system call fails if:

     [EACCES]		The process does not have permission to	register a

     [EFAULT]		There was an error reading or writing the kevent

     [EBADF]		The specified descriptor is invalid.

     [EINTR]		A signal was delivered before the timeout expired and
			before any events were placed on the kqueue for

     [EINTR]		A cancellation request was delivered to	the thread,
			but not	yet handled.

     [EINVAL]		The specified time limit or filter is invalid.

     [ENOENT]		The event could	not be found to	be modified or

     [ENOMEM]		No memory was available	to register the	event or, in
			the special case of a timer, the maximum number	of
			timers has been	exceeded.  This	maximum	is config-
			urable via the kern.kq_calloutmax sysctl.

     [ESRCH]		The specified process to attach	to does	not exist.

     When kevent() call	fails with EINTR error,	all changes in the changelist
     have been applied.

     aio_error(2), aio_read(2),	aio_return(2), poll(2),	read(2), select(2),
     sigaction(2), write(2), pthread_setcancelstate(3),	signal(3)

     The kqueue() and kevent() system calls first appeared in FreeBSD 4.1.

     The kqueue() system and this manual page were written by Jonathan Lemon

     The timeout value is limited to 24	hours; longer timeouts will be
     silently reinterpreted as 24 hours.

FreeBSD	10.2			March 29, 2015			  FreeBSD 10.2


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